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The distribution of families shows that the Aus-

tralasian fauna is particularly distinct (Fig. 4). The

Palaearctic and Nearctic (Holarctic realm) have the

highest-mutual afﬁnity and are linked, at a low level

of similarity, with the Neotropical, Afrotropical and

Oriental regions, with the Neotropical distinctly set

off from the Afrotropical and Oriental.

An analysis of the regional endemism of water

mite taxa (Table 4) shows that four families are

endemic to the Australasian region, a further two are

exclusive to the Palaearctic, Nearctic and Neotrop-

ical, a single family is limit ed to the Oriental and no

endemic families are known from the Afrotropical

region. At the subfamily level, the Neotropical

region is distinctive with 12 exclusive subfamilies,

followed by Palaearctic (4), Nearctic (3) and

Australasian (3). A last consideration can be made

on the distribution of taxa exclusive for pairs of

adjacent biogeographic regions (Fig. 5). Some taxa

possibly reﬂecting an older Laurasian or North-

Pangean distribution have their geographical range

limited to the Holarctic while two subfamilies are

limited to the Palaearctic and Oriental regi ons. In

the Southern Hemisphere, the Neotropical shows a

closer afﬁnity with the Nearctic (two families and

ﬁve subfamilies) than with the Afrotropical region

Table 3 Genera richness of water mite families in each bi-

ogeographic region

Family PA NA NT AT OL AU World

Stygothrombiidae 1 1 0 0 0 0 1

Hydrovolziidae 2 1 0 1 1 0 3

Acherontacaridae 1 1 0 0 1 0 1

Hydrachnidae 1 1 1 1 1 1 1

Limnocharidae 4 3 3 2 2 3 6

Eylaidae 1 1 2 1 1 1 2

Apheviderulicidae 1 1 0 0 0 0 1

Piersigiidae 1 1 0 0 1 2 3

Hydryphantidae 27 23 12 9 13 10 50

Hydrodromidae 1 1 1 1 1 1 1

Teratothyadidae 0 0 0 1 1 0 2

Rhynchohydracaridae 0 1 3 0 0 0 3

Ctenothyadidae 0 0 0 0 1 1 2

Thermacaridae 1 1 1 0 1 0 1

Zelandothyadidae 0 0 0 0 0 2 2

Stygotoniidae 0 0 0 0 0 1 1

Sperchontidae 2 2 4 1 1 1 4

Teutoniidae 1 1 0 0 0 0 1

Rutripalpidae 1 1 0 0 0 0 1

Anisitsiellidae 8 5 11 9 16 6 33

Lebertiidae 2 3 2 1 1 0 4

Acucapitidae 0 0 0 0 1 0 1

Oxidae 2 2 3 3 2 2 4

Torrenticolidae 4 4 5 3 5 1 6

Pontarachnidae 0 0 0 1 0 1 1

Limnesiidae 3 9 20 4 3 4 27

Omartacaridae 0 1 1 0 1 1 2

Hygrobatidae 5 6 33 19 5 24 77

Ferradasiidae 0 0 1 0 0 0 1

Unionicolidae 3 4 8 11 4 6 18

Feltriidae 1 1 1 0 1 0 1

Pionidae 9 9 4 2 3 6 14

Astacocrotonidae 0 0 0 0 0 1 1

Wettinidae 1 1 0 1 0 3 5

Frontipodopsidae 1 1 1 2 1 1 2

Aturidae 19 16 26 18 14 29 62

Lethaxonidae 1 2 1 1 1 0 2

Mideidae 2 1 0 0 0 0 2

Gretacaridae 0 0 0 0 0 1 1

Momoniidae 4 3 4 1 2 6 13

Mideopsidae 1 1 3 3 2 4 8

Nudomideopsidae 1 3 0 0 0 2 3

Uchidastygacaridae 2 3 0 0 0 1 4

Table 3 continued

Family PA NA NT AT OL AU World

Kantacaridae 1 0 0 0 0 0 1

Nipponacaridae 1 0 0 0 0 0 1

Neoacaridae 1 2 1 0 0 0 2

Bogatiidae 1 1 0 0 0 0 2

Chappuisididae 2 1 1 0 0 0 2

Krendowskiidae 1 2 3 1 1 0 4

Acalyptonotidae 1 2 0 0 0 0 2

Athienemanniidae 4 4 1 3 1 4 16

Harpagopalpidae 0 0 0 1 1 0 1

Hungarohydracaridae 3 0 1 1 2 0 4

Arenohydracaridae 0 1 1 0 0 0 1

Amoenacaridae 0 1 0 0 0 0 1

Laversiidae 0 1 0 0 0 0 1

Arrenuridae 2 1 5 8 2 2 12

Total genera 131 131 164 110 94 128 428

