Prorhynchus stagnalis Schultze, 1851

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

Prorhynchus stagnalis Schultze, 1851

Fig. 4

Known distribution.

Species with a worldwide distribution, recorded from South America ( Brazil) ( Marcus 1944; Reyes et al. 2021), North America (Unite States) ( Higley 1918; Kenk 1949; Kolasa et al. 1987; Smith 1991; Glasgow 2021), Faroes Islands ( Steinböck 1931), West Europe ( United Kingdom, Ireland, Sweden, Belgium, Netherlands; Germany, Switzerland, Austria, Poland, Czech Republic, France, Spain) ( Schultze 1851; Graff 1882, 1913; Fuhrmann 1894, 1900; Steinböck 1923; Southern 1936; Rixen 1961; Young 1970, 1972, 1973; Kolasa 1971; Farias et al. 1996; Noreña et al. 2007), East Europe ( Bulgaria, Hungary, Hungary, North Macedonia-Alvania, Russia) ( Graff 1913; Steinböck 1932; An der Lan 1939), Asia ( China and Japan) ( Okugawa 1953; Peng et al. 2007), and Africa ( Kenya) ( Young and Young 1976).

Material.

Six specimens studied alive, two preserved in ethanol for future molecular analyses, four cut in two pieces, the anterior part containing the stylet for whole mounting ( ZMH V 13839 –13842) and the posterior part preserved for molecular analyses; collected in Kirchwerder-Fünfhausen, submerged vegetation and litter in an irrigation channel, 0.1–0.2 m deep.

Remarks.

Animals 2.5–3.5 mm long depending on the contraction stage, opaque, without eyes (Fig. 4 A ). The oral pore opens at the anterior tip; the pharynx (Fig. 4 B, C : ph) is located behind the brain (Fig. 4 B : br) and beside the prostate vesicle (Fig. 4 B, C : pv), and proximally opens into the intestine (Fig. 4 A : i). The intestine of some specimens contained numerous chaetae of oligochaetes; a feeding behaviour previously recorded to the species (Tylet et al. 2018).

The ovary (Fig. 4 A : ov) is very long and runs medially, extending over the posterior two thirds of the body; eggs were observed. The male reproductive system includes the large unpired testis, located at mid body; the seminal vesicle, located posterior to the pharynx; the prostate vesicle running beside the pharynx; and the armed copulatory organ. The prostate vesicle shows thick muscle walls, perforated by the necks of the extracapsular prostatic glands, containing a coarse-granular secretion. Proximally, the prostate vesicle looks empty, maybe because it contains a very fine secretion. A very large duct connects the prostate vesicle with the armed bulb.

The armed male copulatory organ (Fig. 4 D – F ) is located in the anterior part of the body, anterior to the brain. It consists of a central stylet (Fig. 4 B, D – F : st) surrounded by two concentrical rings of spines. The stylet is 64–68 µm long (x ̄ = 66 µm; n = 4), proximally funnel shaped and posteriorly tapering to the distal sharp tip. As described by Tyler et al. (2018), the stylet consist of two pieces, clearly distinguishable in the posterior third of the stylet. In the distal area, only the sharp tip of the stylet (part of the inner tube) could be observed. The number of bars on each ring surrounding the stylet is difficult to observe; however, in two specimens it seems that there are 10 bars in the external and six in the internal ring. This pattern has also been identified in populations from the Unite States and Brazil ( Tyler et al. 2018; Reyes et al. 2021). The external bars (Fig. 4 D – F : eb) are arched, 53–57 µm long (x ̄ = 55 µm; n = 4), proximally broad and posteriorly taper to rounded tips. The internal bars (Fig. 4 D, F : ib) appear straight to slightly arched but the rest of their morphology is hardly distinguishable; they are 47–53 µm long (x ̄ = 50 µm; n = 4).

It has been proposed that P. stagnalis comprises a complex of species primarily distinguished by variations in the morphology of male sclerotised structures. However, comprehensive morphological and molecular investigations are imperative to elucidate the taxonomy of this species group fully. Furthermore, extensive sampling is required, including the type locality of the species (Greifswald, Germany). The specimens we have examined exhibit the smallest stylet (~ 66 µm) compared to those documented in the literature from The Urals (100–124 µm; Rogozin 2015), the United States (~ 90 µm; Tyler et al. 2018), and Brazil (~ 123 µm; Reyes et al. 2021).

Figures

Figure 4. Prorhynchus stagnalis. A. Habitus of a swimming specimen; B – D. Anterior part of the body; E, F. Sclerotised structures. Abbreviations: br brain; eb external bars; i intestine; ib internal bars; ov ovary; ph pharynx; phg pharyngeal glands; pv prostate vesicle; st stylet. Scale bars: 200 μm (A); 100 μm (B – D); 50 μm (E, F).