Stenostomum leucops (Duges, 1828) Schmidt, 1848

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 leucops (Duges, 1828) Schmidt, 1848

Fig. 2

Known distribution.

Species with a broad distribution through North America ( United States and Mexico) ( Higley 1918; Nuttycombe and Waters 1938; Kolasa et al. 1987; Núñez-Ortiz et al. 2016; Glasgow 2021), South America ( Argentina, Brazil, Peru, and Suriname) ( Marcus 1945; van der Land 1970; Noreña-Janssen 1995; Gamo and Leal-Zanchet 2004; Noreña et al. 2005; Damborenea et al. 2011; Reyes et al. 2021), Iceland and Faroe Islands ( Steinböck 1948), West Europe ( United Kingdom, Ireland, The Netherlands, Germany, Switzerland, and Spain) ( Graff 1882; Hofsten 1911; Gieysztor 1931; Young 1970, 1972, 1973; Noreña et al. 2007), East Europe ( Poland, Romania, Serbia, and Croatia) ( Graff 1882; Steinböck 1933; Kolasa 1971; Mack-Fira 1974), Russia ( Nasonov 1926; Steinböck 1932; Timoshkin et al. 2010), Asia ( Thailand and Japan) ( Yamazaki et al. 2012; Ngamniyom and Panyarachun 2016), and Africa ( Tanzania and Kenya) ( Young and Kolasa 1974; Young 1976).

Material.

Three specimens studied alive and stored in absolute ethanol; collected in Kirchwerder-Fünfhausen, submerged vegetation and litter in an irrigation channel, 0.1–0.2 m deep. Three specimens studied alive and stored in absolute ethanol; collected in Groß Glienicker lake, littoral, floating vegetation. Three specimens studied alive and stored in absolute ethanol; collected in Sylt, floating vegetation in a small pond. Two specimens from Kirchwerder-Fünfhausen and one from Sylt were sequenced for the molecular analyses.

Remarks.

Specimens measuring 1120–1490 µm long (x ̄ = 1305 µm; n = 2) and 175–220 µm at widest point (x ̄ = 198 µm; n = 2), with two zooids, anterior end rounded and posterior tapering (Fig. 2 A – C ). The ciliated pits (Fig. 2 D : 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. 2 A, B, D : br) consists of two pairs of lobes, the anterior brain (Fig. 2 F, G : ab) and the posterior brain (Fig. 2 F, G : pb). The anterior brain is more distinct but does not show a clearly compartments. A pair of refractile bodies (Fig. 2 F, G : rb) are connected to the posterior brain via stalked structures. The refractile bodies are 8–9 µm in diameter (n = 3). The number of spherules contained in the refractile bodies is difficult to observe but they are more than 20 in all studied specimens. The refractile bodies can appear either rounded or crescent-shaped, depending on the orientation of these structures.

Our identification of specimens was primarily based on Luther’s (1960) description. However, recent studies have suggested that S. leucops may actually represent a complex of cryptic species, possibly related to S. grande Child, 1902 (see Yamazaki et al. 2012; Rosa et al. 2015). Molecular evidence supports this notion and echoes earlier discussions by Nuttycombe and Waters (1938) and Marcus (1945), who both deemed the available description insufficient for clear recognition of the species. Rosa et al. (2015) identified three distinct clades within S. leucops , each distributed in Brazil, the United Kingdom, and Sweden, respectively, but were unable to find morphological diagnostic traits to differentiate them. The phylogenetic analysis here developed (see section Molecular phylogenetic analyses) shows a close relationship between S. leucops from Germany and Sweden.

Given that the type locality of S. leucops is in the vicinity of New York, United States, a comprehensive morphological and molecular phylogenetic analysis of specimens from that area is essential to stabilize the classification of S. leucops (see Discussion).

Figures

Figure 2. Stenostomum leucops. A – C Habitus of swimming specimens; D. Anterior part of the body; E. Posterior part of the body F, G. Anterior part of the body. Abbreviations: ab anterior brain; br brain; cp ciliated pits; i intestine; m mouth; np nephridiopore; pb posterior brain; ph pharynx; phg pharyngeal glands. Scale bars: 200 μm (A – C); 100 μm (D, E); 50 μm (F, G).