Balian E.V., L?v?que C., Segers H., Martens K. (Eds.) Freshwater Animal Diversity Assessment

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The effect of bias in sampling effort on the bioge-

ography of nematodes can be most clearly demonstrated

by considering the case of Mermithida. The current

picture at the genus and species level (Fig. 2) shows a

pattern where mermithids are much more diverse in the

northern than the southern hemisphere. Consequently,

the family Mermithidae is claimed to have a Holarctic

distribution (Rubtsov, 1974 in Kozhova et al., 2000). In

reality, the current data is inconclusive with regard to

global distribution and the observed pattern may simply

be an artifact of sampling bias: nematological literature

reveals that there has been very little research focus on

mermithids in the southern hemisphere and as a result

we know little about mermithids of that region (see also

discussion above). Furthermore, differences in taxo-

nomic approaches are likely to muddle the picture even

further. Thus, the higher number of mermithid species

reported from Europe and Asia compared to North

America is probably partly due to the tendency of

mermithid specialists in North America to be more

reluctant in proposing new species or accepting known

ones (compare, e.g., Curran & Hominick, 1981 with

Rubtsov, 1981).

Endemism

Discussions of nematode endemism are largely

premature because of the extremely limited efforts

devoted to inventorying freshwater nematodes on a

geographic scale, compounded by the almost inevitable

operational bias and patchy nature of sampling efforts

in hitherto conducted studies. A case in point is the

study of nematodes in ancient lakes. These lakes are

particularly important in view of the fact that they are

natural laboratories for speciation (Rossiter &

Kanawabe, 2000) and a deserved focus on their

nematofauna may be of paramount signiﬁcance to our

understanding of freshwater nematode biogeography.

Decraemer & Coomans (1994) summarized the

nematode species count from the ancient lakes

Malawi, Kinner et, Titicaca, Tanganyika, and Ohrid

to be 3, 7, 11, 14, and 24, respectively. Lake Baikal is

one of the better studied among the ancient lakes with

a rich nematofuana of 92 species in 20 families of

which about 90% are considered endemic to the lake

(Kozhova et al., 2000). Taxonomically, tobrilids

disproportionately dominate the community with

globally unparalleled diversity making this lake the

center of known tobrilid diversity (Tsalolikhin, 1980,

1983). In light of the continuing new species

discovery (e.g., Sh oshina, 2003) and the effects of

sampling effort on species richness estimates (e.g.,

Michiels & Traunspurger, 2004), the actual nematode

species diversity in Lake Baikal could well be much

higher than thus far reported. Furthermore, employ-

ing taxo n delimitation criteria other than morphology

may reveal cryptic species. However, no detailed

population genetic studies were carried out on

freshwater nematodes and o nly few on free-living

marine nematodes (Derycke et al., 2005). The latter

molecular study showed a strong genetic differenti-

ation among populations and an inverse correlation

between dispersal ability and genetic differentiation.

Understanding historical and ecological processes

that led to the evolution of a tobrilid ﬂock in lake

Baikal, for example, would provide clues in under-

standing mechanisms of speciation and biogeography

of nematodes in other freshwater bodies. This being

so, it is plau sible that all ancient lakes could similarly

be centers of diversity for nematode (sub)groups.

Unfortunately nematode communities of many ancient

lakes (Lakes Lanao, Victoria, Malawi, Tanganyika,

Khubsugul, Biwa, Pannon, and Shanwang) have not

been studied in any meaningful way and our knowl-

edge of even these scientiﬁcally famous lakes remains

extremely patchy. This lack of information is even

greater for smaller or much more cryptic freshwater

bodies. Recent discoveries of invertebrate hyperdiver-

sity in freshwater habitats such as the underground

calcrete formations in western Australia (Leys et al.,

2003) or the stromatolite pools in Cuatro Cie

negas,

Mexico (Dinger et al. 2005) stand out as locations that

are likely to reveal an abundance of nematodes,

potentially with a high number of new taxa.

Acknowledgments The ﬁrst author thanks Dr. William K.