Total families 42 44 31 28 33 30

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

Afrotropical; OL: Oriental; AU: Australasian

Hydrobiologia (2008) 595:303–315 309

123

Table 4 Water mite families and subfamilies endemic to each biogeographic region

PA NA NT AT OL AU

Family Kantacaridae Laversiidae Ferradasiidae Acucapitidae Gretacaridae

Nipponacaridae Amoenoacaridae Arenohydracaridae Astacocrotonidae

Stygotoniidae

Zelandothyadidae

Subfamily Stygolimnesiinae Cowichaniinae Epallagopodinae Omanohydracarinae Nicalimnesiinae Schminkeinae

Bogatiinae Chimerathyadinae Tyrrelliinae Bharatoniinae Guineaxonopsinae

Balcanohydracarinae Stygameracariinae Ankelothyadinae Notomundamellinae

Tsushimacarinae Neotorrenticolinae

Cubanohydracarinae

Rheolimnesiinae

Psammolimnesiinae

Mixolimnesiinae

Rhynchohydracarinae

Plaumanniinae

Santiagoacarinae

Apeltosperchontinae

PA: Palaearctic; NA: Nearctic; NT: Neotropical; AT: Afrotropical; OL: Oriental; AU: Australasian

310 Hydrobiologia (2008) 595:303–315

123

(one subfamily). Two families and two subfamilies

have an Afrotropical–Oriental distribution and only

a single family and subfamily are exclusive to the

Australasian–Oriental regions.

Endemism and faunistic similarity at generic and

species level

When described species are considered, the Palae-

arctic is the most species-rich region (27% of the

known species) followed by Neotropical (22%) and

Nearctic (17%). A lower richness is recorded from

Fig. 4 Dendrogram of faunistic similarity between bioregions

based on presence–absence of water mite families (Sørensen

index)

Fig. 5 Water mite families and subfamilies with distributions

actually limited to adjacent biogeographic regions. PA—

Palaearctic; NA—Nearctic; NT—Neotropical; ET—Afrotrop-

ical; OL—Oriental; AU—Australasian; PAC—Paciﬁc Oceanic

Islands; ANT—Antarctic

Table 5 Species richness of water mite superfamilies in each biogeographic region

Superfamily PA NA NT AT OL AU World

Stygothrombioidea 4 6 0000 10

Hydrovolzioidea 16 6 0170 30

Eylaioidea 59 36 35 16 10 20 176

Hydrachnoidea 82 26 8 24 15 16 171

Hydryphantoidea 183 65 51 35 38 38 410

Lebertioidea 375 141 144 150 142 63 1,015

Hygrobatoidea 634 473 789 395 240 434 2,965

Arrenuroidea 289 272 278 166 102 123 1,230

Total species 1,642 1,025 1,305 787 554 694 6,007

PA: Palaearctic; NA: Nearctic; NT: Neotropical; AT: Afrotropical; OL: Oriental; AU: Australasian

Hydrobiologia (2008) 595:303–315 311

123

Australasian and Afrotropical regions (11–13%)

followed by the Oriental region with only 9% of

described species (Table 5).

The total number of genera (Table 3) is not

correlated with species richness and is distinctly

higher in the Neotropical (164 genera); genus rich-

ness is similar in the Palaearctic, Nearctic and

Australasian regions and is lower in Afrotropi cal

and Oriental regions. Moreover, as reported in other

groups (Ephemero ptera, this volume), Northern and

Southern Hemispheres are characterized by a differ-

ent mean number of genera per family: about three

genera per family occur in the Palaeartic, Nearctic

and Oriental regions while an average of more than

four genera characterizes the families of Australasian

and Afrotropical regions and more than ﬁve genera

those of the Neotropical.

