Stenostomum gotlandense Larsson & Willems, 2010

Diez, Yander L. & Schmidt-Rhaesa, Andreas, 2024, Little neighbours in Hamburg: free-living aquatic flatworms (Platyhelminthes), Evolutionary Systematics 8 (2), pp. 279-310 : 279-310

Stenostomum gotlandense Larsson & Willems, 2010

Fig. 1

Known distribution.

Species only known, until now, from Gotland, Sweden ( Larsson et al. 2008; Larsson and Willems 2010).

Material.

Two specimens studied alive and stored in absolute ethanol for molecular analyses, one of them sequenced; collected in Kirchwerder-Fünfhausen, submerged vegetation and litter in an irrigation channel, 0.1–0.2 m deep.

Remarks.

Specimens measuring 768–957 µm long (x ̄ = 863 µm; n = 2) and 90–110 µm at widest point (x ̄ = 100 µm; n = 2), with two zooids, slender, tapering to both rounded extremes (Fig. 1 A ). The ciliated pits (Fig. 1 B : cp) are relative short and open close to the most anterior part of the body. The epidermis is fully ciliated. Larger cilia are distributed along the body, particularly in the anterior and posterior ends. The brain (Fig. 1 B : br) consists of two pairs of lobes, the anterior brain (Fig. 1 C : ab) and the posterior brain (Fig. 1 C : pb). We were not able to determine the exact number of compartments of the anterior brain, but in one specimen there appear to exist seven. Refractile bodies not present. Proximal rim of the pharynx (Fig. 1 A, B : ph) with a number of folds and surrounds the large mouth opening (Fig. 1 A, B : m). The protonephridium (Fig. 1 D : pn) ends in a nephridiopore at the posterior end of the body.

As noted by Larsson and Willems (2010), the folded rim of the pharynx, the large mouth opening, and the small ciliated pits represent a unique combination of morphological traits within Stenostomum . Our morphological identification is further corroborated by the phylogenetic analysis, leading us to confidently report this species for the first time in Germany, specifically in Hamburg. The German specimens are similar in size, considering the length of the first zooid (495–588 µm), to those from Sweden (500 µm). However, Larsson and Willems (2010) observed that their specimens exhibited approximately 10 compartments in the anterior brain, whereas the specimens from Hamburg present about seven. Nonetheless, one specimen illustrated by Larsson and Willems (2010: fig. 3 B) shows only six segments in the anterior brain.

Figures

Figure 1. Stenostomum gotlandense. A. Habitus of a swimming specimen; B. Anterior part of the body; C. Anterior part of the body showing the brain; D. Posterior part of the body. Abbreviations: ab anterior brain; br brain; cp ciliated pits; ep excretophore; i intestine; m mouth; pb posterior brain; ph pharynx; pn protonephridia. Scale bars: 50 μm (A, C); 100 μm (B, D).