Thomas, Hubbard Center for Genome Studies, University of

New Hampshire, for allowing him to use the facilities of the

center during the preparation of this manuscript, and Drs.

Istva

n Andra

ssy (Hungary), Mike Hodda (Australia), Michael

Kidd (USA), and Melissa Yoder (USA) for supplying useful

literature.

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78 Hydrobiologia (2008) 595:67–78

123

FRESHWATER ANIMAL DIVERSITY ASSESSMENT

Global diversity of hairworms (Nematomorpha:

Gordiaceae) in freshwater

George Poinar Jr.

 Springer Science+Business Media B.V. 2007

Abstract Freshwater hairworms (Gordiaceae) and

marine hairworms (Nectonematoidea) comprise the

phylum Nematomorpha. Only the freshwater forms

are discussed here. While freshwater hairworms

develop as parasites of both aquatic and terrestrial

arthropods, they all enter fresh water to mate,

oviposit and produce infective stages (preparasitic

larvae). The global species diversity of freshwater

hairworms based on published descriptions is approx-

imately 326 species and a conservative estimate for

the global species diversity in this group would be

around 2,000 species.

Keywords Nematomorpha  Hairworms 

Gordiaceae

Introduction

There are tw o classes in the phylum Nematomorpha:

the freshwater hairworms or Gordiaceae, comprising

a dozen genera and the marine hairworms or Necto-

nematoidea, with a single genus. The host range,

species diversity and distribution of the Nectonem-

atoidea were summarized by Poinar and Brockerhoff

(2001) and since all of the ﬁve known species are

marine, they will not be included here.

While all freshwater hairworms develop as para-

sites in arthropods, all enter fresh water to mate,

oviposit, and infect paratenic hosts. Freshwater

hairworm adults, eggs and preparasitic larvae occur

in ponds, streams, lakes, and various man-made

structures that retain water. The adults range from tan

to black in color and from several centimeters to over

a meter in length. The body wall is covered with a

hard, opaque, multilayered cuticle that varies in

texture from smo oth to highly sculptured. The surface

projections (areoles), warts and/or bristles on the

epicuticle as well as the male tail, provi de important

taxonomic characters. While the anterior end can be

attenuated, the tail may be rounded, bilobed, or even

trilobed. Hairworm preparasitic larvae are capable of

encysting (without development) in a wide range of

paratenic hosts, including invertebrates and verte-

brates. Development occurs in the body cavity of

invertebrates (especially insects) that have ingested

paratenic hosts (Poinar, 2001; Schmidt-Rhaesa, 1997;

Smith, 2001; Hanelt et al., 2005) Fig. 1.

Species/generic diversity

Some 21 extant and two fossil genera of freshwater

hairworms have been described worldwide (Table 1).

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

que, H. Segers &

K. Martens

Freshwater Animal Diversity Assessment

G. Poinar Jr. (&)

Department of Zoology, Oregon State University,

Corvallis, OR 97331, USA

e-mail: poinarg@science.oregonstate.edu

123

Hydrobiologia (2008) 595:79–83

DOI 10.1007/s10750-007-9112-3

Some of these genera are questionable and further

studies are needed to test their reliability. Species

diversity is difﬁcult to determine since the intraspe-

ciﬁc variation of most species is unknown and some

characters now used for species separation may not

be reliable. The global species diversity based on

published descriptions varies according to authorities,

but is around 326 species (Table 2). An estimate for

the projected global species diversity in this group

would be around 2,000 species (Table 3). The

Nematomorpha is an ancient group that probably

extends back at least to the Carboniferous, although

fossils are extremel y rare, the oldest dating from the

Early Cretaceous (100 mya) (Poinar & Buckley,

2006) Fig. 2.

Phylogeny and historical processes

The hairworms represent a relict group that is not

closely related to any other living phylum. Various

authors have attempted to show phylogenetic

Fig. 1 A typical hairworm

life cycle. (A). Hairworm

adults emerge from an

insect host (Orthoptera).

(B). Hairworms mate and

oviposit in freshwater. (C).

