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Biodiversity, Classification, and Numbers of Species of Protists

John O. Corliss
P.O. Box 2729, Bala Cynwyd, PA 19004

Clear recognition of the great evolutionary gulf between the prokaryotes (essentially bacteria) and the eukaryotes (all other organisms) nearly four decades ago led to numerous studies that preoccupied research biologists' time for some years. But in the 1970s, attention began to refocus on the equally important specific fields of eukaryogenesis (the evolutionary appearance of cells above the bacterial level) and the phylogenetic origin of multicellular-multitissued organisms themselves, with the recognition that filling the gap between bacteria and animals/ plants seemed to require some intermediate level of organismic organization. The hypothetical “gap-fillers”—to the surprise, perhaps, of many experimental biologists but not of field and taxonomic protozoologists and phycologists—turned out to be represented by the largely unicellular eukaryotic microorganisms, a huge assemblage (tens of thousands of species) of widespread but often poorly known forms that now can collectively be called the protists. Thus dawned the interdisciplinary field of protistology, arbitrarily said to have reached a recognizable state in about 1975 (Corliss 1986, 1987). Vast improvements in cytological techniques, including kinds of electron microscopy (Patterson 1994) and the advent of molecular methods (Cavalier-Smith 1995), have since aided greatly in the expansion of such investigations exploiting what may be termed the protist perspective.

Biodiversity, quite new itself as a term and concept in biology, is often linked with conservation in people's minds, and the organisms involved are typically the highly visible plants and animals now living on Earth. The protists—that is, the generally unicellular and microscopic algae and protozoa and the lower fungi—are, like the bacteria, cosmopolitan and ubiquitous; but the healthy abundance

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of many of these microorganisms is absolutely necessary for maintenance of a sustainable world. Their roles at the base of the food chain and in nutrient recycling are known to be of the highest importance, and their potential in treating diseases is under study. Their roles in the preservation, not to mention the (past) evolution, of other organisms have been and are indeed indispensable. In contrast, some of the parasitic species are highly virulent to their hosts and thus can have disastrous effects on human populations, food crops, and domesticated animals. Today's unpredicted increase in appearance of opportunistic protistan parasites in AIDS patients is an example of our need to understand these organisms better. Only recently have all the points such as those mentioned above begun to become appreciated (Andersen 1992, 1998; Colwell 1997; Corliss 1989b, 1991, 1998a; Finlay 1998; Finlay and Esteban 1998; Hawksworth and Colwell 1992; John 1994; Norton and others 1996; Patterson and Sogin 1993; Sogin and Hinkle 1997; Vickerman 1992, 1998). But it is increasingly clear that much further work is required to assess the multiple roles of protists in natural ecosystems.

To speak quantitatively about the numbers of known protistan species, a main aim of this paper, we must first have some idea of the qualitative nature of protists: what are they, and how can they be defined and classified? A further question—what are the probable evolutionary and phylogenetic interrelationships of what to most people is the rather large number of separate high-level taxa commonly recognized as containing protists?—is mostly well beyond discussion in the present brief essay, although it obviously affects the classification of the organisms concerned. For often detailed treatments of major aspects of the last question, the reader is referred to Coombs and others (1998), Hausmann and Hülsman (1996), Hülsmann and Hausmann (1994), Karpov (1990), Katz (1998), Knoll (1992), Ku´znicki and Walne (1993), Lipscomb and others (1998), Patterson (1994), Schlegel (1998), Sleigh (1995), Sogin and others (1996) and the many pertinent references within those works.

What is a Protist?

Even defining the term protist is somewhat controversial, so I shall offer only a broad and general description here, attempting to make clear their essential uniquenesses, in combination, as a great and diverse assemblage of organisms on Earth. Recent comments on this difficult question have appeared in works by Andersen (1998), Cavalier-Smith (1993b, 1998a), Corliss (1994a, 1998b), Hausmann and Hülsmann (1996), Margulis (1996), Patterson (1994), Vickerman (1998).

Protists, typically and mostly, are single-celled, microscopic eukaryotic organisms, occasionally forming a single tissue that can lead to large body size (for example, in some multicellular brown algae). As cells, they may have one to several nuclei; and various other organelles are always present in their cytoplasm. They represent, in general, a structural grade between the bacteria or prokaryotes and the so-called higher eukaryotes. Although eukaryotic themselves, protists do not have multicelled organs or true vascular systems, and ordinarily they do not show complex developmental or embryonic stages in their life cycles (ontogeny). Whereas the ancestors of some contemporary protistan groups very likely gave rise

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to lines leading to such recognized kingdoms as Fungi, Animalia, and Plantae, others—as far as we can surmise at this time—have retained their protistan nature, evolutionarily speaking. It is reasonable to assume that extinction of protistan groups occurred often during past millennia, although the fossil record to date has not been very helpful in this respect (Lipps 1993; Tappan 1980). Sexuality is not recognized in species of many taxa; asexual division is the most common mode of reproduction and allows stability of an adapted genotype.

Overall distribution today of these lower eukaryotes is cosmopolitan; nutritional and locomotive modes are many, and there are amazing structural and functional adaptations (Hausmann and Hülsmann 1996). The single-celled species are wholly independent organisms: the two terms—cell and organism—are thus not mutually exclusive descriptors (Corliss 1989a; Hausmann and Bradbury 1996). Major habitats of free-living forms include soils and bodies of freshwater and salt water; and ectosymbiotic and endosymbiotic species are found in association with numerous animal and some plant species and even other protists. Some parasitic forms are highly pathogenic (some malarial species of the genus Plasmodium are most notable), with hosts that include humans. Useful species (from the human perspective) include the many involved in essential food chains, in nutrient turnover in lakes and seas, in functioning as bioindicators or biomonitors of pollution and potentially as biocontrol agents, in serving as ideal cells in a multitude of biomedical and medical research projects, and in their direct roles in the petroleum, food, medicinal, agricultural, aquacultural, and other commercial industries. It has been said that 40% of global photosynthesis (carbon fixation and oxygen production) is contributed by algae, and the abundant diatoms alone are responsible for nearly half that (Andersen 1992, 1998).

Historical Background on Protistan Taxonomy

We need to understand background information—however brief—on the overall classification of species now known as protists to appreciate their present status. More than a century ago, Ernst Haeckel (1866, 1878; and see Aescht 1998) and a few others proposed that these “lower forms” on the ladder of life should be considered as members of a distinct third kingdom, alongside the established kingdoms of Animalia and Plantae. To shorten a lengthy tale (Corliss 1998c; Lipscomb 1991; Ragan 1997; Rothschild 1989), such ideas, for various involved reasons, did not succeed for a long time, although refined and resurrected by such notable workers as Copeland (1956) and Whittaker (1969).

