Gyratrix hermaphroditus Ehrenberg, 1831

Fig. 5

Known distribution.

This is the microturbellarian species with the widest distribution worldwide (see Artois and Tessens 2008; Diez et al. 2018). However, this distribution corresponds to a puzzle of dozens of cryptic species (Tessens et al. 2021).

Material.

Specimens were found in two locations. One specimen was studied alive and preserved in ethanol for future molecular analyses. It was collected in Wandse river, submerged vegetation with organic matter, 0.1 m deep. Fifteen specimens were studied alive, eleven of them whole mounted and four preserved for future molecular analyses collected in Kirchwerder-Fünfhausen, among submerged vegetation and litter in an irrigation channel, 0.1–0.2 m deep.

Remarks.

The specimen from Wandse river is 1354 µm long and those from Kirchwerder-Fünfhausen are 870–926 µm long (x ̄ = 904 µm; n = 6). They are unpigmented, with a pair of eyes (Fig. 5 A: e) posterior to the proboscis (Fig. 5 A: pr).

The testis (Fig. 5 A, B: t) is located at the left body side, extending from the region of the eyes to about three quarters of body length. Ovary and atrial organs located posteriorly in the body. The prostate stylet type II (Fig. 5 B – F: ps 2) is 189 µm long in the specimen from Wandse river and 155–169 µm long (x ̄ = 164 µm; n = 9) in those from Kirchwerder-Fünfhausen. The prostate stylet type III (Fig. 5 B – F: ps 3) is 147 µm long in the specimen from Wandse river and 150 µm long (x ̄ = 131–167 µm; n = 9) in those from Kirchwerder-Fünfhausen. In the specimens from both localities, the stylet type III distally bifurcates and therefore ends in two opposed tips.

Vitellarium not observed. The ovary (Fig. 5 B, C: ov) is kidney shaped in the specimen from Wandse river, with the oocytes organised in a row, increasing in diameter from proximal to distal. In contrary, in the specimens from Kirchwerder-Fünfhausen, the ovary is globular, with oocytes of dissimilar size. The bursa (Fig. 5 B – D: b) is located completely posterior and overlaps with the stylets; several masses of sperm in digestion were observed.

Based on our findings, it appears that the specimens collected from the two sampled localities correspond to two distinct species within the G. hermaphroditus complex. These species are distinguishable by differences in the size of the sclerotised male structures and the morphology of the ovary. However, differentiation within this extensive species complex poses significant challenges and warrants molecular investigations. Phylogenetic analyses conducted by Tessens et al. (2021) revealed a remarkable diversity of 62–78 species concealed under the name G. hermaphroditus, with many of them confined to single localities while others are more widely distributed.

The morphology of the hard structures in our specimens appears to align with group H as defined by Tessens et al. (2021), characterised by the bifurcate ending of prostate stylet type III. Notably, this group comprises exclusively freshwater species from Australia. However, molecular studies are essential to validate this morphological resemblance. Specimens from Germany included in the aforementioned phylogenetic study fall into groups A and B (comprising freshwater and brackish species) and group L (marine species). Nonetheless, representatives from the latter three groups exhibit a prostate stylet type III ending in a simple, hook-shaped tip. Future integrative studies are imperative to unravel the intricate taxonomy of the G. hermaphroditus species complex, which should encompass the analysis of specimens from its type locality in Berlin, Germany.