The Fauna Europaea is a database of the scientific names and distribution of all living, currently known European land and fresh-water animal species assembled by a large network of experts, using advanced electronic tools for data collations and validation routines. An extended description of the Fauna Europaea project can be found in Jong et al. 2014). A summary is given in the sections below.

The Gastrotricha are one of the 58 Fauna Europaea major taxonomic groups, and currently number 326 freshwater species worldwide, 224 of which reported from Europe. The data were acquired and checked by 5 specialists. No division of the taxa groups to be checked has been set among the specialists, thus all the specialists have to be considered responsible for the available data (Fig. 1​, Table 1).

Figure 1.  

FaEu data set Gastrotricha species per family: the updated number of species is reported in brackets. See Table 1​ for family statistics.

Gastrotricha

The phylum Gastrotricha currently (May 2015) counts 813 species of free-living, microinvertebrates commonly present and actively moving on and into the sediments of aquatic ecosystems where they represent a significant component of meiofauna, with densities up to 364 ind/cm² in marine sands and 168 ind/cm² in freshwater sediments (Nesteruk 1993, Nesteruk 1996, Todaro and Hummon 2008).

Gastrotricha have ‘Aschelminthes’ or ‘pseudocoelomates’ features such as a worm-like body, a complete intestine, and a primary body cavity even if almost vestigial (Hyman 1951). Morphological and ultrastructural studies support a sister-group relationship of Gastrotricha with Cycloneuralia or with Ecdysozoa, whereas molecular analyses mainly carried out on the gene 18S rDNA have suggest to include the phylum into the Platyzoa, close to Gnathostomulida and acoelomate forms (for an overview of the major phylogenetic scenarios concerning Gastrotricha among the Bilateria as well as the internal relationships of the phylum, see Balsamo et al. 2010 and Kieneke and Schmidt-Rhaesa 2015). Recent studies based on a trascriptomic-phylogenomic approach found the gastrotrichs to be the sister taxon of the Platyhelminthes (e.g. Egger et al. 2015).

Classical morphological taxonomy recognizes two orders, Macrodasyida and Chaetonotida, quite different in general morphology, biology and ecology (Balsamo and Todaro 2002, Balsamo et al. 2014, Kieneke and Schmidt-Rhaesa 2015).

The order Macrodasyida includes 356 species (10 families, 35 genera), all interstitial, marine or brackish-water except for only 2 freshwater species. The marine monotypic genus Hemidasys has been considered extinct by Hummon and Todaro (2010). The vast majority of Chaetonotida, 335 species, colonize fresh waters, with a clear preference for eutrophic habitats, where most species known so far live as epiphytic or periphytic. Only about 70 freshwater species have been found in psammic habitats, and less than 35 species are known from sediments of running waters. Two entire families (Dasydytidae and Neogosseidae), with about 50 species, are semipelagic or fully planktonic: the colonization of water column corresponds to specific, characteristic morphological adaptations (Kisielewski 1981, Kisielewski 1990, Schwank 1990, Ricci and Balsamo 2000, Balsamo et al. 2008, Todaro et al. 2013, Kånneby and Todaro 2015).

Gastrotrichs share several morphological and physiological features with other groups of freshwater microinvertebrates: among them are small size, soft body, worm-like body shape, high contractility, adhesive glands, ciliary locomotion, triradiate pharynx with well-developed musculature, short life cycle, common parthenogenic reproduction, production of resting stages (d’Hondt 1971). But the adaptive potential of freshwater Gastrotricha is relatively limited compared to other animal phyla like Rotifera or Nematoda, so that their distribution appears to be narrower and to current knowledge it does not extend to extreme habitats.

The diversity of the phylum Gastrotricha is not very high, but these animals, as a part of the microphagous benthic community, play a significant ecological role in aquatic environments, linking the microbial loop to the higher trophic levels (Balsamo et al. 2014).

Taxonomy and taxonomical issues

The order Macrodasyida includes only 2 freshwater species: Redudasys fornerise, belonging to the family Redudasyidae (which also comprises a marine species, see Todaro et al. 2012), and Marinellina flagellata, which was previously assigned either to Macrodasyida (Ruttner-Kolisko 1955, Kisielewski 1987) or to Chaetonotida, in the freshwater family Dichaeturidae (Remane 1961) and is now considered a Macrodasyida incertae sedis. It is the only Macrodasyida species known from Europe, where it was found only once in Austria (Ruttner-Kolisko 1955).

The order Chaetonotida encompasses 335 freshwater species and 4 strictly brackish-water species. Four families (12 genera) are exclusively freshwater (Dasydytidae, Neogosseidae, Proichthydiidae, Dichaeturidae), and the largest family, Chaetonotidae, includes both marine and freshwater species (2 genera exclusively marine, 5 freshwater and 5 including marine, brackish-water and freshwater species).