Compared to the presence–absence of families, a

quantitative analysis of the richness of gener a among

families (Fig. 6) leads to slightly different results.

The regions are here grouped into two clusters, the

ﬁrst including again the Palaearctic and Nearctic, but

linked at low level of simi larity with the Oriental; the

second is formed by Neotropical and Afrotropical–

Australasian regions.

This pattern seems mainly determined by the

generic diversiﬁcation of some cosmopolitan families

(Fig. 7). The Holarctic and Oriental regions are

characterized by the diversiﬁcation of hydryphantids

and the Oriental furthermore by the massive presence

of anisitsiellid genera. On the other hand, Hygrobat-

idae, Limnesiidae and Aturidae are more diversiﬁed

in the Neotropics, Afrotropical and Australasian

regions with the Neotropical being unique because

of the extreme diversiﬁcation of Limnesiidae.

The percentages of endemic genera (Table 6)

follow the same pattern described above for the

generic diversiﬁcation of the most cosmopolitan

families, with regions of the Southern Hemisphere

having a high degree of endemism with about 50% of

the genera exclusive for each area. Also in this case,

Australasia is distinctive with 62% of the genera

known only from this region. Conversely, in the

Palaearctic, Nearctic and Oriental regions the per-

centages of endemic genera are markedly lower

(between 21 and 27%).

At species level, the Nearctic is characterized by

the dominance of lentic adapted species of the

genera Arrenurus, Unionicola and Piona, which

represent about 30% of the North American water

mite fauna; also remarkable is the diversiﬁcation of

the lotic-adapted g enera Feltria (37 species) and

Aturus (49 species). The Palaearctic fauna is mostly

dominated by lotic species of the genera Atractides,

Lebertia and Sperchon (more than 25% of the whole

fauna) and by lentic-adapted species of the genera

Hydrachna, Piona and Eylais. In the Neotropical,

Fig. 6 Dendrogram of faunistic similarity between bioregions

based on generic richness of water mite families (Bray-Curtis

index)

Fig. 7 Generic diversity of some cosmopolitan water mite

families in each bioregion

Table 6 Numbers of endemic water mite families, subfami-

lies, genera and percentage of endemic genera in each bioge-

ographic region

PA NA NT AT OL AU

Endemic families 2 2 2 – 1 4

Endemic subfamilies 4 3 12 1 2 3

Endemic genera 34 28 83 52 23 80

Percent endemic genera 26.9 21.4 50.6 47.2 24.4 62.0

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

Afrotropical; OL: Oriental; AU: Australasian

312 Hydrobiologia (2008) 595:303–315

123

species of the genera Koenikea, Limnesia, Corticac-

arus and Torrenticola represent about 40% of the

Neotropical mite fauna. The Afrotropical water mite

fauna is mostly represented by species of the genera

Arrenurus, Neumania, Hygrobates, Torrenticol a and

Monatractides (more than 40% of the whole fauna).

The Oriental region is characterized by a relatively

large number of Monatractides, Torrenticola and

Atractides species, while the genus Lebertia is

little represented. Almost 30% of the Australasian

fauna is represented by species of endemic

(Procorticacarus) and Gondwanan genera (Austra-

liobates, Aspidiobates), and by species of the more

widespread genera Limnesia, Axonopsella, Koenikea

and Frontipoda. The latter two genera h ave the

highest-species richness in the austral part of the

world. A number of near-cosmopolitan genera are

absent in the Australasian region i.e. Lebertia,

Atractides and Torrenticola.

Global distribution patterns

From the data presented above we can conclude that

the extant distribution of water mites is in good

agreement with the basic vicariance pattern proposed

by Smith & Cook (1999). A differential distribution

of water mite fauna had been established before the

break-up of Pangea with a distinction between

Northern and Southern Pangean clades. After the

separation, Laurasia and Gondwanaland were both

characterized by the presence of a distinct temperate

and tropical fauna.