When the evolutionist-geneticist-microbiologist Margulis (1974, 1988; Margulis and Schwartz 1982; Margulis and others 1990) came on the scene, her forceful arguments stimulated a great deal of research in cell and evolutionary biology. Her undiluted enthusiasm convinced many a formerly reluctant biologist to appreciate the wisdom and particularly the convenience and pedagogical usefulness of a five-kingdom arrangement for all living organisms: Monera (or Bacteria), Protista (or Protoctista), Fungi, Plantae, and Animalia. A neo-Haeckelian system seemed to have become established for all time, although battles were (and are) incessantly waged over the internal composition of the “new” kingdom: what

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and how many algal divisions and protozoan phyla, for example, are to be subsumed under that heading? What was definitely and irrevocably clear was that protozoa should no longer be treated as “mini-animals” and that most algae must no longer be considered to be merely “mini-plants” (Corliss 1983).

In recent years, considerable evidence has indicated that some major lines of protists share closer relationships with other kingdoms (an outstanding example is green algae with land plants) than they do with formerly neighboring protistan taxa. Such revelations threaten the stability of the whole five-kingdom concept—but this is a complex subject largely beyond further consideration here. However, it is still often convenient and appropriate (as in this paper) to treat the many protistan groups as a single great assemblage, although using a lowercase “p” and writing of “the protists” rather than of “a kingdom Protista”. Incidentally, another problem, not due extended discussion here but deserving mention, is the nomenclatural matter of Protista and Protoctista (the latter is properly pronounced “proto-tista”, in that the “c” is silent in this combination). Arguments for both names exist in the literature, but I believe that today the consensus among research protistologists favors the shorter name; and there is no rule of nomenclature that obliges one to treat the longer word as having any official priority (Corliss 1990, 1994a).

Alternatives to accepting a single formal kingdom Protista have recently been reviewed (Corliss 1994a,b, 1998c; see also Cavalier-Smith 1998a); listing them should suffice here to give the reader an appreciation of possible choices that exist. One is to recognize no separate high-level taxon for the protists, considering them overall to represent but an evolutionary grade or level of cellular organization (between bacteria and the higher eukaryotes) and thus sidestepping a number of high-level taxonomic problems. A second is to view groups of protists as simply mostly independent evolutionary lines or lineages, again leaving aside attempts to define high-level taxonomic interrelationships among such lines; cladistically derived phylogenetic trees (such as those constructed from collected molecular biological data) often support such a choice. Finally, to avoid a single, perhaps highly artificial kingdom for the diverse protist assemblages, some workers have proposed the assignment of these organisms to multiple kingdoms of eukaryotes. Such kingdoms can number from five or six to 18–20 or even more. Some might be composed solely of protists; others might contain various protistan taxa but comprise predominantly taxa of existing major kingdoms of multicellular organisms (such as the Plantae, Animalia, or Fungi).

Classificational Framework for Major Groups of Protists

Although discussion of the evolutionary or phylogenetic relationships of the diverse high-level protistan groups is beyond the scope of the present paper, a taxonomic framework of some sort is necessary for clarity in the treatment of their nature or composition, including numbers and inventories of species. Only naming or identifying major assemblages will make possible our recognizing, comparing, and retrieving information about the different groups (Mayr 1997, 1998),

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many of which, in the case of the protists, have been known (under a variety of names) for scores of years.

Adopting Cavalier-Smith's (1998a) five eukaryotic kingdoms and their names, and using a constellation-of-characters (Corliss 1976) approach as a basis for their taxonomic separateness, I have assigned some 14 phyla of protists to PROTOZOA, 11 to CHROMISTA, six to PLANTAE, and two each to FUNGI and ANIMALIA; see table 1. Thus, I am suggesting that some 35 eukaryotic phyla are required to contain the protists overall—a welcome reduction from the 45 of 15 years ago (Corliss 1984). A very brief description of the taxonomic composition of the kingdoms involved is appropriate here because even the better-known ones might no longer embrace the same phyletic taxa as in years past. A link with the classical systems, at both nomenclatural and taxonomic levels, is needed if we are to understand the present locations and interrelationships of the diverse protistan forms implicated.

PROTOZOA (literally meaning “first animals”) traditionally has embraced species belonging not only to the phyla listed in Protozoa in table 1 but also to other major taxa no longer included there, most notably Cryptomonada, Haptomonada, Labyrinthomorpha, Opalinata, and some lower taxa of Bicosoecae, Chrysophyta, and Dictyochae; I consider these seven phyla to belong to the kingdom Chromista. The phyla Choanozoa, Myxozoa, and Microspora were also treated as protozoan taxa in the past. The first two of these are now assigned to Animalia, and the third to Fungi (see table 1). Even a few well-studied genera of Chlorophyta and Prasinophyta, phyla now both in Plantae in table 1, have been steadfastly embraced by protozoologists in their classification schemes. So, interestingly, the present kingdom Protozoa is considerably more restricted, more refined, and thus more meaningful than the former phylum Protozoa of the literature (see discussions in Cavalier-Smith 1993b; Corliss 1994a).

CHROMISTA never existed in former times as such, before Cavalier-Smith's (1981) proposal of this particular name (see also Cavalier-Smith 1986, 1989,

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1997b, 1998a). Despite its containing some former protozoan groups (see above), it is now largely “algal” in composition, including such major well- and long-known, mainly photosynthetic groups as Chrysophyta, Diatomae, Phaeophyta, and Raphidophyta (with their many well-known classes). Furthermore, botanists have generally claimed Bicosoecae and Dictyochae at the same time that zoologists were considering them to be “first animals”. In much of its composition, the kingdom Chromista of table 1 resembles the rather similar assemblage widely known today as the stramenopiles (Patterson 1989, 1994; Sogin and Hinkle 1997). Both circumscribed groups, chromists and stramenopiles, contain predominantly species of the old and large heterokont algal assemblage of the past botanical and phycological literature (see historical reviews in Corliss 1984, 1994a).

PLANTAE, a kingdom for scores of years, has conventionally been composed not only of the bryophytes, pteridophytes, and higher aquatic and terrestrial species (gymnosperms, angiosperms, and so on), but also traditionally of all the so-called algae, ranging from the prokaryotic cyanobacteria (blue-green algae) through the algal classes and divisions (or phyla) listed in this paper under various kingdoms, not to mention all fungal and even bacterial taxa. Here (see Corliss 1994a, 1998c), I have restricted the plant kingdom to the vascular (multicellular) photosynthetic eukaryotes plus four phyla of green algae, one of red algae, and one for the enigmatic glaucophytes (table 1).

FUNGI, separated from Plantae by various workers during the last 40–50 years (recently more vigorously; see Barr 1992), is sometimes persistently considered basically as a “plant” group. It has long included phyla of the so-called higher fungi (Ascomycota, Zygomycota, and Basidiomycota), but it has conventionally also laid claim to various lower fungi, including diverse kinds of slime molds (now under Protozoa or Chromista; see table 1) and the water molds or so-called motile zoosporic fungi (the chytrids, which are true fungi, and members of the Pseudofungi, which are quite different in many taxonomic characteristics and now assignable as heterokontic algae to Chromista). Very recent molecular studies add the curious and possibly ancient “protozoan” group of Microspora (the microsporidians), minute intracellular parasites with unique spores, to the Fungi (Canning 1998; Edlind 1998; Keeling and McFadden 1998).