A preparasitic hairworm

larva ready to hatch from its

egg. (D). Using anterior

stylets and posterior spines,

a larva penetrates the gut of

a paratenic host. (E). Larvae

encyst in the body cavity of

paratenic host (here a caddis

ﬂy larva). (F). A hairworm

larva encysted in the tissues

of its paratenic host.

Development of the

hairworm is completed

when the paratenic host is

eaten, together with the

encysted hairworm larvae,

by the ﬁnal (deﬁnitive) host.

Not drawn to scale

80 Hydrobiologia (2008) 595:79–83

123

relationships between hairworms and othe r ‘pseudo-

coelomate’ phyla (summarized by Schmidt-Rhaesa,

1998). However, it is difﬁcult to determine the

phylogenetic relationships of a group when there are

no completely free-living members and the morphol-

ogy and development are incompletely known

(Bresciani, 1991). Also since the preparastic larvae

are quite different from the adults, a phylogenetic

position based on morphology will differ depending

on the character states chosen. The morphology of

adult and preparasitic larval freshwater hairworms is

unique and supports the view that hairworms sepa-

rated from the main body of the Aschelminthes at an

early stage. Perhaps molecular studies will reveal if

hairworms are related to the kinorhynchs, priapulids,

rotifers, or nematodes, the groups considered by most

to be their closest relatives.

Present distribution and main areas of endemicity

A worldwide study of the biodiversity of hairworms

developing in ground beetles (Carabidae: Coleoptera)

showed that at least 70 species of beetles are

parasitized by 47 species of hairworms belonging to

at least ﬁve genera (Poinar et al., 2004). The

environmental conditions for hairworm survival are

Table 1 Freshwater hairworm families and genera

Gordiidae May 1919

Acutogordius Heinze, 1952

Gordius L. 1766

Chordodidae May 1919

Beatogordius Heinze, 1934

Chordodiolus Heinz, 1935

Chordodes Creplin, 1847

Cretachordodes Poinar & Buckley, 2006 (fossil genus)

Dacochordodes Ca

puse, 1965

Digordius Kirjanova, 1950

Euchordodes Heinze, 1937

Gordionus G. W. Mu

ller, 1927

Lanochordodes Kiryanova, 1950

Neochordodes Carvalho, 1942

Noteochordodes Miralles & Villalobos, 2000

Paleochordodes Poinar, 1999 (fossil genus)

Pantachordodes Heinze, 1954

Parachordodes Camerano, 1897

Paragordius Camerano, 1897

Paragordionus Heinze, 1935

Progordius Kirjanova, 1950

Pseudogordius Yeh & Jordan, 1957

Pseudochordodes Carvalho, 1942

Semigordionus Heinze, 1952

Spinochordodes Kirjanova, 1950

Table 2 Approximate number of hairworm species in geo-

graphical regions (total # of species = 326)

Regions Number of

hairworm

species

Major references

Africa 64 Camerano (1915);

Sciacchitano (1958)

Asia 28 Sciacchitano (1958)

Australia 9 Schmidt-Rhaesa et al.

(2003);

Schmidt-Rhaesa (2002);

Schmidt-Rhaesa &

Bryant (2004b)

China 8 Camerano (1915)

Europe 99 Schmidt-Rhaesa (1997)

India 31 Camerano (1915)

Japan 11 Schmidt-Rhaesa (2004a)

Madagascar 5 Sciacchitano (1958)

Malaysia 3 Poinar (2004)

New Zealand 5 Poinar (2006)

North America 18 Poinar & Chandler (2004)

Paciﬁc Islands 13 Sciacchitano (1958);

Schmidt-Rhaesa (2003)

South &

MesoAmerica

32 Camerano (1915);

Sciacchitano (1958);

Schmidt-Rhaesa &

Menzel (2005)

Table 3 Estimated species diversity of hairworms (total

# = 2000)

Zoogeographic

Region

Estimated number

of species

Palaearctic 250

Nearctic 200

Neotropical 500

Afrotropical 400

Australasian 300

Oriental 300

Paciﬁc Oceanic Islands 50

Hydrobiologia (2008) 595:79–83 81

123

more complex than that for the host, since most

hairworms need both an aquatic paratenic and a

terrestrial developmental host.