In Laurasia a prevalent temperate fauna evolved

and diversiﬁed from the original stock with tropical

clades mainly conﬁned to the southern part of the

supercontinent. These elements may have persisted

until today as localized endemism in non-glaciated

areas of both North America and southern Eurasia

(possible extant representatives could be the family

Apheviderulicidae and the genera Javalbia, Utaxatax

and Momonisia). Palaearctic and Nearctic endemics

are mainly localized in Eastern Eurasia and Western

North America which represented the western and

eastern borders of Laurasia. Areas of particular

interest with endemic species and genera are also

localized in southwestern North America and in the

Mediterranean area in southern Europe.

In the Southern Hemisphe re the situation is more

complex. Gondwanaland was subjected to multiple

fragmentation and extensive migration (drifting) of

continental blocks. India separated ﬁrst and moved

largely northward experiencing dramatic climatic

change. Most of the original fauna disappeared and

only a few Gondwanan elements persist today (Terat-

othyadidae, Harpagopalpidae). The extant Oriental

fauna is mostly dominated by immigrant Southern

Palaearctic elements and by some Australasian taxa.

The water mite fauna of currently southern biore-

gions is mainly derived from ancient tropical

Gondwanan clades. The long-term isolation and

phylogenetic diversiﬁcation of most of these clades

in Australia and New Zealand could explain the

distinctive fauna of Australasian. Remnants of older

temperate elements could have persisted at high

elevation or high latitude in South America, Australia

and New Zealand.

The separation of the Oriental and the Australasian

regions has always been of great interest to science.

Many boundary lines have been proposed between

these two faunal regions (for a review see Simpson,

1977 and Cox, 2001). When one considers the water

mite fauna, Lydekker’s Line (Lydekker, 1896) seems

to match most closely the boundary of the Austral-

asian fauna. Lydekker’s Line corresponds with the

edge of the continental shelf of New Guinea and

Australia. East of this line a typical Australasian

fauna is

found, with characteristic genera including

Australiobates and Procorticacarus. Sulawesi has a

predominantly Asian fauna, with member of the

family Torrenticolidae dominating. Moreover, spe-

cies of Atractides, a genus absent from Australasia,

have been reported from Sulawesi. No records have

been published from the intermediate area between

Sulawesi and Australia.

In the Paciﬁc, species of islands north of the

Equator are of Asian origin, while those south of the

Equator are of Australasian origin. Only one endemic

genus is known i.e. Fijilimnesia from Fiji. Two

Aspidiobates species , a genus with a Gondwanan

distribution, are found on Vanuatu, which has never

been a part of Gondwana. The number of species

decreases from west to east, and from Western Samoa

eastward streams are devoid of water mites (Smit,

2005).

The fauna of South America also conserves a

typical mixture of tropical and temperate elements.

The recent establishment of the Central American

land bridge did not alter the original composition of

Hydrobiologia (2008) 595:303–315 313

123

the fauna except for the northernmost part and the

Caribbean area. Conversely, a number of taxa

migrated northward and invaded North America

(Smith & Cook, 1999; Smith et al., 2001).

These considerations suggest that the actual scenario

of water mite diversity and distribution reﬂect essen-

tially the basic vicariance pattern. Isolation, phyloge-

netic diversiﬁcation, 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.

Human related issues

Water mites represent a robust component of aquatic

communities in terms of both abundance and species

richness. Their high specialization to particular micro-

habitats and the strength of biotic interactions as

predators (deutonymphs and adults) and parasites

(larvae) are important features not shared by other

components of macroinvertebrate communities. Per-

haps because of this speciﬁcity and interconnected-

ness, they reﬂect natural changes and human induced

modiﬁcations of freshwater communities (see Di

Sabatino et al., 2000, 2002 and references therein).

