Two dimensions of biodiversity research exemplified by nematomorpha and gastrotricha

Integrative and Comparative Biology, Jun 2002 by Schmidt-Rhaesa, Andreas

Apart from these new approaches to phylogenetic relationships among gastrotrichs, the whole taxon seems to be monophyletic. Possible autapomorphies are the multilayered epicuticle (Nielsen, 2001) and the presence of helicoidal muscles along the digestive tract (Hochberg and Litvaitis, 2001). The position of Gastrotricha within the Bilateria, however, varies. Gastrotrichs have long been associated with "Aschelminthes" or pseudocoelomates. These animals were loosely grouped and could not be assigned to other taxa. In the mid-1990s, several workers and working groups reexamined phylogenetic relationships within the "Aschelminthes" (Nielsen, 1995; Ehlers et al., 1996; Wallace et al., 1996). They concluded that Gastrotricha is the sister-taxon of a monophyletic clade comprising Nematoda, Nematomorpha, Priapulida, Kinorhyncha and Loricifera (Fig. 5). Ahlrichs (1995) named this clade Cycloneuralia, but Nielsen (1995, 2001) applied the term Introverta. For all six taxa (including gastrotrichs), Ahlrichs (1995) applied the name Nemathelminthes, while Nielsen (1995) named them Cycloneuralia. Here I use the names introduced by Ahlrichs (1995) (see Fig. 5). Possible autapomorphies of Nemathelminthes (and therefore synapomorphies of Gastrotricha and Cycloneuralia) are the possession of a cuticle composed of a fine fibrillar proteinaceous layer and an outer trilaminate epicuticle (Fig. 6) and probably also the muscular sucking pharynx. A "peripharyngeal brain with three regions" (Nielsen, 2001) is difficult to evaluate, because it is not clear whether this character is realized in Gastrotricha. The three regions appear to be present in Turbanella cornuta (Teuchert, 1977), but are absent in Cephalodasys maximus (Wiedermann, 1995). A muscular sucking pharynx is widely distributed among Nemathelminthes, but different orientations of the lumen occur which led Neuhaus (1994) to assume a non-triradiate lumen as the plesiomorphic condition. However, if one assumes that the shifting of the pharyngeal lumen of 180 is not a complicated process during development, the assumption of a triradiate lumen as an autapomorphy of the Nemathelminthes appears to be the parsimonious solution.

In the recently developed hypothesis that arthropods are the sister-group of Cycloneuralia (within a taxon named Ecdysozoa) (Aguinaldo et al., 1997; SchmidtRhaesa et al., 1998; Garey, 2001), Gastrotricha would be the sister taxon to Ecdysozoa. The interpretation of characters as discussed in the following is not influenced by this hypothesis and Ecdysozoa will therefore not be discussed or mentioned further.

The Nemathelminthes appear as the sister group of Spiralia in a taxon Protostomia (synonymous with Gastroneuralia) (Ehlers et al., 1996) (Fig. 7). Nielsen (1995, 2001) and Nielsen et al. (1996) come to generally comparable results, although sometimes the additional taxa Rotifera, Acanthocephala and Chaetognatha were included in Aschelminthes. With the exception of Zrzavy et al. (1998), the published analsyses of 18S rDNA data and combined molecular/ morphological analyses that include sequences from gastrotrichs (only chaetonotoids) do not support the monophyly of Nemathelminthes. In the analyses of Winnepenninckx et al. (1995) and Giribet et al. (2000) gastrotrichs appear closely associated with Platyhelminthes (Fig. 7). The implications of these conflicting positions are discussed for the evaluation of the characters cuticle, sucking pharynx and hermaphroditism for the Protostomia (Gastroneuralia).