Stenostomum heebuktense, Larsson, Karolina & Willems, Wim, 2010

Larsson, Karolina & Willems, Wim, 2010, Report on freshwater Catenulida (Platyhelminthes) from Sweden with the description of four new species, Zootaxa 2396, pp. 1-18 : 7-9

Stenostomum heebuktense n. sp.

( Fig. 4 )

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

Localities. Loc. 9 (type locality) and 33.

Material. Several individuals studied alive. Photograph (fig. 4A) designated holotype ( SMNH no. 7370), one serially-sectioned specimen designated as paratype ( SMNH no. 7371). There are DNA-sequences (accession numbers: FJ196330 , FJ196341 , FJ384929 , FJ384887 , FJ384831 , FJ384959 ), which will facilitate unambiguous identification (see Larsson et al. 2008).

Etymology. Species name refers to the type locality Heebukten, Bohuslän.

Description. The animals are 0.5 mm; animals with one or two zooids are found. They are white in incident light with a dark and spotted gut caused by ingested material. Excretophores are absent. The body is evenly shaped and cylindrical with the posterior end slightly tapering.

The epithelium is covered with short cilia and basally comprises secretory vesicles, possibly rhabdoids. The body wall musculature consists of very weak, hardly visible inner circular and very strong outer longitudinal muscles. The small ciliated pits are lateral and situated a bit close to the anterior tip. The anterior brain lobes are clearly visible and dentate but not divided into separate ganglia. Refractile organs are absent.

The large mouth opening is located at 1/4 and is surrounded by small eosinophilic glands (fig. 4 C, E: phg), creating a lip-like structure in live animals. The pharynx is very long and extremely conspicuous in live animals, because of its bluish colour (under incident light) and continuous contractions whereby its width is reduced by 2/3 (fig. 4 D). Its total length is slightly variable, ranging between 2/3 and 1/2 of the total body length. Both the pharynx’ distal (fig. 4 E: sph1) and proximal (fig. 4 E: sph2) ends have a sphincter. The distal one is extremely thick and situated very close to the mouth. In between both sphincters the pharynx is surrounded by inner longitudinal and outer circular muscles. It is connected to the body wall by very strong radial muscles (fig. 4 E rm). The pharynx lumen is lined with a high, nucleated epithelium, which is slightly degenerated forming a so-called pseudociliation.

The protonephridium is visible in live specimens, and the nephridiopore is situated terminally at the posterior end.

Individuals with a developed genital system were not found.

Diagnosis. Stenostomum heebuktense n. sp.: species of Stenostomum with one or two zooids, small ciliated pits, anterior brain lobes with a dentate appearance, an extremely long and muscular pharynx. Refractile organs and excretophores absent.

Discussion. Based on the previously mentioned diagnostic characters this species can be assigned to the taxon Stenostomum . However, this species is characterized by the presence of a remarkable and very conspicuous pharynx, absent in all other species of Stenostomum .

This very long and barrel-shaped, muscular pharynx is highly characteristic for this species and, at first sight, in live animals, looks similar to the muscular gut (“Muskeldarm” in Luther 1960), present in species of Myostenostomum Luther, 1960 . However, this muscular gut has a completely different anatomy, as can be seen on sectioned individuals. It consists of two half-circular large muscle bundles on each side of the distal part of the pharynx ( Luther 1960; Rogozin 1992).

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

Figures

FIGURE 4. Stenostomum heebuktense n. sp. (A – B: photographs of live specimens; C: habitus of live specimen; D: live individual illustrating pharynx constriction; E: sagittal reconstruction of the pharynx from the left hand side (pharynx appears shorter in comparison with live animals due to constriction during fixation); F: anterior end of sagittal section).