ANIMALIA, long recognized as the haven for numerous invertebrate and vertebrate phyla, is little affected by protistan studies and reclassifications. However, the always-enigmatic Myxozoa (protozoan myxosporidians of the literature) are now thought to be animals of some sort (Anderson 1998; Cavalier-Smith 1998a; Corliss 1998b; Schlegel and others 1996; Siddall and others 1995; Smothers and others 1994). One might add to the kingdom, as I have controversially done (Corliss 1998c), the choanoflagellates, definitely considered a link to the sponges of Animalia and now to the Fungi as well (Cavalier-Smith 1998a,b).

Obstacles to Deriving Reliable Estimates of Numbers of Protists.

There are many reasons why our knowledge of the kinds and numbers of protistan species generally lags far behind that for numerous other groups of organ-

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isms; these deserve brief mention here. In general, and above all, their tremendous diversity—combined with their often microscopic size, cosmopolitan nature, lack of overt sexual processes, and no helpful fossil record—renders precise study of the morphology, taxonomy, and evolution of most of them very difficult. Furthermore, protists for scores of years have been described by a motley array of naturalists, zoologists, botanists, mycologists, cell biologists, ecologists, limnologists, microscopists, parasitologists, and, more recently, geneticists and evolutionary and molecular biologists—persons with highly diverse backgrounds and conceptual outlooks and, in many cases, without rigorous taxonomic training or even proper awareness of the relevant systematic literature in protistology. Other, more specific reasons for our continuing ignorance and uncertainty about numbers of protists in existence include the following (sometimes overlapping) factors:

• The lack of a universal definition of a species among largely asexual eukaryotic microorganisms. There are few guidelines to assist taxonomic protistologists in their choice from a veritable smorgasbord of kinds (some overlapping) of species in the biological literature: morphological, phenetic, nominal, ecological, cryptic, endemic, taxonomic, parataxonomic, biological, asexual, sexual, genetic (sibling or syngenic), molecular, and chimaeric. The morphospecies concept seems reliable for numerous protists (Finlay and others 1996). But what are the criteria for recognition of the separateness of presumably closely related species? And to what extent does polymorphism (common and often striking in many groups of protists) complicate the problem, not to mention the historical acquisition of some endocytoplasmic inclusions or organelles by engulfment of (or invasion by) “foreign” microorganisms in eons past (Bardele 1997; Cavalier-Smith and Lee 1985; Gray 1992; Margulis 1993, 1996; Sapp 1994; Taylor 1987a)?

• An abundance of nomenclatural problems, exacerbated by lack of clarity in recognizing boundaries at the species and much higher taxonomic levels (Corliss 1993). Different protists have inadvertently been given identical names, and the same protist might have been described independently under different names: this has happened especially in cases of the so-called ambiregnal protists (Corliss 1984, 1986, 1995; Patterson 1986b; Taylor and others 1986). Names of species not accepted by later revisers (for example, of a genus or family) fall into synonymy with the oldest name available (rule of priority); but the senior synonym itself could be associated with an inadequately described organism. Problems are compounded by lumpers and splitters (Corliss 1976) in taxonomic protistology. And, in recent years, with the seemingly constant shifting about in the assignment of various groups to individual higher taxa, anyone tabulating species must be careful not to count the same organism twice under different headings in the ever-changing scheme of higher protistan classification (Corliss 1998b).

• The justification of new species continually being described in the literature on practically all protistan taxonomic groups (see the Zoological Record and relevant botanical and algal lists and monographs). The lack of appropriate techniques of study in the past might often have been the cause of proliferation of what are now deemed unnecessary or unacceptable species; but, today, is it the availability of improved cytological, biochemical, and molecular methods that fuels

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the persistent description of new forms? Yet, as habitats and niches in diverse geographic and ecological areas (including new host species for symbiotic forms) are more thoroughly explored (perhaps even for the first time), is it not reasonable to anticipate the finding of at least some novel species of algae and protozoa? In general, taxonomic protistologists are in widespread agreement that this is inevitably the case, while they understandably bewail the shortage of trained students and funds to investigate such habitats (Andersen 1992; Vickerman 1992). For the large protozoan phylum Ciliophora, in particular, Finlay and others (1996) argue that the great majority of the free-living, free-swimming, phagotrophic forms from freshwater and salt water habitats probably have been discovered and described already and that the number of these considered acceptable reaches only a few thousand. Foissner (1998), in contrast, claims that hundreds of additional species of ciliates inhabiting such edaphic habitats as diverse soils, with perhaps as many as 75% of them not living elsewhere, have been largely and unfairly neglected.

• The different ways in which different workers categorize the areas covered in their own studies or reviews. For example, members of most protistan high-level taxa might be thought of, by some, as falling into only three major groupings, with scant attention to overlappings: free-living species, symbiotic-parasitic forms, and fossilized species (of extinct or contemporary taxa). But the extent to which “symbiotic” and “free-living” forms can coincide is often blurred; consider the cases of mutualistic and commensalistic forms versus “true” endoparasites and ectoparasites, or even those of symphorionts (basically independent organisms merely carried about by nonspecific “hosts”). Other investigators might divide protistan groups on the basis of their being found in different major ecosystems: freshwater, marine, estuarine, or terrestrial habitats. Numerous workers emphasize what seems to be the preference of their organisms for specific geographic areas, raising the problem of endemism versus cosmopolitanism in recognition of “new” species. Still another popular general categorization highlights modes of nutrition: autotrophs (via photosynthetic pigments) versus heterotrophs (phagotrophs and osmotrophs or saprotrophs), with bacteria and algal protists serving as the most commonly engulfed prey organisms. Unfortunately, authors have sometimes not specified the limits or boundaries used in arriving at their particular “total numbers” of species assignable to a given higher taxon.

Is it any wonder that few published works make reliable overall estimates of the total numbers of species of protists? Keeping in mind the difficulties mentioned above, I am attempting here to overcome most such obstacles and arrive as objectively as possible at reasonably accurate figures of known protists as of 1998.

Numbers of Protists in Major Groups

In the following sections, I purposely arrange major taxa of protists under widely known “tried and true” top-level conventional headings, using vernacular titles for such broad categories—“protozoa,” “algae,” “fungi,” “plants,” and “animals.” In each section, the formal names of protistan phyla acceptable to me (see table 1)

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are given in boldface, often with an indication of synonymous names and with cross-references, as needed, to the kingdom (names are in all capital letters for easy recognition) in which a given phylum, in my opinion, seems best assigned today. The final location of a phylum might not match the title of the section; and for some groups, the reader is referred to fuller treatment under one of the other sections.

