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as well as phylogenetical ly. Some phylogenetic

analyses place it weakly with the European species

(which makes more biogeographic sense), but a good

sampling of Cambaroides to place into a robust

phylogenetic analysis has yet to occur. The Astacidae

are distributed west of the Rocky Mountains (mainly

in the Paciﬁc Northwest) and in Europe (Table 1). In

the superfamily Parastacoidea, 10 of the 15 genera

are found in Australia. The remaining genera are

distributed in southern South America (three endemic

genera with 12 species distributed in southern Chile,

Uruguay, and south ern Brazil), New Zealand (with

one endemic genus and two described species), and

Madagascar (with one endemic genus and nine

described species).

The greatest species diversity in the fre shwater

crayﬁshes occurs in the southern Appalachian

Mountains of the southeastern United States

(Table 1). This region is also home to a number of

other highly endemic and highly endangered stream

species including freshwater ﬁshes, salamanders,

snails, and mussels. Based on phylogenetic results

and biogeographic analyses, much of the species

diversity in the Cambaridae is of relatively recent

origin (compared to the Parastacidae) and seems to

have been driven by isolating effects of pre- and

post-Pleistocene river drainage changes (Crandall

and Templeton 1999). Population genetic studies of

species groups are beginning to unravel the evolu-

tionary mechanisms associated with driving this

amazing dive rsity (Buhay and Crandall 2005; Fetz-

ner and Crandall 2003).

The Southern Hemisphere crayﬁshes have a center

of diversity in south–east Australia. These taxa (at

least the genus-level divisions) appear to be much

older in origin relative to the Northern Hemisphere

taxa. These genera appear to form well-deﬁned

monophyletic groups (e.g., Schull et al. 2005) as a

result. The species richness of many of thes e genera

appears to be the result of vicariance and dispersal

with isolation events (e.g., Ponniah and Hughes 2004;

Schull et al. 2005).

Fig. 2 Geographic distribution of the global freshwater crayﬁsh diversity (species number/genus number). PA––Palaearctic; NA––

Nearctic; NT––Neotropical; AT––Afrotropical; OL––Oriental; AU––Australasian; PAC––Paciﬁc Oceanic Islands; ANT––Antarctic

298 Hydrobiologia (2008) 595:295–301

123

Fig. 3 Phylogenetic relationships among the global freshwater crayﬁsh showing the strongly supported division between the two

superfamilies of the Northern Hemisphere (Astacoidea) and the Southern Hemisphere (Parastacoidea). (Sinclair et al. 2004)

Hydrobiologia (2008) 595:295–301 299

123

Human related issues

Freshwater crayﬁsh are a much sought after food item

in many cultures (Fig. 4) and a signiﬁcant aquacul-

tural commodity. Unfortunately for many of the

charismatic species (e.g., the Tasmanian Giant Lob-

ster, Astacopsis gouldi), over-harvesting coupled with

degradation of habitat has resulted in the endanger-

ment of many crayﬁsh species (e.g., Horwitz 1994).

Other areas, such as Madagascar, support a sustain-

able harvest of crayﬁsh, yet these populations are

typically imperiled by stream degradation (Jones

et al. 2005). While all 640+ species of freshwater

crayﬁsh have yet to be categorized for conservation

status relative to the IUCN Red List Criteria (IUCN

2001), a large number of species already occur on

that list. Taylor et al. (1996) found that over 50% of

the United States species of freshwater crayﬁsh are

endangered or threatened to some degree. Yet,

surprising, very few of these species are under

national protection with a few more under local

(state, provincial) protection. For example, in the

United States, 20 species are known from less than

ﬁve localities (with 15 known only from a single

locality) and well over 210 species are considered

endangered or threatened, but only four species are

listed by the federal government on the Endangered

Species List. Native crayﬁsh play an essential role in

their native habitat and may act as keystone species

for the freshwater ecosystems where they occur. Yet

this biodiversity is at great risk of loss if national

governments (especially in the US and Australia

where the bulk of the species diversity occurs) do not

take appropriate measures to protect the highly

endangered freshwater ecosystems that house these

and many other amazing species.

Acknowledgments We thank Estelle Balian and Koen

Martens for their invitation to participate in this exciting

project and for their helpful comments on our article. We also

thank the anonymous reviewers for their helpful comments to

improve the article. Our work was supported by NSF grant EF-

0531762.