Human related issues

There is no direct economic or medical importance of

freshwater hairworms. Hairworms can serve as

natural biological control agents by reducing plant-

feeding orthopterans, however they also parasitize

predatory insects (Poi nar et al., 2004).

Freshwater hairworms are rarely collected since

they normally occur in small numbers and are well

camouﬂaged. However in som e localities, such as the

South Island, New Zealand, one species (Euchord-

odes nigromaculatus) occurs along forest streams in

the hundreds at certain times of the year.

The main threats to freshwater hairworms are

habitat changes that destroy their hosts and water

sources, polluted waters that kill their eggs and newly

hatched larvae and predators, parasites and pathogens

that attack their paratenic and developmental hosts,

and in some cases, hairworms themselves (Poinar,

2001). Most hairworm species proba bly would be

candidates for the Red Data Book species, although a

thorough sampling to determine the natural distribu-

tion of most species has not been undertaken.

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Fig. 2 Biodiversity of hairworm species/genera in each

zoogeographic region. Since hairworm genera are widespread,

many of the 21 known genera occur throughout the world.

There are some 326 described hairworm species. The high

level of biodiversity in the Palaearctic probably indicates the

higher number of studies undertaken rather than an especially

high level of biodiversity. PA—Palaearctic; NA—Nearctic;

NT—Neotropical; AT—Afrotropical ; OL—Oriental; AU—

Australasian; PAC—Paciﬁc Oceanic Islands; ANT—Antarctic

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FRESHWATER ANIMAL DIVERSITY ASSESSMENT

Global diversity of gastrotrichs (Gastrotricha) in fresh waters

Maria Balsamo Æ Jean-Loup d’Hondt Æ

Jacek Kisielewski Æ Lara Pierboni

 Springer Science+Business Media B.V. 2007

Abstract The global diversity of inland water

Gastrotricha is poorly known, and information is

extremely heterogeneous. Gastrotricha have been

studied most widely in Europe and America, whereas

data from the other continents are scatte red or not

even available. This scanty information is related to

several reasons, ﬁrst of which is the technical

difﬁculty in collecting and studying microscopic

and soft-bodied species. In addition, the research

has been limi ted mostly to the epibenthos and

periphyton in lentic waters, and the gastrotrich

taxonomy is still under discussion mainly because

of the great intraspeciﬁc variability. Three of the ﬁve

freshwater families are widespread or cosmopolitan,

and most genera have been reported from at least two

continents. There is strong evidence of a high

diversity in genera and species in tropical areas.

Nearly a half of the freshwater species are known

from only one country or even only from one site, but

the insufﬁcient faunistic knowledge does not allow

deﬁning them as endemic. The phylogenetic relation-

ships and possible evolutionary trends of inland water

species of Gastrotric ha are outlined.

Keywords Gastrotricha  Chaetonotida  Freshwater

species  Global biodiversity  Geo graphic

distribution

Introduction

Gastrotricha are aquatic microinvertebrates compos-

ing a constant, important component of the benthic

communities in marine and freshwater habitats. In

spite of the often high number of populations,

gastrotrichs are not yet well known, possibly due to

their minute size and body fragility, which make

studying them very difﬁcult. The phylum consists of

nearly 690 named species, grouped into two orders,

Macrodasyida and Chaetonotida, greatly different in

morphology, reproductive biology and ecology. Mac-

rodasyida are about 240 worm-like species, all

interstitial in marine and estuari ne habitats except

for two freshwater ones. Th e roughly 450 species of

Chaetonotida are tenpin-shaped, interstitial or epi-

benthic in marin e, brackish, but mainly freshwater

habitats (Balsamo & Todaro, 2002; Fig. 1). Most of

the 318 inland waters gastrotrich species have been

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

que, H. Segers &

K. Martens

Freshwater Animal Diversity Assessment

M. Balsamo (&)  L. Pierboni

Centro Ricerche per la Biodiversita

e la Conservazione,

Universita

di Urbino, Urbino, Italy

e-mail: maria.balsamo@uniurb.it

J.-L. d’Hondt

Muse

um National d’Histoire Naturelle, Paris, France

J. Kisielewski

Museum and Institute of Zoology, Polish Academy

of Science, Warsaw, Poland

123

Hydrobiologia (2008) 595:85–91

DOI 10.1007/s10750-007-9006-4

reported from the periphyton and the surface layer of

organic sediments in eutrophic, lentic biotopes

(Kisielewski, 1998). About 70 species are known

from an interstitial habitat, a half of which can be

found in lotic waters (see Ricci & Balsamo, 2000).

