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Global Diversity of Mites

R. B. Halliday
Australian National Insect Collection, CSIRO Entomology, GPO Box 1700, Canberra Act 2601, Australia
B. M. OConnor
Museum of Zoology, University of Michigan, Ann Arbor, Michigan, 48109-1079
A. S. Baker
Department of Entomology, The Natural History Museum, Cromwell Road, London SW7 5BD, United Kingdom

Introduction

The mites and ticks make up the order Acari (or Acarina) within the class Arachnida. They differ from other arachnids in that, in virtually all cases, all traces of body segmentation have disappeared. The body of a mite is divided into the gnathosoma (mouthparts) and the idiosoma. There is no recognizable head, and the structures normally associated with the head, such as eyes and brain, are incorporated into the idiosoma. There are no antennae, but in many groups the first pair of legs is long and slender and serves a sensory function. Their ancestral arachnid relatives were predatory, but the mites have diversified from this origin to the extent that they now occupy an extraordinarily diverse range of niches. Many have remained predatory, but other groups have adapted to plant feeding and scavenging on dead plant matter and, alone among the arachnids, have developed into parasites of vertebrate and invertebrate animals. They have developed a wide range of associations with other organisms, including phoretic relationships, some of which have produced morphological and behavioral adaptations in both the mites themselves and the animals that they use for transportation.

The acari are usually subdivided into seven suborders. Astigmata includes forms that occur in patchy habitats, such as dung, carrion, decaying wood, fungi, and the nests of mammals and birds; the latter have evolved into many families of vertebrate parasites, the common pests of stored products that occur in the

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home and in food storage, and the dust mites that are implicated in allergy and asthma. Oribatida contains essentially feeders on dead plant material and fungi; these mites play an important part in litter decomposition and soil formation, and a few are important as intermediate hosts of cestode parasites. Prostigmata is a diverse assemblage of predators, plant parasites, algal feeders, and parasites of vertebrate and invertebrate animals. Mesostigmata also includes parasites, but most of its members are predators in soil and decomposing organic material. Ixodida contains the ticks, which are exclusively parasites of vertebrates that feed by drawing blood from their hosts with specially adapted mouthparts. The members of the remaining suborders, Opilioacarida and Holothyrida, are less diverse and less well known; they occur in damp habitats, such as forest leaf litter, and are believed to be predators.

Mites are small. The smallest adults are plant parasites about 80 mm long; the largest are predators about 13 mm long. Most are 400–800 mm long. Their small size has contributed to the diversity of their life cycles and basic biology and has allowed them to exploit an extraordinary variety of niches. The taxonomic and ecological diversity of mites is accompanied by structural diversity. Some exhibit specialized adaptations of the mouthparts, from elongated attenuated chelicerae for sucking plant or animal fluids in parasitic groups to very heavy robust chewing mouthparts in families that feed on fungi or dead plant material. Associations with other organisms have produced specialized structures and organ systems used for grasping and holding—such as hypertrophied claw-like setae, modified mouthparts, and adhesive sucker plates—and modifications of the life cycle to synchronize with other species on which they depend for feeding and dispersal.

Mites are equally diverse in their modes of reproduction. Some copulate with direct sperm transfer from the male to the primary or a secondary genital aperture of the female. In others, males transfer sperm to the female with their chelicerae or deposit a spermatophore on the substrate for females to pick up. Each of those systems is associated with specific structural and behavioral adaptations, some of which are highly complex.

In the face of the tremendous diversity of this group of organisms, generalizations about taxonomic and ecological diversity are difficult. We do not attempt a comprehensive overview of all these varied phenomena here. Instead, we attempt the more modest objective of assessing the evidence of how many species of mites exist. It would be easy to make sweeping generalizations about the existence of millions of species on the basis of our acknowledged ignorance of most mite groups, but we have resisted that temptation. Instead, we present an analysis of the state of knowledge of the mite faunas of Great Britain, Australia, and North America based on the results of recent taxonomic studies of selected taxa. We use these data as the basis of an extrapolation to an estimate of the number of species that remain to be described in these regions and then, more speculatively, to an estimate of the total world fauna of mite species. Previous estimates of the number of described mite species are summarized in table 1, and our current estimate is 48,200 described species.

