Polycelis tenuis Ijima, 1884

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

Polycelis tenuis Ijima, 1884

Fig. 14

Known distribution.

Species recorded from The Netherlands ( Young 1972), Finland, United Kingdom and Ireland ( Luther 1961), Gernany ( Schwank 1981; Martin and Brunke 2012), Macedonia ( Kenk 1978), Romania ( Felicia 2018), and Russia ( Luther 1961).

Material.

Six specimens studied alive, preserved in absolute ethanol for future molecular analyses; collected in Kirchwerder-Fünfhausen, submerged vegetation and litter in an irrigation channel, 0.1–0.2 m deep.

Remarks.

Mature specimens measuring 0.5–1.2 mm, dark coloured (Fig. 14 A, B ). Marginal eyes (Fig. 14 C, D : e) distributed over the anterior third of the body. The pharynx (Fig. 14 B, C ) is located over the midbody and the mouth (Fig. 14 C, E : m) opens anterior to the male copulatory organ. The male copulatory organ (Fig. 14 B, E : mco) is 940–1100 µm long (n = 1; varying according to the relaxing stage) and 620 µm at widest. The male organ is spiny over its distal 300–480 µm, and forms a penial papilla (Fig. 14 E : pp). The spines (Fig. 14 F ) are 17–18 µm long (n = 10). Two adenodactyls (Fig. 14 E : ad) are located posterior to the male bulb and open into the common atrium, oriented forward, and exhibiting a glandular lumen.

Three species of Polycelis have been documented in Germany, and they are widespread across Europe: P. felina , P. nigra , and P. tenuis ( Volk 1903; Ronneberger 1975; Schwank 1981; Müller and Faubel 1993). Among these, only P. nigra has been reported in Hamburg ( Volk 1903). Species within the genus Polycelis are primarily distinguished by the structure of the male bulb and adenodactyls. Polycelis tenuis shares with P. felina the presence of two adenodactyls, structures absent in P. nigra . However, P. felina is easily identifiable by the presence of two tentacles in its anterior body region. The penial papilla of P. tenuis is armed with spines along its distal half, whereas P. nigra exhibits two to three spine rows distally, and P. felina lacks any spines in this region ( Hansen-Melander et al. 1954; Luther 1961; Harrath et al. 2012). Volk (1903) did not provide detailed morphological information about the specimens of P. nigra recorded in Hamburg. Given the necessity of studying the morphology of atrial organs for accurate identification of these triclads, this record requires confirmation.

Figures

Figure 14. Polycelis tenuis. A. Habitus of swimming adult specimen; B. Squeezed adult specimen; C, D. Squeezed juvenile; E. Atrial organs; F. Spines of the penial papilla. Scale bars: 200 μm (D); 600 μm (E); 100 μm (F).