Sources of data leading to my estimated numbers have been many, beginning with those cited, directly or indirectly, in my own first publications on the subject (Corliss 1982, 1984). All such figures have naturally required considerable updating to take into account new species descriptions and to accommodate revisions in which former species might have been rejected. Too numerous to list here have been the useful taxonomic monographs, books, compendia, and authoritative individual papers that I have consulted. But I should mention the most helpful single modern source of information, the volume edited by Margulis and others (1990), a prodigious work that contains 36 scholarly chapters contributed by some 60 specialists on the diverse high-level taxa of protists.. For some groups, our knowledge of numbers is still frustratingly fragmentary. Also distressful is the continuing instability of the exact composition of various higher taxa involved in overall protistan megasystematics, which makes exact placement of some implicated genera and their species difficult. Generic names that are representative of particular taxa, incidentally, are generally not included in the present paper, because of space limitations, illustrative of diversity though they would be. For the interested reader some 1,100 of them have recently appeared elsewhere (Corliss 1994a; and see many more in specialized phycological and protozoological textbooks and in Lee and others 1985, Margulis and others 1990, Margulis and Schwarz 1998, Parker 1982, and Tappan 1980, although genera might be quite differently classified at the highest levels in such works).

Conventional Protozoan Phyla

The taxa below follow the usual arrangement commonly found in well-known biological and more-specialized protozoological textbooks. That is, forms mostly amoeboid, although also including some amoeboflagellates, with pseudopodia of various kinds (the old rhizopod and actinopod “sarcodinids”) make up the first grouping; the numerous taxa whose species are predominantly biflagellated or multiflagellated (both pigmented and nonpigmented arrays, roughly the “phytoflagellates” and “zooflagellates,” respectively, of old) come next; spore-forming parasitic taxa (sporozoa and the former “cnidosporidian” groups) are then treated; and finally the ciliates, a large collection of species that represents one of the most circumscribed and noncontroversial protistan taxa of all, are mentioned.

Names given first (and in boldface type) follow those used in table 1; but explanations and brief descriptions plus major synonymous names are supplied when deemed helpful. Note that, although some two dozen phyletically named taxa are considered below as, in effect, conventionally known “protozoan-like groups,” nearly half have been reassigned to kingdoms other than the PROTOZOA of the present paper, as pointed out in appropriate places.

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Archamoebae (synonym Karyoblastea): The pelobionts (such as Pelomyxa, a free-living, freshwater, benthic “giant amoeba” reaching 5 mm in diameter), embracing some five or six genera if the parasitic Entamoeba is also accepted here. Not long ago, these amitochondriate protists were placed in a separate kingdom, the “ARCHEZOA,” along with the Metamonada and the Parabasala (see below). Although descriptions of quite a few species have appeared in the literature, there is now wide agreement that the number of acceptable ones is probably less than 12. The conservative figure that I am using as a total here is 10.

Neomonada: A group of often small, free-living, marine heterotrophic flagellates and amoeboflagellates (Cavalier-Smith 1998b), still ill-defined, many formerly in Cavalier-Smith's (1993a, 1997a) phylum “Opalozoa”. Depending on the workers involved, the number can range from a dozen or two to several score (including some of the “unassignable” forms of Patterson and Zölffel 1991); many genera are monotypic (that is, they have only a single species). At this time, I estimate 30 as a possible total number of valid species here.

Rhizopoda (synonym Amoebozoa, in part, of Corliss 1984): Predominantly typical amoeboid forms, including ones with tests, shells, or thecae, but some small heterotrophic flagellates here as well (see Patterson and Zölffel 1991). Some workers put the enigmatic algal Chlorarachnion here; others, 40 or more species of plasmodiophorans (endoparasitic slime molds). Separation from the following phyletic group is not always clear. There are at least 5,000 species, with some to be dropped (for example, hundreds of poorly described testaceous amoebae might be rejected by future workers), but predictably with many new “small naked amoebae” awaiting discovery (Vickerman 1992). A few fossil forms—and possibly 250 symbiotic species—have been described.

Mycetozoa (synonyms Eumycetozoa and Myxomycetes): A “lower fungal” plasmodial slime mold group containing both cellular and “acellular” species. Some plasmodia can be longer than 3 m. Exact boundaries are uncertain (see remark under Rhizopoda, above). Some 800–900 species are assigned here, with probably more to be moved in from other taxa and still others to be found and described as new. Possibly a few fossils and a number of symbiotic forms belong here as well (for example, the necrotrophic plasmodiophorans, the soil protists infecting cabbage and other plants).

Foraminifera (synonym Granuloreticulosea): The foraminifers in the broadest sense (Lee and Anderson 1991). Perhaps as many as 45,000 species have been described, with nearly 40,000 as fossilized forms (many represent extinct lines and make up the “globigerine ooze” on ocean floors and are invaluable in dating strata for the petroleum industry). The diameter of some extinct fossil shells or tests may reach 15 cm; of living extant species, up to 6.5 cm. No end of new forams is in sight, although some workers question the taxonomic significance of some minor differences in morphology of the calcareous test. A few taxonomists include 15 genera of xenophyophorans (body diameters, up to 25 cm) and 12 genera of komokiaceans here.

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Labyrinthomorpha: Net slime molds of mycologists; unique parasitic forms (for example, on eelgrass); also some saprotrophic on dead tissues. These are now better placed in the kingdom CHROMISTA than in PROTOZOA or FUNGI (Cavalier-Smith 1998a). The group includes labyrinthuleans proper plus thraustochytriaceans. It totals about 50 species.

Heliozoa: Mostly a freshwater group of the classical “actinopod sarcodinids”. The amazing marine Sticholonche zanclea, a single species formerly considered to make up the separate heliozoan class Taxopoda, is perhaps better assigned to membership in the next phylum (Radiozoa, below). Some 180 species have been described as heliozoa, but only about 100 might be acceptable to today's specialists on the group, which itself could still be a polyphyletic taxon (Smith and Patterson 1986).

Radiozoa (synonym Radiolaria): Spherical marine planktonic “actinopods”, producers of great depths of “radiolarian ooze” on ocean floors. There are three major subgroups, of which the first two are closer taxonomically to each other than to the third (all are sometimes treated as separate phyla): Acantharia, 500 species (possibly only half valid), of which no fossils have been described; Polycystina, 10,000 species (possibly only half valid), nearly 75% of which are found as fossils; Phaeodaria, without the endosymbionts found in preceding groups, 1,150 species (possibly only 60% valid), few of which have been found as fossils.

Percolozoa: Small heterotrophic flagellates or amoeboflagellates; a considerably smaller group than when originally circumscribed (Cavalier-Smith 1993b). Some former heterolobosean genera are here, some “unassignable” forms of Patterson and Zölffel (1991), and, controversially, the ciliate-turned-flagellate (Lipscomb and Corliss 1982; Patterson and Brugerolle 1988) Stephanopogon. There are more than 100 species.