References

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monophyletic origin of freshwater crayﬁshes estimated

from nuclear and mitochondrial DNA sequences. Pro-

ceedings of the Royal Society of London, Series B 267:

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and centers of origin of the crayﬁsh subgenus Proceri-

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the nested clade analysis: An empirical evaluation of

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fossil, paleo- and neo-ichnological evidence, and the

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tions to Zoology 105: 1–84.

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& O. R. Ravoahangimalala, 2005. A multidisciplinary

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crayﬁsh stand in Madagascar with both Astacoides madagas-

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Hydrobiologia (2008) 595:295–301 301

123

FRESHWATER ANIMAL DIVERSITY ASSESSMENT

Global diversity of water mites (Acari, Hydrachnidia;

Arachnida) in freshwater

Antonio Di Sabatino Æ Harry Smit Æ

Reinhard Gerecke Æ Tom Goldschmidt Æ

Noriko Matsumoto Æ Bruno Cicolani

 Springer Science+Business Media B.V. 2007

Abstract The Hydrach nidia (wate r mites) represent

the most important group of the Arachnida in fresh

water. Over 6,000 species have been described

worldwide, representing 57 families, 81 subfamilies

and more than 400 genera. The article analyzes extant

water mite diversity and biogeography. Data on

distribution and species richness of water mites are

substantial but still far from complete. Many parts of

the world are poorly investigated, Oriental and

Afrotropical regions in particular. Moreover, infor-

mation among different freshwater habitats is

unbalanced with springs and interstitial waters dis-

proportionately unrep resented. Therefore, more than

10,000 species coul d be reasonably expected to occur

in inland waters worldwide. Based on available

information, the Palaearctic region represents one of

the better investigated areas with the highest number

of species recorded (1,642 species). More than 1,000

species have been recorded in each of the Neotropical

(1,305 species) and Nearctic regions (1,025 species).

Known species richness is lower in Afrotropical (787

species) and Australasian (694 species) regions, and

lowest in the Oriental region (554 species). The total

number of genera is not correlated with species

richness and is distinctly higher in the Neotropical

(164 genera); genus richness is similar in the

Palaearctic, Nearctic and Australasian regions (128–

131 genera) and is lower in the Afrotropical and

Oriental regions with 110 and 94 genera, respec-

tively. A mean number of about three genera per

family occur in the Palaeartic, Nearctic and Oriental

while an average of more than four genera charac-

terizes the families of Australasian and Afrotropical

regions and more than ﬁve genera those of the

Neotropical. Australasian fauna is also characterized

by the highest percentage of endemic genera (62%),

followed by Neotropical (50.6%) and Afrotropical

(47.2%) regions. Lower values are recorded for the

Palaearctic (26.9%), Oriental (24.4%) and Nea rctic

Guest editors: E. V. Balian, C. Le

que, H. Segers &

K. Martens

Freshwater Animal Diversity Assessment

A. Di Sabatino (&)  B. Cicolani

Dipartimento di Scienze Ambientali,

University of L’Aquila, Via Vetoio,

L’Aquila 67100, Italy

e-mail: adisab@univaq.it

H. Smit

Zoological Museum, University of Amsterdam,

Plantage Middenlaan 64, 1018 DH Amsterdam,

The Netherlands

R. Gerecke

Biesingerstr. 11, D-7024 Tu

bingen, Germany

T. Goldschmidt

Zoologisches Institut, University of Karlsruhe,

Kornblumenstr. 13, D-76128 Karlsruhe, Germany

N. Matsumoto

Division of Biological Sciences,

Graduate School of Science, Hokkaido University,

Sapporo, Japan

123

Hydrobiologia (2008) 595:303–315

DOI 10.1007/s10750-007-9025-1

(21.4%). The Palaearctic and Nearctic have the

highest faunistic similarity, some minor afﬁnities are

also evident for the generic diversiﬁcation of Holarc-

tic and Oriental families. The faunas of Southern

Hemisphere bioregions are more distinct and charac-

terized by the presence of ancient Gondwanan clades

with a regional diversiﬁcation particularly evident in

the Neotropics and Australasia. This scenario of water

mite diversity and distribution reﬂect the basic

vicariance pattern, isolation, phylogenetic diversiﬁca-

tion, recent climatic vicissitudes and episodes of

dispersal between adjacent land masses together with

extant ecological factors can be evoked to explain

distribution patterns at a global scale.

Keywords Biodiversity  Biogeography 

Freshwaters  Global assessment  Water mites 

Hydrachnidia

Introduction

The Hydrachnidia (water mites), also called Hyd-

rachnellae, Hydracarina, Hydrachnida, represent the

most important group of the Arachnida in freshwater.