Thanks to recent studies, the current systematics is relatively stable for the order Macrodasyida (Todaro et al. 2006, Hummon and Todaro 2010, Todaro et al. 2011, Todaro et al. 2012, Guidi et al. 2014, Todaro et al. 2014), except for the monotypic genus Marinellina (still incertae sedis). By contrast, the taxonomy of the order Chaetonotida has been repeatedly revised in the last decades and is still unstable especially at the species level (Schwank 1990, Kisielewski 1991, Kisielewski 1997, Kisielewski 1998, Leasi and Todaro 2008, Balsamo et al. 2009, Hummon and Todaro 2010, Balsamo et al. 2014). Furthermore, a phylogenetic study based on molecular markers found the largest chaetonotidan family, Chaetonotidae, and most of the genera included in it to be non-monophyletic (Kånneby et al. 2013).

Actually, the taxonomy of the phylum has been founded on morphological traits both of the external structure and the internal anatomy, like the shape and size of the cuticular elements, the organization of the reproductive system and the fine structure of spermatozoa (Marotta et al. 2005, Hummon and Todaro 2010, Guidi et al. 2014). However, especially in Chaetonotida, several of these taxonomical characters appear to vary, even considerably, at species level and in some cases also at genus level, which makes taxonomic identification quite problematic (Schwank 1990, Balsamo et al. 2009).

It must be said that Gastrotricha, small and diaphanous, should be observed alive with a very good microscopical equipment in order to recognize and measure all the morphological details taxonomically important (Balsamo et al. 2014). That is not always possible, considering the technical problems of collecting samples, maintaining them in the lab, searching for in them in the sediments under a stereomicroscope, and then isolating single specimens be anesthetized in some manner and to allow proper observation. However, several freshwater species can be suitably fixed and observed later with good results (Giere 2009).

The recent introduction of molecular analyses to Gastrotricha has opened new perspectives in the study of phylogenetical relationships; an integrative taxonomical approach using both morphological and molecular methods seems essential to effectively revise the current classification according to phylogenetic relationships (Kånneby et al. 2012, Paps and Riutort 2012, Todaro et al. 2012, Kånneby et al. 2013, Todaro et al. 2014).

We may expect a future increase in species numbers of Chaetonotidae and possibly of Dasydytidae. A reliable assessment of the estimated species number cannot be advanced due to the sporadic nature of faunistic findings and samplings, but a reasonable minimal estimate of possible increase in number of species known is advanced based on the current knowledge of unpublished data (Table 1).

European freshwater species

The Fauna Europaea database (version 2.4, January 2011) includes 215 species, 214 of which belong to the order Chaetonotida (4 families) and a single species incertae sedis is a member of the order Macrodasyida.

Eight additional species of Chaetonotida have been described and another one has been recorded first for Europe after the release of the database, so that currently European freshwater species of Gastrotricha known so far add up to 224 overall (223 Chaetonotida + 1 Macrodasyida), that are considered in the present paper.

A synthesis of European taxa is presented below.

Family Chaetonotidae (Fig. 2 A-F)

Figure 2.  

Common and rare representatives of European freshwater gastrotrichs. A-F, Fam. Chaetonotidae. A, Chaetonotus (Captochaetus) robustus (insert shows the peculiar scales); B, Chaetonotus (Chaetonotus) polyspinosus; C, Chaetonotus (Lepidochaetus) zelinkai; D, Ichthydium skandicum (insert shows the scales of the furcal base); E, Lepidodermella squamata (insert shows the dorsal scales); F, Polymerurus nodicaudus; G, Fam. Neogosseidae, Neogossea antennigera; H, Fam. Dasydytidae, Stylochaeta fusiformis. Scale bars: A, F =100 µm; B-E, G, H = 50 µm.

Original images can be found here: Suppl. materials 2, 3, 4, 5, 6, 7, 8, 9.

Tenpin-like body, 84-770 μm in length.Two adhesive tubes forming the caudal ‘furca’. Four cephalic ciliary tufts. Cuticle generally provided with ornamentations of various shape and size, in some cases smooth. Two longitudinal ventral ciliary locomotory bands. Pharynx generally cylindrical, slightly widened posteriorly. Parthenogenic; rudimentary testes or aberrant spermatozoa recorded in several species.

This family is the largest one of the order, and includes most of the epibenthic and periphytic species colonizing standing waters as well as the few interstitial species known from psammic habitats.

At global scale the family Chaetonotidae includes 277 freshwater species (10 genera).