Bicosoecae: Small nonpigmented heterotrophic flagellates, some colonial. This group has long been claimed by protozoologists, but see the treatment under “Algae,” below. It is assigned to the kingdom CHROMISTA.

Dictyochae: Silicoflagellates, some known from the fossil record; long claimed by protozoologists; but see the treatment under “Algae,” below. It is assigned to the kingdom CHROMISTA.

Cryptomonada: Mostly pigmented species, although many are heterotrophic. The group has long been claimed by protozoologists, but see the treatment under “Algae,” below. It is assigned to the kingdom CHROMISTA.

Haptomonada: Pigmented, but claimed also by protozoologists. It is treated here under “Algae,” below. It is assigned to the kingdom CHROMISTA.

Opalinata (synonyms Protociliata, Paraflagellata): Protozoologists' well-known opalinid parasites (in the strictest sense) plus Karotomorpha and Proteromonas (Delvinquier and Patterson 1992; Patterson 1986a). More than 400 species are

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reported in the literature, but many of the opalinids described are from ill-fixed material; perhaps only 200 are acceptable as valid today. The group was in PROTOZOA and is now assigned to the kingdom CHROMISTA (Cavalier-Smith 1998a,b).

Euglenozoa: Two principal subgroups (Triemer and Farmer 1991; Vickerman and others 1991): The Euglenophyta of the algal literature, more than 1000 species, mainly free-living, freshwater, and photosynthetic, although also phagotrophic, colorless, and some symbiotic-parasitic (and rare fossil) forms are known; and the Kinetoplastidea of the protozoological-parasitological literature, more than 600 species, ranging from pathogenic blood and tissue parasites of human beings (trypanosomatids) to free-living, freshwater or salt-water biflagellated species (bodonids).

Dinozoa (synonyms Dinoflagellata, Pyrrhophyta, and Peridinea + Syndinea): A major group of unique biflagellated protists, the dinoflagellates, long claimed by both phycologists and protozoologists. The pigmented species, some also heterotrophic, are a major component of marine plankton, but 10% occur in fresh-water habitats; about 50% of the species are nonpigmented; some dinos are thecate and some colonial. About half the described species have been found as fossils, exclusive of 400 genera of acritarchs (a fossil group also assigned here by some workers) but including the small taxa of ebriideans and ellobiophyceans. Some species are important symbionts of other organisms; others exhibit toxic blooms (for example, red tides) with direct and indirect effect on humans. There is a distinct taxonomic subdivision of osmotrophic, endosymbiotic forms in diverse marine hosts. A primitive group might now include the nonpigmented former apicomplexan (see below) parasite Perkinsus (Siddall and others 1997). There are some 4,500 species, with perhaps nearly 2,500 as fossils of some extant but mostly extinct forms (Fensome and others 1993; Taylor 1987b).

Metamonada (synonym “polymastigotes,” in part): Biflagellated to multiflagellated forms, typically gut parasites of diverse hosts (from insects to humans), allegedly (with the following phylum) primitive protists (Vickerman and others 1991). They have no mitochondria but hydrogenosomes (latest review, Müller 1998). There are about 300 species, but some are in need of restudy.

Parabasala (synonym “polymastigotes,” in part): Mostly parasitic multiflagellated forms (called trichomonads and hypermastigotes), amitochondriate, and with striking parabasal (Golgi) apparatus. They share enough characteristics with the above phylum (Metamonada) to be joined with it (and the Archamoebae) under the one-time kingdom “ARCHEZOA” of Cavalier-Smith (1993b, 1998a, and references therein). The group has more than 400 species, some in need of restudy; doubtless more will be found, especially in the inadequately explored insect (woodroach) digestive tract.

Choanozoa (synonyms Choanoflagellata, Craspedophyceae): Planktonic (mostly marine) nonpigmented “collar-flagellates” with a single smooth anterior flagellum, often stalked or loricate. The group was placed in the protozoan phy-

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lum Neomonada by Cavalier-Smith (1998b) and tentatively transferred to the kingdom ANIMALIA by Corliss (1998c) and here. There are about 150 species.

Apicomplexa: Popular name for what is essentially the still-valid “Sporozoa” of old (Ellis and others 1998). An “apical complex” is made visible only by electron microscopy. The species are all symbiotic in a great variety of hosts, many as harmful endoparasites (Levine 1988; Perkins 1991). They include some of the smallest protists (intracellular forms with diameters less than 1mm), although others can be up to 10 mm long. The major subgroups are gregarines (some large), coccidians (Toxoplasma and others in humans), and haematozoeans (malarial organisms and others). Perkinsus has been transferred to the phylum Dinozoa (see above). The “Ascetospora” of the literature is tentatively placed here. There are more than 5,000 species, some questionable today because of inadequate past accounts; but parasitologists predict numerous yet-to-be-described species. Levine (1973) once estimated, on the basis of potential combinations of numbers of sporocysts and sporozoites in the oocyst (which represent important differentiating taxonomic characters), that there could be, hypothetically, more than a million species in the second sporozoan subgroup (the coccidians) alone!

Microspora (synonym Microsporidia): A highly unusual group, with very small spores (diameters less than 1 mm) containing a complex extrusome and with a chitinous cell wall. The group consists of obligate intracellular parasites found in other protists, insects, fishes, and, opportunistically, human AIDS patients. Unicellular forms long considered as protozoa, they are here placed in the kingdom FUNGI on the basis of recent molecular findings (see citations in a preceding section of this paper). There are more than 800 species.

Myxozoa (synonyms Myxosporidia and Myxospora): Formerly grouped with Microspora as “cnidosporidians”. These are histozoic or coelozoic parasites, mainly of cold-blooded vertebrates (the cause of great economic losses in the commercial fish industry). They have valved multicellular spores with polar capsules that include extrusible filaments. They were long considered as protozoa but here are placed in the kingdom ANIMALIA mainly on the basis of molecular data (see citations in a preceding section of this paper). There are more than 1,200 species. Some species in invertebrates, formerly assigned to independent status in a (second) major class, Actinomyxidea, are now being identified as simply stages in the life cycle of well-known myxosporidian fish parasites (Kent and others 1994).

