Stenostomum handoelense, 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 : 6-7

Stenostomum handoelense n. sp.

( Fig. 3 C–E)

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

Localities. Loc. 23 (type locality) and 7.

Material. Several individuals studied. Photograph (fig. 3D) designated holotype ( SMNH no. 7369). There are DNA-sequences (accession numbers: FJ384823 , FJ384864 , FJ384951 , FJ384905 , FJ384824 , FJ384865 , FJ384952 , FJ384906 , FJ384815 , FJ384857 , FJ384945 , FJ384816 , FJ384858 , FJ384946 ), which will facilitate unambiguous identification (see Larsson et al. 2008).

Etymology. Species name refers to the type locality Handöl, Jämtland.

Remark. This species is clearly identifiable based on morphological characters, photos and drawings of live specimens.

Description. The animals are 0.4 mm long. Specimens with one or two zooids were found. The colour is white in reflected light and the gut is darker with excretophores evenly distributed along both sides of the gut. The excretophores are white in reflected light and black in transmitted light. The body is slender, evenly shaped and cylindrical with both ends tapering. The anterior part of the body is long and more mobile than the rest of the body creating a head-like appearance (see fig. 3 D, prostomium in Luther 1960).

The epidermis is completely covered with short cilia. Rhabdoids could not be observed. The deep ciliated pits are long, extending from the anterior tip to approximately 1/8 of the body length.

The brain consists of two pairs of brain lobes; the anterior lobes are more distinct and consist of 10–12 metamerically arranged ganglia, giving a striped appearance to the prostomium. Refractile organs are absent.

The mouth opening is more or less U-shaped and situated at approximately 1/6. The pharynx is muscular and slightly wrinkled and the distal rim is constricted around the small mouth opening. The length of the pharynx is approximately 1/5 of the total body length. A pair of glands is present along the pharynx. The gland bodies are situated close to the proximal end of the pharynx, but their long and slender necks open close to the mouth.

The protonephridium is slightly sinuous and visible in live specimens and ends in a nephridiopore at the posterior end.

Individuals with a developed genital system were not found.

Diagnosis. Stenostomum handoelense n. sp.: species of Stenostomum with one or two zooids, a prostomium, long ciliated pits ranging from the anterior tip to approximately 1/8, anterior brain lobes with 10–12 “metamerical” compartments, a barrel-like pharynx with a small mouth opening, two club-shaped pharynx glands and excretophores. Refractile organs absent.

Discussion. S. handoelense can easily be placed within the taxon Stenostomum . A remarkable feature of this species is the long and mobile anterior end. This so-called prostomium has a striped appearance due to the “metamerically” arranged anterior brain lobes. A similar feature is also present in Stenostomum anatirostrum Marcus, 1945 , Stenostomum glandulosum Kepner & Carter, 1931 and Stenostomum bryophilum Luther, 1960 . In S. anatirostrum the prostomium is spatulate or sharply arched, constricted proximally, separating it from the pharynx ( Noreña et al. 2005). In S. bryophilum a large number of pharynx glands are present, clustered around the pharynx ( Luther 1960). Furthermore the brain of this species has a lower number of “metamerical” brain compartments (4–6 in Luther 1960), although in one of Luther’s figures, 10 metamerical brain compartments are depicted ( Luther 1960: Fig. 9F). S. glandulosum , the third species showing a prostomium similar to that of S. handoelense , is clearly identifiable by the presence of three pairs of refractile organs and epidermal inclusions (= rosette-shaped glands in Kepner & Carter 1931)

In the molecular analyses of Swedish Catenulida (Larsson et al. 2008.) S. handoelense forms a clade with Stenostomum heebuktense n. sp. and Stenostomum steveoi n. sp. (see fig. 2). This clade is always strongly supported, although no morphological synapomorphy could be found.

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).