Water mites are mostly absent from springs, running

waters, ponds and lakes whose natural sediment

structure has been destroyed and/or water quality

reduced by organic waste waters, heavy metals or other

poisonous com pounds. Moreover, due to the high

number of taxa specialized for life in spring habitats

(Di Sabatino et al., 2003) or in the upper part of the

interstitial zone, their study offers an important but

seldom exploited tool for understand ing and monitor-

ing human impacts in groundwaters (Boulton et al.,

2003) and groundwater–surfacewater interactions.

Acknowledgements We wish to thank H. Proctor (Alberta,

Canada) and three anonymous reviewers for their helpful

comments on an earlier draft of this article. ADS wish to thank

P. Vignini (L’Aquila, Italy) for her valuable help during the

preparation of the manuscript. ADS and BC were in part

ﬁnancially supported by the Italian MIUR (ex 60%).

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FRESHWATER ANIMAL DIVERSITY ASSESSMENT

Global diversity of halacarid mites (Halacaridae: Acari:

Arachnida) in freshwater

Ilse Bartsch

 Springer Science+Business Media B.V. 2007

Abstract Halacarid mites have successfully invaded

the sea and approximately 56 species colonized the

freshwater. Invasion from the sea into continental

waters probably started in the Mesozoic or Pre-

Mesozoic and went on in the following epochs. The

number of genera (14) and species (34) recorded from

the Palaearctic is remarkably higher than that of other

geographical regions. These numbers do not imply that

the Palaearctic is a centre of origin, they reﬂect the

sampling activity rather than reliable data on diversity.

Keywords Halacaridae  freshwater  distribution 

phylogeny  endemicity

Introduction

Most representatives of the family Halacaridae

(Order Prostigmata) are marine, to date slightly more

than 1,000 marine species are described versus about

56 from continental freshwater. Halacarid mites,

marine, as well as freshwater taxa, are benthic

throughout their life; they are unable to swim;

plankton stages are not known.

Halacarids are small-sized, rarely more than

500 lm in length, and in general less conspicuous

than the majority of water mites, they are never as

intensely coloured as many hydrachnids and never as

heavily sclerotized, as oribatids and mesostigmatids.

In halacarid mites, legs I and II are directed forward,

legs III and IV backward, the epimeres of the two

anterior pairs of legs by a gap separated from the

posterior epimeres. The legs of adults are six-

segmented, the palps four-segmented. The idiosoma

generally bears four dorsal plates, though in some

species the plates may be fused or reduced.

Freshwater halacarids are found in subterranean

and surface waters; they live in springs, wells, the

hyporheic zone of rivers and ﬂocculent ooze of lakes,

in artiﬁcial ﬁlters, sandy deposits, amongst colonial

organisms, gill chambers, mosses and vascular plants,

in humic as well as in brackish coastal waters. Many

of the freshwater halacarid species proved to be

euryoecious and very tolerant towards short-term

physical and chemical ﬂuctuations in their environ-

ment. A stable and predictable habitat and successful

competition seem to be more important for coloni-

zation than environmental parameters.

The mites run through one larval and tw o nymphal

stages before the ﬁnal moult to adults. The fecu ndity

is low, often a single generation per year and less than

50 offsprings per female. Adults and juveniles live in

the same substratum. Migration, dispersal or resting

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

que, H. Segers &

K. Martens

Freshwater Animal Diversity Assessment

I. Bartsch (&)

Forschungsinstitut Senckenberg, Deutsches Zentrum fu

marine Biodiversita

tsforschung, Notkestr. 85, Hamburg

22607, Germany

e-mail: bartsch@meeresforschung.de

123

Hydrobiologia (2008) 595:317–322

DOI 10.1007/s10750-007-9026-0

stages are not known. Reproduction can be dioecious

or parthenogenetic.

Halacarid mites are often classiﬁed as predators or

scavengers, feeding on small metazoans and proto-

zoans, but some species feed on algal and plant cells,

and probably bacteria and fungi are used by halacarid

mites as well. One species is expected to be a

parasite.