Originating from terrestrial ancestors, they have

colonized all kinds of freshw ater habitats. Water

mites are highly diversiﬁed both in lotic and lentic

habitats, as well as in springs and interstitial waters

(Di Sabatino et al., 2000, 2003; Smith et al., 2001).

Hydrachnidia belong to the cohort Parasitengona

(Actinedida), a group whose species are characterized

by a complex life cycle involving a heteromorphic

parasitic larva, two pupa-like resting stages (proto-

and tritonymph) and free-living predacious deu-

tonymphs and adults. The resting stages provide an

adaptation for avoiding unfavourable conditions in

unstable environments, and larval parasitism on

ﬂying insects confers substantial advantages ensuring

dispersal and rapid exploitation of new habitats

(Smith et al., 2001).

Water mites are characterized by bright colours

and a highly diversiﬁed morphology. Plesiotypically,

body shape is globular but it may also be ﬂattened

dorso-ventrally or laterally, or elongated into a worm-

like form (Fig. 1). Length ranges from 0.2 mm up to

10 mm, although mos t species are between 0.5 and

1.5 mm. As in all Acari, the body of a water mite is

divided into two principal units, gnathosoma and

idiosoma. The gnathosoma is a complex of trophic

and sensory structures composed of a sclerotized

capsule (capitulum) and two pairs of appendages

(palps and chelicerae). The idiosoma, or body proper,

may be soft-skinned or the integumental muscle-

attachment sites are transformed to more or less

extended sclerotized plates with tendencies to devel-

op complete sclerotization. The idiosom a is also

characterized by the presence of series of defensive

glands and mechanoreceptive slit organs. The ventral

side includes four pairs of sclerotized coxal plates

(insertion points for legs and leg muscles), the genital

ﬁeld and the opening of the excretory system. The

four pairs of legs of adults are six-segmented and

usually bear one pair of terminal claws. Size and

chaetotaxy of leg segments are modiﬁed in relation to

modes of locomotion and reproduction. For more

detailed information see Di Sabatino et al. (2000,

2002), Smith et al. (2001) and references therein.

Water mites are grouped into eight superfamilies

with more than 50 families. Ove r 420 genera and

about 6,000 species are described (Viets, 1987; Smit,

in http//www.watermite.org). However, this number

likely greatly underestimates global richness of water

mites. For example, about 5,500 species are calculated

to occur in the Neotropics alone (Goldschmidt, 2002).

Species diversity

The most recent catalogue of water mites (Viets, 1987)

included more than 5,000 described species in 50

families and 310 genera. Since then, our knowledge on

taxonomy, diversity and distribution has signiﬁcantly

improved. More than 1,000 species have been

described and 65 new genera, nine subfamilies (three

of which were reinstated) and ten new families (six of

which were reinstated) were established (Table 1).

Most of these new species have come from the

southern hemisphere. More than 230 species have

been added to the list of the Neotropical freshwater

fauna with about 140 additional species recorded for

South America and 100 for Central America. Aus-

tralasian is the second region in terms of new species

discovered after 1986, with 115 species from Aus-

tralia, 35 from New Zealand and 34 from western

Paciﬁc islands. A considerable number of species

were also reported from the still poorly known

freshwater fauna of the Eastern Palaearctic with more

304 Hydrobiologia (2008) 595:303–315

123

than 150 species described from China and Russia.

More than 100 species were also added to the faunal

lists of North America and Europe. At higher

taxonomic levels, new families were described from

the western Palaearctic, Nearctic, Oriental and

Australasian, with Australia and USA being the

countries with the highest number of new genera.

These considerations suggest that the actual num-

ber of water mite species in the world is large but

cannot be readily estimated. A species-accumulation

Fig. 1 Scanning electron microphotographs of some water mite

genera. (A)—Le bertia (Lebertiidae), ventral view. A widespread

genus presents in all bioregions except Australasian. (B)—Feltria

(Feltriidae), ventral view. A genus particularly diversiﬁed in the

Northern Hemisphere, not reported from Afrotropical and Austral-

asian. (C)—Apheviderulix, dorsal view. The only genus of the

enigmatic family Apheviderulicidae with distribution limited to

South Western Palaearctic and South Western North America.

(D)—Harpagopalpus (Harpagopalpidae), frontal view. The few

species of this genus are only known from the Afrotropical and

Oriental regions. (Photos (A)and(D) are from Gerecke; (B) from

collection Carl Bader, Basel; (C) from Gerecke et al. (1999)

Hydrobiologia (2008) 595:303–315 305

123

curve calculated for the western Palaearctic, one of

the better investigated areas of the world, indicates

that the number of new species discovered is still

increasing and there is no indication that it is reaching

a plateau (Fig. 2).