Ciliophora (synonym Heterokaryota): Multiciliated (usually), colorless (with exceptions), relatively large cilioprotists (general range, 10–500 mm; a few up to 5,000 mm; and some colonies up to 15 cm in diameter). They exhibit nuclear dualism (the heterokaryotic condition—two kinds of nuclei, macronuclei and micronuclei; see Raikov 1996 for latest review), and are often phagotrophs, freeliving in widely diverse habitats, although many groups are symbiotic-parasitic (including Balantidium in humans) or symphoriontic (the latter usually stalked). This is a large phylum (ranking fifth among all protists, behind diatoms, forams, charophytes, and radiolarians), with 8–10 classes and many orders. The total

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number of species is often said to be at least 8,000, including about 200 fossil forms (all of tintinnids) and an estimated 2,600 symbiotic species, with many more presumably awaiting discovery (Corliss 1979; Lynn and Corliss 1991; Small and Lynn 1985). But more conservative figures have recently been offered by Finlay and others (1996), who estimate a maximum of 4,300 for their pragmatic “morphospecies” of cosmopolitan free-living, phagotrophic forms primarily from major freshwater and salt-water habitats (calculating this to be 70% of all ciliates, including symbiotic species) and who suggest that careful taxonomic revisions might reduce their number to about 3,000. The matter is controversial (there might be many more valid soil-dwelling species than is often appreciated: see Foissner 1998).

Additional “protozoan” groups: Treated in the following section as conventional green or golden-brown “algal”taxa, are three phyla from within which selected (mostly motile) subgroups have long been of interest to protozoologists: the Chlorophyta (the volvocine line) and the Prasinophyta (both assigned here to the kingdom PLANTAE) and the Chrysophyta, assigned to the kingdom CHROMISTA. Treated in the later section on conventional “fungal” phyla are members of the Chytridiomycota, a number of species of which have been routinely included in protozoological textbooks. But that phylum, in its entirety, is placed in the kingdom FUNGI in this paper. Finally, the “Ascetospora” or “Haplosporidea” of both old and more recent literature (for example, CavalierSmith 1993b; Corliss 1994a) is tentatively placed within the Apicomplexa (above) here, on the basis of reasoning found in Cavalier-Smith (1998a).

Conventional Algal Phyla

Deliberately omitted here is further mention of the prokaryotic (cyanobacterial) divisions or classes of algae, the “Cyanophyta” or blue-green algae, with some 2,000 species, and the “Prochlorophyta” with fewer than six. Many botanists and phycologists recognize three “true” major broad algal assemblages, the red algae, the green algae, and the chromophyte algae. The latter vast group (containing numerous classes, depending on the author) has been known by a variety of names, including Chromobionta, Heterokontae, Heterokontophyta, and even Chrysophyta (in its broadest usage). Andersen (1992), whose summarizing table on numbers of algal species overall has been especially helpful to me (see also John 1994; Norton and others 1996), considered those three diverse assemblages to be taxonomically and phylogenetically “the major algal lineages,” with four additional “minor lineages” (dinoflagellates, euglenophytes, cryptophytes, and glaucophytes) listed in his table below his classes of chromophytes.

Here I recognize some 16 eukaryotic algal groups, at phylum (division) rank, eight of which I assign to the kingdom CHROMISTA, six to PLANTAE, and two to PROTOZOA (Euglenozoa, Dinozoa: see above). The chromistan phyla contain the majority of the species broadly classified as “chromophyte algae” by botanists, and most of their species are pigmented (that is, carry out photosynthesis). My order of presentation below more or less follows the conventional arrangement used by many phycologists. Space does not permit specific mention of the names

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of many often traditionally well-recognized algal taxa usually treated (and here generally so retained) at the level of class or below, not as divisions or phyla. Their numbers of species have not been left out of my overall count but are included, as appropriate, in totals given for such large, all-embracing phyla or divisions as the Chlorophyta and the Chrysophyta. Some former classes have been elevated to phyletic status, or their names have here been considered more or less synonymous with preferred different names for the higher rank of phylum. “Taxonomic inflation”, bringing about a concomitant increase in names, has been as inevitably rampant, in recent years, among the “algal” protists as among their “protozoan” and “fungal” counterparts—perhaps a consequence of our increasingly precise methods of study and analysis of the systematics and phylogeny of these highly diverse eukaryotic microorganisms (Corliss 1998b).

Rhodophyta: Nonflagellated, mostly marine, macroalgae (red seaweeds), but some minute unicells as well, and some parasitic species. Meter-long multicellular parenchymatous forms appear along rocky shores. The two principal classes or subgroups, Bangiophyceae and Florideophyceae, each have several or many orders. Species encrusted with CaCO3 fossilize well. Red algae are a source of commercially valuable agar and of maerl, widely used as a fertilizer. This taxon (containing some of the oldest fossil algae known) has been given very high independent ranking taxonomically or, as here, has been assigned a unique place in the kingdom PLANTAE. There are well over 5,000 species (about 750 as fossils), and more than 100 species have been described as parasites of other red algae.

Glaucophyta (synonym Glaucocystophyta): A small algal group, all with cyanelles, all freshwater, and most biflagellated. They are placed in or near Rhodophyta by many workers; here, they are tentatively assigned to separate phyletic status in the kingdom PLANTAE. Depending on the number of accepted genera, the species counts range from a few to about 15.

Prasinophyta (synonym Micromonadophyceae): Grass-green scaly algae, freshwater, mostly small (and possibly primitive) biflagellated unicells. One of the tiniest free-living protists belongs to the picoplanktonic genus Micromonas (diameter, 1 mm). These species are assigned to the kingdom PLANTAE with other green algae. Some 400 species have been described (but perhaps only half that number are fully acceptable). About 100 have been found as fossils (some of which were originally identified as acritarchs; see the comment under Dinozoa, above).

Chlorophyta: The green algae of the botanical literature, mostly unicells or colonial in freshwater, many nonmotile. The celebrated “zoochlorellae” (symbionts of many ciliates) of the classical protozoological literature (and see Reisser 1992) belong here. There are many separate classes or orders; some phycologists conservatively include here members of some of the other phyla described below (such as Ulvophyta). This evolutionarily important and ecologically widespread group is assigned to the kingdom PLANTAE. It contains perhaps more than

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3,500 species, depending on the inclusiveness of the phylum and thus on the workers making the counts.

Ulvophyta: Includes macroscopic seaweeds from tropical marine waters. They are sessile, with coenocytic or multicellular thalli. The group is assigned to the kingdom PLANTAE and contains at least 300 species, a few of which are fossils.

Charophyta (synonyms Conjugatophyceae, Gamophyceae, and Zygonematophyceae, and others): Mostly (including the ubiquitous desmids) unicellular or filamentous in freshwater, vegetative stage nonflagellated, and with conjugation often involving amoeboid gametes. Larger forms—far fewer in species—are placed in a separate class, which includes the well-known stoneworts, with macroscopic thalli typically scale-covered. Several charophyte characteristics are clearly reminiscent of land plants, their evolutionary descendants. This group of green algae is assigned to the kingdom PLANTAE. It has some 12,000 species, but about 9,000 are desmids alone (of which half are of uncertain validity); stoneworts number fewer than 400 species, about 300 of which have been found only as fossils.

Dinozoa: Long (and still) claimed as algae, but treated in this paper as PROTOZOA (see preceding section).

Euglenozoa: Long (and still) claimed as algae, but treated in this paper as PROTOZOA (see preceding section).