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Mite Fauna of Australia

We have conducted a comprehensive survey of the literature associated with the Australian mite fauna. By August 1997, about 2,700 described species of mites in Australia were known—Astigmata, 330; Oribatida, 330; Prostigmata, 1,270; Ixodida, 80; Holothyrida, 3; and Mesostigmata, 675 (Halliday 1998). We may then ask ourselves how to extrapolate this figure to derive an estimate of the size of the total fauna. One approach is suggested by the figures presented in table 2. The table lists a series of mite groups in which the Australian fauna has been the subject of modern revision and shows the numbers of species before and after revision. The totals show that the previously known fauna was multiplied by a factor of about 2.9 as a result of revision. That might lead to an expectation that the total Australian mite fauna could include about 7,800 species if the same trend is repeated in other groups.

However, we believe for several reasons that a multiplying factor of 2.9 is a serious underestimate. The water-mite survey published by Cook (1986) essentially reported the results of a single collecting expedition by a single person, and many

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other species of water mites have been reported in Australia since then (for example, Harvey, 1987, 1989, 1990a,b,c,d, 1996). Schicha's (1987) revision of Phytoseiidae included mainly species associated with economic crop plants. The fauna associated with native plants, such as those occurring in rain forests, has only recently been studied (for example, Schicha and O'Dowd 1993) and is likely to be a rich source of new species. The study of the Ascidae reported by Halliday and others (1998) deliberately excluded many undescribed species and documented only enough species to record the presence of each genus in Australia.

It is also informative to examine the state of knowledge of some groups that are not in table 2. The known Australian fauna of Eriophyoidea includes only 53 described species of a world total of 2,884 (Armine and Stasny 1994). The United States has 635 described species of Eriophyoidea (Baker and others 1996), described from 579 host plant species. The Australian flora is very likely to include over 25,000 species of plants (George 1981), so if the same relationship between plant species and mite species numbers exists in Australia, the eriophyoid fauna of Australia might exceed 5,000 or even 10,000 species. A total of 64 species of described feather mites has been recorded from Australia. The number of bird species in Australia is about 700 (Slater 1970), and the feather mites demonstrate a high degree of host specificity (Gaud and Atyeo 1996), so the number of feather mite species might exceed 1,000. Hirschmann and Wisniewski (1993) listed some 2,000 described Uropodina species worldwide, but only 67 from Australia are known. The uropodine fauna of Australian rain forests is extremely rich and is likely to yield hundreds of species. Until the 1980s, the Australian fauna of Halacaridae had been very little studied and included fewer than 20 described species. However, a single collecting trip to a small island (Rottnest Island, WA; area, 1,900 ha) in 1991 yielded over 80 species (Bartsch 1996), most of which remain undescribed. The Australian fauna now comprises 80 described species, almost all of which were first described in the last five years. The Australian coastline of over 36,000 km is certain to yield hundreds of species of Halacaridae. About 300 described species of Tarsonemidae are known worldwide (Lindquist 1986), but only eight from Australia are recorded. Bearing all those factors in mind, we conservatively estimate that the Australian mite fauna is likely to exceed 20,000 species, more than seven times as many as the known described species.

Mite Fauna of Great Britain and Ireland

On the basis of the checklists of Turk (1953) and Luxton (1996), the recent monograph by Hillyard (1996), and a search of the Zoological Record, we obtained a total of 1,740 species for the mite fauna of Great Britain and Ireland—Astigmata, 265; Oribatida, 303; Prostigmata, 675; Ixodida, 22; and Mesostigmata, 475. However, the regularity with which acari new to science or newly recorded are still being found (for example, Luxton 1996; Skorupski and Luxton 1996) is evidence that the fauna is incompletely known.

