Pseudexogone Augener 1922

Genus Pseudexogone Augener, 1922

Pseudexogone Augener, 1922: 191. — Fauchald 1977: 84.

TYPE SPECIES. — Pseudexogone backstromi Augener, 1922, by original designation.

POTENTIAL APOMORPHY BASED DEFINITION. — Bidentate curved notospines.

SPECIES INCLUDED. — Pseudexogone backstromi Augener, 1922, P.dineti (Katzmann, Laubier & Ramos, 1974) n. comb., P. helmuti n. sp., P. imajimai n. sp. and P. williamsae n. sp.

DISTRIBUTION. — Cosmopolitan.

EMENDED DIAGNOSIS. — Synelminae with palps simple, each with a ventrolateral papilla. Three antennae. Eyes present or absent. Two pairs of tentacular cirri. Parapodia with bidentate curved notospines, neurochaetae include pectinate and denticulate capillaries and furcate chaetae.

REMARKS

Although the anterior end of P. backstromi resembles species of Exogone Ørsted, 1845, its general appearance is very similar to some pilargids that have been included in Synelmis Chamberlin, 1919 (Salazar-Vallejo 2003); besides, the differences in the structure of the alimentary canal, especially the presence of a proventricle in syllids, are so important that Augener had some doubts about its placement among syllids. Synelmis is usually defined by having biarticulate palps and straight notospines, but its perceived similarity to syllids is depicted by the generic placement that some species received in the past, e.g., Ancistrosyllis albini Langerhans, 1881 and A. rigida Fauvel, 1919. Synelmis is the type genus for Synelminae, which has been defined by having a smooth integument, body cylindrical, and straight or curved notospines (Salazar-Vallejo 2003). Pseudexogone belongs to this subfamily and differs from Synelmis especially by having simple palps, and bidentate sigmoid notospines. These two pilargid synelmin genera are apparently closely allied. Further, Pseudexogone is closely allied to Litocorsa Pearson, 1970; they share a thin transparent body and simple palps. However, they differ because Litocorsa has straight emergent notospines with tapering tips, instead of having notospines with bidentate tips. Further, Litocorsa has the potential autopomorphy of having neurospines (Darbyshire & Mackie 2003).

In the original diagnosis, Augener (1922: 191) indicated that Pseudexogone was syllid-like, had a nematoid body with simple palps, simple hooks in median and posterior chaetigers, and an unarmed pharynx. After the description of P. backstromi, he tentatively placed the animal close to syllids (Augener 1922: 194). Augener (1922: 193) confused the position of the bidentate notospines, since he regarded them as ventral (neuropodial). This could be explained because of either the polychaete body was too transparent, or because he confused the typical morphology of the pygidium; i.e. anal cirri are generally ventral or ventrolateral, which is shown in his original drawings, but if the specimen was twisted, then there would be some confusion about which surface was dorsal or ventral in the posterior end. An important missing detail in the original description is the start of notospines; since this is a rather conservative useful feature (see below), this species could be separated by using additional materials or by employing other morphological features.

Further delay in recognizing this genus as a member of the Pilargidae resulted because Pseudexogone was regarded as a syllid in two major reviews (Hartman 1959: 220; Fauchald 1977: 84). However, the same morphological features such as body pattern and structures (nematoid body, simple palps, bidentate emergent spines) were recognized in a pilargid by Katzmann et al. (1974), when they described Synelmis dineti (see below). As herein defined, Pseudexogone includes species living in subtidal depths and rarely from deeper waters (c. 1000 m depth). Besides the type species, it includes P. dineti n. comb., P. helmuti n. sp., P. imajimai n. sp., and P. williamsae n. sp.