Freshwater Gastrotricha are presently grouped into 21

genera in 5 families of Chaet onotida, and into two

genera ‘incertae sedis’ of Macrodasyida. Many

species have been discovered in the last 50 years,

but the list of the species accepted in the current

systematization of the group is currently under review

following recent extensive and careful faunistic

surveys (see for references Schwank, 1990; Kisie-

lewski, 1991, 1998; Naidu & Rao, 2004).

Global species diversity

Past research

First descriptions of freshwater gastrotrich species go

back to the XVIII–XIX centuries in detailed studies

carried out by famous zoologists, but the systematic

research started in late XIX century leading to the

ﬁrst, important monograph by Zelinka (1889) on the

European and North American freshwater species. In

the beginning of the 1900s scattered faunistic and

systematic information was gathered mainly in

Europe, but as far away as Africa, India, Ceylon,

Tibet, Japan, New Guinea, Jamaica, Paraguay, and

America. Since 1970 a signiﬁcant rise of interest

brought about a series of studies mostly in Europe but

also in Israel, India, Japan, Korea, US, Argentina and

Brazil. Research has touched almost only continental

biotopes, but some data are also available for insu lar

fresh waters (Tuscan Archipelago, Italy; Azores,

Portugal; Jam aica; see Schwank, 1990; Kisielewski,

1998). The history of the knowledge of brackish-

water species is much shorter. Scattered, occasional

ﬁndings in low-salinity environments have concerned

European coastal lagoons, estuaries, deltas, Brazilian

mangroves and Amazonian estuaries, even hundreds

of kilometres from the sea (see Kisielewski, 1991).

Brackish-water gastrotrichs are mostly marine, chae-

tonotidan species, clearly adapted to great salinity

variations. Only a minority of fre shwater species can

survive salinity, and very few are exclusive to

brackish waters and may b e endemic of these habitats

(Kisielewski, 1991; Tongiorgi et al., 1999).

Estimated global diversity

At a high taxonomical level, the Chaetonotida

families Dasydytidae, Neogosseidae and espec ially

Chaetonotidae (subfamily Chaetonotinae) appear to

be widely distributed. The other families show a

limited distributio n: the rare Dichaeturidae have been

occasionally found in few European sites, the ditypic

Proichthydiidae are only known from South America

and Asia, and the subfamily Undulinae (Chaetonot-

idae) is reported from one site in Amazonia (see

Schwank, 1990; Kisielewski, 1991). The only two

freshwater monotypic genera of the order Macrod-

asyida, each recorded in one site, are known only in

Europe and South America, respectively (Ruttner-

Kolisko, 1955; Kisielewski, 1987). At a super-

speciﬁc level, almost a half of the genera show an

intercontinental distribution. A high diversity of

endemic genera in the Brazi lian fauna, but not in

the European and Levantine ones has been evidenced

by Kisielewsk i (1991). Abou t 1/3 of European

species and 1/3–1/2 of South American ones appear

cosmopolitan. The distribut ion of the other species, as

well as that of subgenera and gener a seems to be

restricted to a single continent or, if intercontinental,

to the tropical zone. The few studies carried out in the

tropical area show a very high generic and speciﬁc

diversity even of families rare in temperate zones

(Kisielewski, 1991). This strongly suggests that

freshwater fauna, especially the highly specialized

families, will be much richer in the tropical regions.

Detailed and reliable faunistic comparisons were

Fig. 1 Chaetonotus schultzei, S.E.M

86 Hydrobiologia (2008) 595:85–91

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