A multiplying factor of 1.49 was obtained with the method described above (table 3), and this gives a projected total fauna of 2,590 species. Examination of the taxonomic attention that the various mite taxa have received in Great Britain

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and Ireland, however, suggests that far more are likely to occur there. The Ixodida have been extensively studied, and it is unlikely that additional native species will be discovered (Arthur 1963; Hillyard 1996). The Oribatida have also been the subject of much taxonomic work over many years (for example, Michael 1884, 1888; Luxton 1996), but new records are still being uncovered (Luxton 1996). Some families of Mesostigmata, such as the Macrochelidae, have been reviewed more than once in the last 50 years (Evans and Browning 1956; Hyatt and Emberson 1988), but most families have not been studied in great detail. Of all the orders, the Astigmata and Prostigmata have been the least studied; apart from Green and Macquitty (1987) and Gledhill and Viets (1976), there are no modern taxonomic monographs or reviews of the British members of either taxa. More often, descriptions or records appear in studies of particular habitats, such as human dwellings and food stores (Hughes 1976) or hosts (Hyatt 1990).

Many localities and habitats remain to be comprehensively sampled. Green and Macquitty (1987) acknowledged that their monograph on the marine mites could not be regarded as a complete account of the British fauna, because so much of the coastline was unexplored for halacarids. Similarly, Gledhill (1979) anticipated additional records of freshwater mites when the hyporheic zone of superficial riverine gravels and sands was more exhaustively sampled. Surveys of terrestrial mites—such as the Eupodidae, Rhagidiidae, and Phytoseiidae (table 3)—have concentrated on the faunas of woodland soils and plants, but even such a narrow range of situations has yielded many new species and records. Such habitats as tree-hole litter, fungi, mosses, and intertidal areas are still to be thoroughly examined.

It can be expected, therefore, that surveys of the majority of mite taxa will result in large numbers of species being added to the fauna of Great Britain and Ireland, as will a close examination of the many habitats for which the mite fauna is not known in any detail.

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Mite Fauna of North America.

OConnor (1990) reviewed the status of the mite fauna of North America north of Mexico by using data derived from species lists maintained by systematists working with each major taxonomic group. At that time, 5,106 described species had been recorded for the region—Opilioacarida, 1; Ixodida, 83: Mesostigmata, 869; Prostigmata, 2,803; Oribatida + “Endeostigmata”, 930; and Astigmata, 420. Contributors to that dataset were asked to estimate the total number of species expected in the North American fauna. The estimates were Opilioacarida, 1; Ixodida, 84; Mesostigmata, 2,827; Prostigmata, 7,977; Oribatida + “Endeostigmata”, 15,300; and Astigmata, 3,611. That is a total estimated fauna of 29,800 species, which means a multiplying factor of almost six. Since those data were reported, several revisionary works and compilations have added to the fauna. Farrier and Hennessey (1993) cataloged the free-living Mesostigmata, recording over 1,300 species in “North America”, including Mexico and parts of Central America. Baker and Tuttle (1994) revised the spider-mites (Tetranychidae) of the United States, and Baker and others (1996) revised the Eriophyoidea of the United States. Numerous smaller-scale revisions of families and genera have also been published, most notably for several groups of Oribatida (for example, Behan-Pelletier 1986, 1989, 1990, 1993, 1994; Norton and others 1996) and water mites (for example, Smith 1989a,b, 1990a,b, 1991a,b, 1994). The latter data provide something of a test of the earlier estimates of faunal diversity. With the method described above (see table 4), an increase factor of 3.24 was obtained for the oribatid groups recently revised and 2.65 for the water mites. The prior published estimates would predict increase factors of 16.5 for the oribatids and 1.79 for the water mites. The much lower observed increase factor for the oribatids might be explained by the fact that the recent revisions dealt primarily with faunas occurring in boreal Canada, where species diversity might be expected to be lower than

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in North America as a whole. The larger than predicted increase for the water-mite taxa might reflect the fact that many of the included groups specialize in stream and hyporheic environments, which have received much less attention than lotic environments. The data again point out the danger in extrapolating too much from limited observations.

