Stenostomum gotlandense, 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 : 4-6

Stenostomum gotlandense n. sp.

( Fig. 3 A–B )

Synonyms: Stenostomum island sp. 4 (Larsson et al. 2008.)

Localities. Loc. 20 (type locality)

Material. Several specimens studied alive. Photograph (fig. 3A) designated holotype ( SMNH no. 7366), two serially-sectioned specimens designated as paratypes ( SMNH no. 7367, SMNH no. 7368). There are DNA-sequences (accession numbers: FJ384944 , FJ384902 ), which will facilitate unambiguous identification (see Larsson et al. 2008).

Etymology. Species named after the Baltic island Gotland, where the type locality is situated.

Description. The animals are 0.5 mm long; only specimens with one zooid were found. The colour is white in reflected light and the gut is darker without any excretophores. The body is evenly shaped and cylindrical with both ends slightly tapering. The margin of the dark intestine is slightly dentate and paler close to the edges.

The epidermis contains dark secretory vesicles visible on sectioned material, rhabdoids, and is completely covered with short cilia. The body wall musculature consists of very weak inner circular muscles and stronger outer longitudinal muscles. The two small ciliated pits are deep and situated at approximately 1/7 of the total body length.

The anterior brain lobes are more conspicuously developed and consist of approximately ten metamerically arranged compartments, giving this part of the brain a dentate impression. Refractile organs are absent.

The proximal rim of the pharynx shows a number of folds and surrounds the very large mouth opening. The length of the pharynx is 1/6 of the total body length.

The protonephridium is visible in live specimens, not sinuous and ends in a nephridiopore at the posterior end of the body.

Individuals with a developed genital system were not found.

Diagnosis. Stenostomum gotlandense n. sp.: species of Stenostomum with small but deep ciliated pits at 1/ 7 of the body length, anterior brain lobes with a dentate appearance, a large mouth opening surrounded by a pharynx showing wrinkles proximally. Refractile organs and excretophores absent.

Discussion. Stenostomum gotlandense clearly is a member of the taxon Stenostomum as it shows the following diagnostic features ( Noreña et al. 2005): a large brain consisting of two pairs of brain lobes and paired ciliated pits, no statocyst.

The folded rim of the pharynx together with the large mouth opening and the deep and small ciliated pits are a unique combination of characters, not found in any other species of Stenostomum , making S. gotlandense easily recognisable. Small ciliated pits are present in numerous species: e.g. Stenostomum constrictum Luther, 1960 , Stenostomum heebuktense n.sp., Stenostomum sphagnetorum Luther, 1960 and Stenostomum steveoi n. sp. (see Luther 1960 and present contribution). However, in all these cases the ciliated pits are situated much closer to the anterior tip than in S. gotlandense .

In the molecular analyses of Swedish Catenulida (Larsson et al. 2008.) S. gotlandense forms a separate clade with three undescribed species of Stenostomum (see fig. 2 and separate section further on). Apart from the fact that all these species are so far only found on Gotland ( S. gotlandense , sp. 1 and sp. 2) or Öland ( Stenostomum sp. 3), they have no features in common with S. gotlandense .

Figures

FIGURE 3. A – B: Stenostomum gotlandense n. sp., C – E: Stenostomum handoelense n. sp., F – H: Stenostomum steveoi n. sp. (A, C, D, F, G: photographs of live specimens; B, E, H: freehand drawings of habitus of live specimen).