In European fresh waters the family is currently represented by 195 species in 8 genera: 5 genera include also marine species Aspidiophorus (15 species), Chaetonotus (125), Heterolepidoderma (18), Ichthydium (19), Lepidodermella (7), whereas 3 genera are exclusively freshwater: Fluxiderma (3), Polymerurus (7), Rhomballichthys (1).

These numbers include 8 new species and a single new record for Europe that have been reported after the release of the Fauna Europaea database in January 2011.

Newly described species

Chaetonotus (Chaetonotus) eximius Kolicka, Kisielewski, Nesteruk & Zawierucha, 2013

Chaetonotus (Chaetonotus) pravus Kolicka, Kisielewski, Nesteruk & Zawierucha, 2013

Chaetonotus (Primochaetus) veronicae Kånneby, 2013

Chaetonotus (Tristratachaetus) rhombosquamatus Kolicka, Kisielewski, Nesteruk & Zawierucha, 2013

Heterolepidoderma acidophilum Kånneby, Todaro & Jondelius, 2012

Heterolepidoderma joermungandri Kånneby, 2011

Heterolepidoderma trapezoidum Kånneby, 2011

Lepidodermella intermedia Kånneby, Todaro & Jondelius, 2012

New species record for Europe

Chaetonotus (Primochaetus) soberanus Grosso & Drahg, 1983

Family Dasydytidae (Fig. 2 H)

Tenpin-like or bottle-shaped body, 75-291 μm in length (caudal spines excluded). Caudal furca absent: rounded or truncated caudal end with 2 lobes bearing bristles or 2 to few long spines. Two cephalic ciliary tufts and a transversal band extending from ventral to dorsal side. Cuticle smooth, rarely with few faint scales. Ventrolateral spines short to very long, simple or more often barbed, isolated or in groups, inserted on the trunk region and sometimes movable. Few oblique ciliary locomotory series or tufts, rarely 2 bands (Haltidytes). Pharynx cylindrical with1-2 bulbs. Parthenogenic; rudimentary testes or aberrant spermatozoa recorded in several species.

All species of Dasydytidae are freshwater, semiplanktonic and planktonic in standing waters.

At global scale the family Dasydytidae includes 42 species in 7 genera.

In European fresh waters the family Dasydytidae is represented by 21 species (6 genera): Anacanthoderma (2), Chitonodytes (3), Dasydytes (3), Haltidytes (3), Setopus (6) and Stylochaeta (4).

Family Dichaeturidae

Cylindrical body, 98-150 μm in length. Two pairs of adhesive tubes forming the caudal furca. Cephalic ciliature uniform and continuous with 2 ventral ciliary locomotory bands. Cuticle smooth. A dorsal, transverse series of some thin, straight bristles or spines anterior to the furca. Pharynx cylindrical. Sexuality unknown.

All species of family Dichaeturidae are freshwater, very rare, semiplanktonic in standing waters.

At global scale the family Dichaeturidae includes 4 species of a single genus.

In European fresh waters the family Dichaeturidae is represented by 3 species in the single genus Dichaetura.

Family Neogosseidae (Fig. 2 G)

Tenpin-like body, 90-310 μm in length (caudal spines excluded). Caudal furca absent: trunk end rounded or truncated with 2 short, caudal protuberances and 2 pairs or an unpaired median group of long simple or barbed spines. One dorsal and 2 ventral cephalic interrupted transverse ciliary bands. Cuticle smooth or with numerous fine spined scales. Several pairs of ventral ciliary locomotory tufts or oblique bands. Thick pharynx with 1-4 bulbs. Parthenogenic.

All the species of family Neogosseidae are freshwater, semiplanktonic or planktonic in standing waters.

At global scale the family Neogosseidae is composed of 9 species in 2 genera.

In European fresh waters the family is represented by 4 species in 2 genera: Neogossea (3) and Kijanebalola (1 species, finding doubtful).

Family Proichthydiidae, the only other family of Chaetonotida exclusively freshwater, includes 2 species of 2 monotypic genera (Proichthydium, Proichthydioides): it is known from Brazil and Japan but has not yet been reported from Europe.

Order Macrodasyida

Family incertae sedis

Marinellina flagellata Ruttner-Kolisko, 1955

Cylindrical body, 220 μm in length. Two pairs of adhesive tubes forming the caudal furca. Two cephalic ventrolateral adhesive tubes. Numerous long sensory bristles on the head and along the body. Cuticle smooth. Ventral locomotory ciliature undescribed. Thick, cylindrical pharynx. Sexuality unknown.

This single species of the genus was found only once from Austrian interstitial fresh waters.

Currently living animals in stable populations, largely excluding (1) rare/irregular immigrants, intruder or invader species, (2) accidental or deliberate releases of exotic (pet) species, (3) domesticated animals, (4) foreign species imported and released for bio-control or (5) foreign species largely confined to hothouses.