Data on present distribution and species richness

of water mites are substantial but still far from

complete (Table 2). Many parts of the world, Afro-

tropical and Oriental regions in particular, are poorly

investigated. Moreover, information on distribution

and species diversity among different freshwater

habitats is unbalance d, with particularly scanty

documentation of taxa that dwell in springs or

hyporheic-interstitial habitats. Therefore, more than

10,000 species coul d be reasonably expected to occur

in inland waters world-wide.

Phylogeny and historical processes

Hydrachnidia probably originated during the Mid-

Palaeozoic era (400 Mya) from terrestrial Parasit-

engona-like ancestors, but see also Smith & Cook

(1999) for an alternative hypothesis. From plesio-

typic unstable and intermittent habitats (temporary

pools, seepage areas and small springs) they

invaded all kind of freshw ater habitats. The only

two fossil records from Tertiary deposits refer to

larval specimens of highly derivative modern clades

(Poinar, 1985). No attempt has been made so far to

produce a hypothesis about the age and phyloge-

netic relationships of the major clades with the

support of molecular data. Differentiation of extant

superfamilies started from early Mesozoic

Table 1 New water mite taxa described after 1986 (data from Smit in http//www.watermite.org)

W Palaearctic E Palaearctic Nearctic Neotropical Afrotropical Oriental Australasian

Families 1

–2

–– 11

Subfamilies – – 3

21 –1

Genera 4 2 17 9 6 6 23

Species 118 181 110 239 49 133 184

The family Apheviderulicidae is distributed throughout N-America and W-Palaearctic

The subfamily Cyclomomoniinae is represented both in Nearctic and Australia

Fig. 2 Cumulative

numbers of water mite

species described from the

Western Palaearctic region

(data from Gerecke in http//

www. watermite.org)

Table 2 Taxonomic diversity of water mites at global and

biogeographic region scales

PA NA NT AT OL AU World

Families 42 44 31 28 33 30 57

Subfamilies 40 69 55 44 51 50 82

Genera 131 131 164 110 94 128 428

Species 1,642 1,025 1,305 787 554 694 >6,000

PA: Palaearctic; NA: Nearctic; NT: Neotropical; AT:

Afrotropical; OL: Oriental; AU: Australasian

306 Hydrobiologia (2008) 595:303–315

123

(240 Mya), Stygothrombioidea, Hydrovolzioidea,

Hydrachnoidea, Eylaoidea and Hydryphantoidea

are by most authors considered ‘‘lower water

mites’’. Members of these superfamilies share

plesiotypic features such as aerial larvae bearing

six-segmented legs and parasitism of a wide variety

of insect hosts. However, also within these groups,

numerous adaptations to particular environmental

conditions are realized. Probably from hydryphan-

toid-like ancestors, the ‘‘higher water mites’’ (Le-

bertioidea, Hygrobatoidea and Arrenuroidea)

evolved and radiated, perhaps in part as a response

to the diversiﬁcation of nematoceran insect hosts.

The systematics of higher groups is still in a state

of ﬂux. For alternative systems of superfamilial and

familial classiﬁcations see Tuzovskij (1987), Witte

(1991), Harvey (1998) and Panesar (2004).

Present distribution and main areas of endemicity

The currently available information do es allow us to

present a preliminary scenario on water mite diversity

at a global scale (Fig. 3).

Palaearctic

Due to a long tradition of European water mite

research and more recent intensive ﬁeld-work in

southern and central Europe, the Western Palaearctic

is one of the better investigated areas of the world

(Gerecke & Lehmann, 2005—see also Gerecke in

http//www.watermite.org). A total of about 1,600

species in 42 fam ilies, 40 subfamilies and 131 genera

have been documented. More than 1,100 species (114

genera and 40 families) are recorded from the

Western Palaearctic, most of them (about 800

species) from countries surrounding the Mediterra-

nean Sea (Di Sabatino & Gerecke, unpublished

database). From central Europe a recent list reports

620 species (Gerecke & Lehmann, 2005). Data from

the Eastern Palaearctic are more scanty and mostly

refer to some Russian territories (Sokolow, 1940;

Tuzovskij, 1997) and China (Jin, 1997). Ongoing

research and unpublished records suggest that more

than 3,000 species are potentially present in this

zoogeographic region.