Bicosoecae: Freshwater and marine nonpigmented flagellates, some with loricae. These were formerly placed within Chrysophyta. The group is assigned to the kingdom CHROMISTA and contains about 40 species.

Dictyochae (synonym Dictyochophyceae): Silicoflagellates, formerly in Chrysophyta, with a number of fossil marine forms. Dictyocha is the only genus with extant species (as the phylum is restricted here). The group is assigned to the kingdom CHROMISTA. It has fewer than 12 species (excluding Actinomonas and Pedinella and close relatives that are placed here by some phycologists).

Cryptomonada (synonym Cryptophyta): Well-known freshwater and marine mostly pigmented biflagellated protists, some phagotrophic, some endosymbiotic. The group is controversially assigned to the kingdom CHROMISTA but not within the large heterokontic moiety. There are about 200 species.

Haptomonada (synonyms Coccolithophora, Haptophyta, and Prymnesiophyta): Yellow-brown algae, typically marine flagellates with unique haptonema arising between a pair of polar flagella and a body usually covered with layers of scales, some known as coccoliths. The group is controversially assigned to the kingdom CHROMISTA but not within the large heterokontic moiety. Some 500 living and 1,200 fossil species have been described; the celebrated white cliffs of Dover are composed mostly of coccoliths.

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Chrysophyta: The golden-brown algae, numerous freshwater species, some with delicate loricae, and many producing a unique statospore as a resting stage. The group is assigned to the kingdom CHROMISTA as a major phylum of the heterokontic moiety there. There are perhaps more than 1,500 species, several classes of which are given independent phyletic status by some workers. The total includes about 250 fossil forms.

Diatomae (synonyms Bacillariophyta, Diatomea, and Diatomophyceae): The diatoms. “Bacillariophyceae” is the most popular name used for this taxonomic group. They are yellow-brown unicells, widespread planktonic and benthic forms in salt-water amd today especially freshwater habitats and are also found in moist soil; a few are endosymbionts of the protozoan foraminifers (Lee 1992). They are nonflagellated in the vegetative stage. Diatoms have the characteristic two-valved siliceous test or frustule, which is readily fossilizable and the main component of commercially useful diatomite (“diatomaceous earth”). The group is assigned to the kingdom CHROMISTA. The number of recorded forms has apparently reached 100,000, including fossils of both extinct and extant forms, according to Round and others (1990), or even 200,000 according to Mann and Droop (1996). But some conservative phycologists have estimated that only about 25% (or less) might be acceptable as truly separate extant species. The ratio of living to fossil forms has been given as 2:3. Some diatom specialists (personal communications acknowledged in Andersen 1992 and Norton and others 1996; see also John 1994) predict that the “real” (potentially describable) number of species of these highly abundant and very important autotrophic protists might reach an amazing total of 10,000,000!

Raphidophyta (synonym Chloromonadophyceae and inappropriately known as “the chloromonads”, but Chloromonas is a genus of the green algal phylum Chlorophyta in the kingdom PLANTAE): Small group of yellow-green algae, from freshwater and salt-water habitats, formerly placed in Chrysophyta by some workers. The group is assigned to the kingdom CHROMISTA. It has fewer than three dozen species.

Phaeophyta (synonyms Fucophyceae and Melanophyceae): The brown heterokont algae, with multicellular filaments or thalli. These are large seaweeds (kelp) of intertidal or subtidal habitats, gigantic protists reaching lengths of up to 60 m. Many are of commercial value, directly as food or as sources of alginates, fertilizers, vitamins, and minerals. The group, sometimes closely linked to the Chrysophyta, is assigned to the kingdom CHROMISTA. It has more than 1,600 species, a few described as fossils and a few as symbionts on other algae or seagrass.

Conventional Fungal Phyla

Under consideration in this paper are only the basically unicellular “fungus-like” protists, not the long-accepted “higher” fungal taxa. As implied in earlier sections, botanists (that is, mycologists) formerly claimed many protozoan-protistan groups as “lower” fungi, particularly the slime molds (including the labyrinthulids and plasmodiophorans) and the zoosporic taxa. In recent years, there has been

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growing acceptance of removal from the kingdom FUNGI not only of the various slime molds but also of two of the three flagellated (independently motile) groups (see Pseudofungi, below), leaving only the chytrids as true (although unicellular and flagellated) fungi.

Myxomycetes (synonym Myxomycota): See account under the “protozoan” phylum Mycetozoa, above. The group is assigned to the kingdom PROTOZOA.

Labyrinthomorpha: See the group, by the same name, above (with other “protozoan” phyla). But the group is assigned in this paper to the kingdom CHROMISTA.

Pseudofungi (synonyms, at least in part, Mastigomycetes, Oomycota, Phycomycetes, and Pseudomycota): Zoosporic protists separable into two subphyletic zoosporic taxa, the Oomycetes (synonym Oomycota) and the Hyphochytriomycetes (synonyms Hyphochytrea and Hyphochytridiomycota) of the literature. Both groups are assigned to the kingdom CHROMISTA. These small but numerous freshwater “water molds,” whose zoospores have an anteriorly projecting flagellum bearing mastigonemes, parasitize hosts ranging from other protists and aquatic plants to fishes and, via the soil, grapes and potatoes. Many species are also saprotrophic on detritus and dead tissues in aqueous and terrestrial habitats. More than 800 species of oomycetes have been described, although some are now considered doubtful; about two dozen species are known from the second taxon.

Chytridiomycota: A third taxon of zoosporic protists, but with posteriorly projecting smooth flagellum (no mastigonemes) and taxonomically remaining in the kingdom FUNGI. They have several fungal characteristics, including chitinous cell walls in their hyphal stage, although they are basically unicellular. They are symbionts or saprobes in soil and freshwater habitats (Powell 1993); a few, treated as protozoa in past years, are found in the digestive tract of horses and ruminants. Some 900 species have been described.

Microspora: As pointed out above (under conventional “protozoan” phyla), only very recently have true fungal affinities been discovered for these tiny intracellular parasites presumably of ancient phylogenetic origin. See the protozoan section (above) for data on the group; but recall that it now properly belongs here in the kingdom FUNGI as the second phylum of fungal protists (the first being the Chytridiomycota, see immediately above).

Conventional Plant Phyla

Considering here only the basically unicellular (or multicellular but not truly multitissued) “lower” plants, I hardly need to point out that formerly all “algal” taxa, including some claimed also by protozoologists, were treated as members of the kingdom PLANTAE. Because all fungi were under this banner, too, it follows that the “lower” fungi, most groups of which are now considered to be members of the totally protistan kingdoms PROTOZOA and CHROMISTA, were also formerly claimed by botanists as plants. Today, I assign or retain essentially only

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the red and (some of) the green algal groups as protistan assemblages in the kingdom PLANTAE (see conventional “algal” section, above).