Species diversity

The ﬁrst record of a marine mite (‘insecta marina’)

from the seashore was published more than 200 years

ago (Baster, 1758: pl.10, Fig. 1). A century later, Gosse

(1855) introduced the genus Halacarus and Murray

(1877) established the family Halacaridae for ‘mites

living habitually under the sea’. The ﬁrst halacarid

mite from freshwater, Leptognathus violaceus, was

described by Kramer (1879). Viets (1927, 1933)

separated between marine and freshwater genera and

set up the family Porohalacaridae Viets 1933 for

freshwater species with external genital acetabula, in

contrast to the Halacaridae which included marine

species with internal genital acetabula. Since of

numerous discrepancies between genera belonging to

the Porohalacaridae on the one hand and characters

shared with marine genera, the division based on

presence or absence of external genital acetabula was

abandoned (Newell, 1947; Petrova, 1974; Bartsch,

1989b, 1996). Nowadays, freshwater halacarid genera

are incorporated in halacarid subfamilies, which

Fig. 1 Habitus of Lobohalacarus. Scale = 100 lm

318 Hydrobiologia (2008) 595:317–322

123

include either fresh or mostly marine species, some of

the freshwater halacarid mites are assigned to else

marine genera.

Approximately 56 halacarid species (Table 1) are

known from freshwater habitats. This ﬁgure includes

species of genera in general restricted to fresh or

diluted brackish (0.5–5%) water, as well as limnic

representatives of genera widespread in marine

habitats. Not included are species commonly found

in refreshed brackish water (about 0.5% or less) but

most abundant in coastal waters (salinity 5–30 %).

Subspecies are not mentioned separately. Some of the

species, as well as genera are in need of re-

examination to prove their identity. Not included in

the table (Table 1) and map (Fig. 2) are two species

found in aquaria in Europe, but most likely being

introduced from overseas.

Phylogeny and historical process

Arachnids have an evolutionary age dating back to

the Cambrian and probably all major orders of Acari

have existed, since the Silurian (Vitzthum, 1943).

The mites in the sea, the halacarid mites, are

expected to have evolved from aquatic or semi-

aquatic prostigmatid ancestors which colonized the

sea shores. The taxon most likely exists since the

Pre-Mesozoic or Mesozoic (Bartsch 1982, 1989a,

1996). Freshwater halacarids are polyphyletic,

descendants of marin e or brackish water ancestors

(Petrova, 1974; Bartsch 1982, 1996). For halacarid

mites, living in a wide range of substrata, both an

epigean and hypogean way was open to migra te

from the sea into freshwater. The increased fresh-

water inﬂux and hence need for osmoregulation was

met by either moving the genital acetabula to an

external position or increasing the genital acetabula

or epimeral pores. Both are osmoregulatorily active

areas, and an external position and/or enlargement

will be of competitive advantage in a life in diluted

brackish or freshwater.

Several of the present-day freshwater genera and

even species may have invaded continental waters in

the Mesozoic or even Pre-Mesozoic (Bartsch, 1996).

These probably ﬁrst immigr ants are today restricted

Table 1 Freshwater Halacaridae (Acari), genera of and number of species recorded from each zoogeographical region

PA NA NT AT OL AU PAC ANT World

Astacopsiphagus 0000010 0 1

Acarothrix 0100000 0 1

Copidognathus 4100000 0 5

Caspihalacarus 1000000 0 1

Halacarellus 100000 0 0 1

Hamohalacarus 0100000 0 1

Himejacarus 100000 0 0 1

Limnohalacarus 2114220 0 13

Lobohalacarus 2121021 1 6

Lohmannella 500000 0 0 5

Porohalacarus 3100010 0 3

Parasoldanellonyx 3100000 0 3

Peregrinacarus 0010000 1 2

Porolohmannella 1100000 0 1

Ropohalacarus 1100000 0 1

Soldanellonyx 8302111 0 9

Stygohalacarus 1000000 0 1

Troglohalacarus 100000 0 0 1

Total 34 12 4 7 3 7 2 2 56

PA: Palaearctic region; NA: Nearctic region; NT: Neotropical region; AT: Afrotropical region; OL: Oriental region; AU:

Australasian region; PAC: Paciﬁc Oceanic Islands; ANT: Antarctic region

Hydrobiologia (2008) 595:317–322 319

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