Worldwide, the situation for known diversity of turbellarian platyhelminths parallels that for several other meiofauna groups. As with nematodes, for instance (Lambshead, 2003), and harpacticoids (Baguley & Montagna in Baguley et al., in press), most species are described from northern Europe, and relatively small numbers are known from the rest of Europe and North America or the rest of the world. These numbers, in other words, are a reflection of taxonomic effort rather than actual diversity. Effort in turbellarian taxonomy has definitely centered on northern Europe where prominent contributors to turbellarian taxonomy, such as Graff (nearly 500 species described), Luther, Westblad, Karling, and Ax based their work; special circumstances apply for South America where the prolific taxonomists Ernst and Eveline du Bois-Reymond Marcus described quite a few species (nearly 400).
The number of extant species of meiofaunal taxa is estimated to be at least an order of magnitude greater---even four or five orders greater by some metrics---than the number of species described. Baguley & Montagna (in Baguley et al., in press), for instance, estimate the actual number of harpacticoid species to be between 105 and 106 compared to the 4 500 species described so far; and Lambshead & Boucher (2003), extrapolate from species counts in the equatorial Pacific to arrive at a global species richness of 105--107, which is two to four orders of magnitude greater than the currently recognized approximately 12 000 species.
No such estimates have been made for interstitial turbellarians, but the patterns identified for nematodes and harpacticoids may hold. In a database we have assembled of described species of turbellarian platyhelminths (Turbellarian Taxonomic Database), we find records of 6 210 species. These numbers include non-meiofaunal species, such as large-bodied polyclads, freshwater and terrestrial triclads (though there are a few interstitial species among triclad and polyclad turbellarians), and such a strictly freshwater group as the Temnocephalida. Discounting, then, approximately 3 600 species (about 1 930 triclads, 1 340 polyclads, 90 temnocephalans, and roughly 160 other freshwater and parasitic species), the number of described marine meiofaunal turbellarians would be about 2 600.
Purely as an educated guess, we expect that the actual number of turbellarian species is at least an order of magnitude larger than the number of known turbellarian species---that is, that there are probably more than 26 000 species of meiofaunal turbellarian species. We base this judgement on our sampling outside of Europe for kalyptorhynch and acoel turbellarians, representatives found predominantly in sandy and muddy sediments, respectively. For example, the kalyptorhynch family Polycystididae currently comprises 191 known species---154 described species and a further 37 species known but not formally described in the literature (Artois 2001)---but we (Artois & Schockaert, unpublished observations) estimate that it consists of about 2 000 species in actuality. This is a world-wide estimate, taking into account the number of places where collecting has been done, over what time period, and the abundance of completely unsampled locations likely to harbor species in this family.
As another indicator, the diversity of species of Acoela we (Hooge & Tyler 2001, Hooge & Tyler, unpublished observations) have found along the coast of Maine is at least three times the number recorded in the literature for this region. Of 13 species described before this year, 10 appear to be the same as European species and 2 were described from more southern locales along the North American Atlantic coast. We (Hooge & Tyler) have begun describing new species we encounter in just the intertidal and shallow subtidal zone---so far limiting our descriptions to those new species that we have used in various comparative morphological studies---and with only modest sampling effort have more than doubled the number of known species for the Maine coast (Hooge & Tyler 2001, Tyler in prep, describing 17 new species). The actual number of species we have encountered is likely to be as many as 100 in this kind of habitat just in Maine. Further south, in sediments of subtropical and tropical regions known to have greater diversity in turbellarians, the situation is likely to generate far higher proportion of undescribed-to-described species, especially since overlap with northern European fauna would not apply to any extent. In our opinion, then, a reasonable estimate of the actual number of acoel species is 10 times the number described so far. Other groups of meiofaunal turbellarians are also likely to be at least an order of magnitude larger than are currently known.
We refrain from assuming numbers higher than one order of magnitude, as may apply for nematodes, because we do not know what diversity may exist in the deep sea. In most deep-sea sampling for meiofauna, animals are extracted after bulk fixation of the sediment, and in such situations turbellarians are likely to be overlooked: being softbodied and typically reacting poorly to conventional fixatives, they become unrecognizable lumps in bulk fixations of sediments. Rieger and Sterrer (in Coull et al. 1977), with their pioneering sampling of living samples from the deep sea (400, 800, and 4 000 m) off North Carolina, found relatively few turbellarians in samples that provided high numbers of nematodes, copepods, and foraminiferans, but these numbers may be specific to the types of sediment there, namely silt and fine sand. We (Artois et al. 2000; Schockaert & Artois, unpublished observations) have recovered live turbellarians from certain sediments (with bryozoan colonies) off Antarctica at depths as great as 2 000 m (as deep as we were able to sample), and our experience has been that they are quite fragile, readily disintegrating under conditions of sampling and observation.
Besides the problem of the paucity of taxonomists working on turbellarian platyhelminths, the largest obstacle to counting species living in the meiofauna, then, may be the difficulty in obtaining reliable samples from the deep sea. Our estimate of the total number of turbellarian species can only be a wild guess, therefore, but this guess---26 000 species in the meiofauna---will have to suffice for now.