Nearctic

About 1,700 species are estimated to occur in North

America, north of Mexico (Smith et al., 2001). So

far, 1,025 species have been reported, in 44 families,

69 subfamilies and 131 genera. Over 800 species

(100 genera, 35 families) occur in Canada (Smith

et al., 1998). Distributional data and check-lists are

also available for some Canadian ecoregions (Smith

Fig. 3 Distribution of

freshwater mite

(Hydrachnidia) species and

genera (SP/GN) per

zoogeographic region:

PA—Palaearctic;

NA—Nearctic;

NT—Neotropical;

AT—Afrotropical;

OL—Oriental;

AU—Australasian;

PAC—Paciﬁc Oceanic

Islands; ANT—Antarctic

Hydrobiologia (2008) 595:303–315 307

123

et al., 1998) and North America (Mitchel l, 1954;

Habeeb, 1967).

Afrotropical

Older data (Viets, 1970; Van Rensburg, 1974) and

more recent papers on the distribution of Afrotropical

water mites, record about 800 species belonging to 110

genera, 44 subfamilies and 28 families from this region

(Gerecke, unpublished). Liberia (180 species), South

Africa (160 species) and Cameroon (150 species) are

the countries best investigated (Cook, 1966; Viets,

1970; Harrison, 2000). From Madagascar, a country

known as particularly rich in species and endemic taxa,

only 65 water mite species have been documented so

far, with the fauna of springs, running waters and

subterranean habitats mostly unknown (Gerecke,

2004). The real diversity is likely to be one order of

magnitude higher (Goldschmidt & Gerecke, 2003).

Neotropical

About 1,300 species in over 160 genera, 55 subfa m-

ilies and 31 families have been recorded from Central

and South America, with large differences in the state

of knowledge of different regions and a high percent-

age of new species discovered also in the better

investigated areas (Goldschmidt, 2002; Rosso de

Ferradas & Fernandez, 2005). The number of known

species ranges from more than 260 from Southern

Brazil (Lundblad, 1941, 1942, 1943a, 1943b, 1944),

220 from Southern Mexico (Cook, 1980) and about

180 from Central Chile (Cook, 1988). Conversely,

only 5 species are reported from Bolivia and no data

exist from Jamaica, Nicaragua or French Guiana.

Oriental

In the Oriental region more than 500 species in 94

genera, 51 subfamilies and 33 families have been

found. Our knowledge of the region is far from

complete, many areas have hardly been explored (e.g.

many Indonesian islands, Southeast Asia, Himalayas,

Philippines) but see also Wiles (2004) and references

therein. A more detailed documentation exists only

from India with 230 species recorded, in 26 families

and 61 genera (Cook, 1967).

Australasian

One of the most distinct water mite faunas in the

world has evolved in Australasia. About 780 species

in 128 genera, 50 subfamilies and 30 families have

been found. Australia (24 families, 89 genera, 440

species) and New Zealand (19 families, 52 genera,

143 species) are the best examined countries of the

region (Cook, 1983, 1986; Harvey, 1998; Smit,

2005), but our knowledge is still insufﬁcient. New

Guinea is very poorly studied, and hardly any data are

available from the adjacent islands (e.g. Solomon

Islands, Bismarck Archipelago, New Britain).

Oceanic islands/paciﬁc

Only eight families, 13 genera and 27 species are

known from Paciﬁc islands (Smit, 2005). Three

species are reported from Vanuatu, 12 from Fiji and

14 from the Caroline Islands. Only two species are

known from Wester n Samoa and Hawaii. Water

mites are absent from the Galapagos Islands (Gerecke

et al., 1996).

Antarctic

No water mites have been reported so far from this

region. Their absence is conﬁrmed by recent studies

from sub-Antarctic islands (Pugh & Dartn all, 1994;

Dartnall, 2005).

Endemism and faunistic similarity at family and

subfamily level

A preliminary analysis of the extant differential

distribution of families and their generic diversiﬁca-

tion (Table 3) shows that 19 of the 57 known families

(33%) occur in all bioregions, three families (5%)

have a near cosmopolitan distribution being absent

only from Australasia, 12 (21%) are restricted to two

bioregions and 11 are endemic to single regions

(Table 4). About 75% of the families occur both in

the Palaearctic and Nea rctic, 30–33 families (more

than 50%) have been documented from Australasian,

Oriental and Neotropical regions, and only 27 (47%)

are represented in the Afrotropical region.

308 Hydrobiologia (2008) 595:303–315

123