Stenostomum steveoi Larsson & Willems, 2010, n. sp.

Stenostomum steveoi n. sp.

(Fig. 3 F–H)

Synonyms: Stenostomum smallpit (Larsson et al. 2008.)

Localities. Loc. 11 (type locality), 16, 33.

Material. Several individuals studied alive. Photograph (fig. 3 G) designated holotype (SMNH no. 7372), one serially-sectioned animal designated paratype (SMNH nos.7373). There are DNA-sequences (accession numbers: FJ196331, FJ384890, FJ384851, FJ384939, FJ384897, FJ384872), which will facilitate unambiguous identification (see Larsson et al. 2008).

Etymology. Species named after the first author’s dog Steve-O, a devoted fieldwork companion.

Description. The length of the animal is 0.3 mm and animals with one or two zooids were observed.

Specimens are white in incident light with a dark gut containing excretophores, which are more numerous close to the pharynx. The excretophores are white in incident light and appears black in reflected light. The body is slender and evenly-shaped with the posterior end slightly tapering. The anterior end is short, mobile and prostomium-like.

The epidermis is entirely covered with short cilia. Epidermal rhabdoids absent. The ciliated pits are small and situated close to the anterior end.

The anterior brain lobes are distinct with “metamerical” arranged ganglia consisting of six compartments, giving a striped appearance. The posterior brain lobes do not have this striped appearance and are situated posterior to the ciliated pits. Refractile organs are absent.

The large ventral mouth opening is situated at approximately ¼ of the body length, and can be closed by a strong sphincter. When open, the mouth often is oval or rectangular in shape. The pharynx is simple and muscular and clearly visible in the living animal. The length of the pharynx is 1/5 of the total length.

The sinuous protonephridium starts at the anterior brain lobes and ends in a nephridiopore in the posterior end of the animal.

Individuals with a developed genital system were not found.

Diagnosis. Stenostomum steveoi n. sp.: species of Stenostomum with one or two zooids, a short prostomium-like anterior end, small ciliated pits close to the anterior end, anterior brain lobes with six metamerically arranged compartments, large oval to rectangular mouth opening, which can be closed by a sphincter, pharynx conspicuous in live animals. Refractile organs absent.

Discussion. Stenostomum steveoi is without doubt a member of the taxon Stenostomum.

The most striking character of this species is the shape of the large mouth opening with a conspicuous sphincter, as seen in live individuals. This type of elongated mouth opening is also seen in Stenostomum membranosum Kepner & Carter, 1931 and Stenostomum saliens Kepner & Carter, 1931 (see Kepner & Carter 1931). S. membranosum lacks excretophores and the cilated pits are longer than in S. steveoi (Kepner & Carter 1931). In S. saliens excreteophores are absent, the cilated pits are small and close to the anterior end and the brain lobes are larger and more conspicuous than in S. steveoi (Kepner & Carter 1931).

The short prostomium-like anterior end with its six metamerical compartments of the anterior brain lobes gives it a striped appearance. This is somewhat similar to the longer prostomium of S. anatirostrum (Noreña et al. 2005), S. bryophilum (Luther 1960), S. glandulosum (Noreña et al. 2005) and S. handoelense (see discussion of S. handoelense) but S. steveoi has much smaller cilated pits than those species.

According to the molecular analyses of Swedish Catenulida in Larsson et al. 2008 (see figure 2), the closest relatives of S. steveoi are two other newly described species, S. handoelense and S. heebuktense.