As has been the case for other regions, North American taxa of economic and medical importance have received more attention. For example, in the first compilation of North American Phytoseiidae, Cunliffe and Baker (1953) reported 26 species. Farrier and Hennessey's 1993 catalog lists 150, for an increase factor of 5.77. Baker and Tuttle's 1994 review of the thoroughly studied North American Tetranychidae lists 218 species, of which only 12 were newly reported. The fauna of Ixodida is essentially completely known. However, many other taxa have received scant attention in North America; relatively little information is available on such major faunal elements as the free-living Prostigmata and Mesostigmata and the arthropod-associated Mesostigmata and Astigmata. Evidence of the latter includes OConnor's (1991) report of 57 species of insect-associated Astigmata collected at a single forested site in northern Michigan; of those, only four were previously described. Later collecting at the same site has yielded an additional 25 undescribed species (OConnor, unpublished data). Most taxa that have received some taxonomic treatment have not been thoroughly surveyed over the entire continent; most published treatments are at best regional. Major areas of endemism—such as California, the Pacific Northwest, the arid Southwest, and subtropical Florida—remain poorly collected for most free-living taxa.

How Many Species of Mites have been Described?

Various figures have been quoted for the number of mite species described worldwide (table 1). We now present a total of 48,200 nominate species (to June 1997). This figure was obtained from three main sources: an index of species at The Natural History Museum, London, which was maintained until 1977; the Zoological Record from 1978 on, which showed that an average of 788 new species were being described each year during that period (table 5); and, for Ixodida, Keirans (1992). The search of the Zoological Record from 1978 to 1996 also showed that mite species names were being synonymized at the rate of about 40 per year during that period, so this figure should be moderated slightly. Nevertheless, the figure of 48,200 should be regarded as a realistic assessment of the total number of valid mite species known worldwide.

Conclusions

It is evident from the estimates of biodiversity in the geographic regions considered above that the true number of mite species in the world fauna is much higher than 48,200. New species of Ixodida are now found only infrequently. The groups in which there are likely to be the greatest increases are the Astigmata, Mesostigmata, Oribatida, and Prostigmata (table 6). Many parts of the world have no active specialists in mite taxonomy or have not been the subject

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of faunal surveys. Similarly, the mites in many habitats and associations are poorly known, such as the soils of tropical rain forests and species associated with other arthropods (Welbourn 1983). Taxonomic attention is sometimes focused on a group when its economic importance is recognized, with the resulting description of huge numbers of new species. For mites, perhaps the best example is seen in the family Phytoseiidae. In the 1950s, when the potential of some phytoseiid species as biological control agents was first noticed, 165 species were listed in the world fauna (Chant 1959). By 1994, no fewer than 1,745 species had been described (Kostiainen and Hoy 1996), and another 55 new species have been found since then. That represents an increase factor of more than 10; if applied to our figure for valid species in other groups, the factor would yield a total of over a half-million.

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We can suggest several strategies that will help to increase our knowledge of the world's mite fauna. It would be useful to document and publicize the existence of collections of unsorted material, such as Berlese funnel samples, so that material collected incidentally by nonacarologists and samples of no immediate interest to acarologists is not lost but is available for later study. It would also be useful if entomologists, mammologists, and ornithologists were encouraged to retain the parasitic and phoretic mites that they find, rather than discarding them, and to draw them to the attention of acarologists. The specimens should either be kept with the host or have collection data included with them if separation is necessary. But the availability of specimens is not likely to be the most important limiting factor in the progress of systematic acarology. Serious assessment of mite biodiversity will continue to be inhibited by the shortage of trained taxonomists, especially in tropical areas, where species diversity is likely to be much greater than in the areas we have examined here.

Acknowledgments

We thank Emma de Boise, Department of Entomology, The Natural History Museum, London, for help in the assessment of numbers of valid mite species and Dave Walter for generously providing access to unpublished information.

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