Conventional Animal Phyla

Considering the protozoa as basically unicellular organisms, it is common knowledge that they were long treated as animals, as a single phylum (or eventually at best a subkingdom) of the kingdom ANIMALIA. It follows that all subtaxa of such protozoan protists were considered taxonomically as microscopic “first” animals. Some algal groups were also included, mostly under the title of “Phytomastigina” or “Phytomastigophora”, as well as two phyla (the chytrids and microsporidians—see above) now treated as true fungi. In this paper, I append only two protistan phyla to the kingdom ANIMALIA (see above, under conventional “protozoan” phyla), namely the Choanozoa (controversially) and the Myxozoa.

Some Summarizing Observations

Using (with appropriate caution) data given on the preceding pages, we can draw several conclusions concerning total numbers of species of protists (see also table 2). A grand total of at least 213,000 species, distributed among the 35 phyla recognized in this paper, have been described in the literature to date. Interestingly enough, about 113,000 of these are fossil forms. Five of the 18 phyla known to have any fossils at all contain 98% of the known fossil protists; these groups, in order of richness in fossil species, are the diatoms, foraminifers, radiozoa, dinoflagellates, and haptomonads. In fact, the diatoms and forams alone are responsible for 90% of them. Still, fossil forms also represent an important percentage of the species of some of the smaller phyla (for example, 15–25% of chrysophytes, prasinophytes, and rhodophytes).

Among the extant contemporary forms, numbering some 100,000 species, only about 14% can be labeled as symbionts in the broadest sense, ranging from symphoriontic, commensalistic, and mutualistic forms to obligate ectoparasites and endoparasites (with the latter including some highly pathogenic microorganisms) on and in all kinds of protistan, plant, fungal, and animal hosts. Free-living species would thus seem to outnumber greatly the symbiotic forms. The percentage figure given above, however, is somewhat misleading. If, in addition to fossils, we also leave to one side the huge number (40,000) of nonfossil diatoms (many controversial anyway?), the roughly 14,000 symbiotic species become nearly one-fourth (23%) of all other extant protists.

Incidentally, 95% of the 14,000 symbiotic species are members solely of the 10 following phyla: Apicomplexa (all), Ciliophora (one-third), Myxozoa (all), Chytridiomycota (all), Pseudofungi (all), Microspora (all), Metamonada plus Parabasala (essentially all), Euglenozoa (some euglenids plus essentially all trypanosomatids), and Opalinata (all). But the remaining 5% include scattered important species found among dinoflagellates, cryptophytes, chlorophytes, and rhodophytes, with the majority pigmented; among amoebae, mycetozoa, and amoeboflagellates; and among members of various other usually smaller protistan groups.

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Additional calculable totals and other comments can be found in table 2, where the phyla of the five kingdoms are in a different arrangement from that found in table 1, in keeping more closely with the order of their presentation on the preceding pages.

Briefly, the totals (here rounded off) per kingdom of all species of protists contained therein (be they fossilized; free-living or symbiotic; autotrophic, heterotrophic, or mixotrophic; benthic or planktonic; from aquatic or terrestrial habitats; and so on) are as follows: PROTOZOA (as restricted in this paper), 82,700 species; CHROMISTA (with its mixture of many traditional algal phyla and some others), 106,400 (but 94% of these are diatoms); PLANTAE (six algal phyla), 21,200; FUNGI (two phyla), 1,700; and ANIMALIA (two phyla), 1,350.

With respect to described species versus putatively valid or acceptable species, I have despaired of solving all such problems here. In my calculations (and in table 2), I have generally used numbers from the first category—that of described forms—on the basis of the original literature (or reliable second-hand sources). For the great majority of protistan phyla, there has seldom been to date a significant difference between the two sets of figures, so I have not cited the latter numbers in this paper. However, there are two striking examples of disparity or discrepancy between the numbers—described versus acceptable species—in the cases of Diatomae and Radiozoa. Of the 100,000 (or more!) diatom species (extant and extinct) allegedly established in the literature, are as few as 10,000–12,000 the maximal number acceptable to many phycologists today? Or are authors of the lower figure excluding fossil (and some other) forms from their estimates without clearly informing their readers of the fact? For the radiozoa, are about half the 11,650 described species now to be considered by protozoologists to be invalid or uncertain? Or do some papers on the subject seem confusing only to the unsophisticated reader? I suggest that specialists, not generalists like me, should discuss and eventually solve or at least clarify such serious problems to everyone's satisfaction.

Whereas there is little doubt that many species of protists have not been carefully enough described in a comparative way (and thus really are “lumpable”) and that endemism has been overused as a basis for newness (including that old parasitological dictum, “A new host means a new species”), is it possible that only a relatively few truly new species remain undetected in the largely unexplored biomes of eukaryotic microorganisms?

On the basis of personal communication with many protistologists, I am obliged to draw the conclusion that, for numerous groups, vast numbers of unique protists do await description. Perhaps we have only scratched the surface regarding the biodiversity of these organisms. Thus, with rare exception, I have not attempted to include estimates of the probable numbers of species assignable to the phyla described to date.

Outlook and Goals for the Future

The roles of protists in natural ecosystems are, in a general way, beginning to be appreciated, but they are hardly yet understood to a very helpful degree, one

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applicable to humankind's many environmental challenges. Awareness of their potential is only the first step in the process of getting to know them better. Several major needs are becoming clear, as exposed very briefly below:

• The biodiversity of protistan groups must be studied in greater depth. That is, we need to understand their distribution—and functions—on a global scale to focus on their diverse interactions with other organisms in a wide variety of habitats. To investigate their ecology, we must improve our knowledge of their taxonomy (and vice versa: Corliss 1992). More-thorough comparative studies need to be carried out, with use of the most precise sampling and cytological techniques now available.

• Reaching widespread agreement on the nature of a protistan species is imperative. If we do not understand the dimensions of a species definition, taxonomic and nomenclatural problems will continue to plague our progress. And we can hardly prepare inventories without knowing the identity of our material in considerable depth.

• More-extensive work on the phylogeny of the prorists will throw light on their evolutionary relationships with the prokaryotic bacteria and with the other eukaryotes, the latter assemblages all supposedly having had protistan origins. An interdisciplinary approach thus needs to continue to be taken in studying protists because of the value of viewing major problems from different points of view. Cladistic trees and taxonomic classification systems must be refined and become more supportive of one another.

• Practical reasons for studying many protistan groups more intensely are related to their direct and indirect effects on human welfare, ranging from their basic food-chain involvement (nutrient and mineral recycling), their roles in agriculture and aquaculture, and their commercial, medicinal, and biomonitoring uses to their being causative agents of major diseases.

• Clearly, more financial support is needed for protistological research, for teaching and training more students and technicians, for maintenance and expansion of culture collections and gene banks, and for preparing appropriate inventories or censuses of species numbers. All these activities are necessary for determining future avenues worthy of exploration in the vast field of protistan biodiversity.

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