Castrella alba Luther, 1955

Figs 6, 7

Known distribution.

Species recorded from Finland and Sweden (Luther 1955).

Material.

Seven specimens studied alive, two of which were whole mounted afterwards (ZMH V 13831 –13832); collected in Kirchwerder-Fünfhausen, submerged vegetation and litter in an irrigation channel, 0.1–0.2 m deep. Four specimens studied alive and preserved in absolute ethanol, collected in Groß Glienicker lake; littoral, floating vegetation. Two specimens from Kirchwerder-Fünfhausen and one from Groß Glienicker lake used for molecular phylogenetic analyses.

Remarks.

Live specimens about 1 mm long, anterior margin rounded and posterior pointing, translucent, pinkish-brownish colouration due to parenchymal glands (Figs 6 – 7). A pair of eyes (Figs 6 A – C, 7 A: e) anterior to the pharynx; the eyes are formed by two pigmented spots linked by a bridge, and sometimes look like two independent structures. Barrel-shaped pharynx (Figs 6 A – C, 7 A: ph) 122–183 µm long (x ̄ = 147 µm; n = 5). Testes located postero-lateral to the pharynx. The male copulatory organ includes a seminal vesicle (Fig. 7 E, F: sv), a prostate vesicle (Fig. 7 E, F: sv), and the stylet. The stylet (Figs 6 D – F, 7 C – F: st) is 43–74 µm long (x ̄ = 60 µm; n = 3) and displays a proximal handle and a distal spiny part; a hook-shaped, well-differentiated, largest spine is 14–30 µm long (x ̄ = 22 µm; n = 3). The vitellaria (Fig. 6 B: vi) run beside the pharynx until the posterior third of the body, fusing before opening into the oviduct. The oval eggs are 64–120 µm long (n = 2) and present a 95–168 µm long stalk (n = 2) (Fig. 7 A – C: eg).

As noted by Luther (1955), identifying the male sclerotised apparatus in C. alba is particularly challenging. In our study, we were able to distinguish a fully developed stylet in only three out of 11 specimens, while this structure was indistinguishable in the whole mounted material. The stylet in this species is generally delicate and, at first glance, appears underdeveloped. Consequently, Luther (1955) speculated that this species might be synonymous with C. vernalis Beklemischev, 1921 . However, the newly studied material, which includes fully developed specimens, suggests that C. alba bears a closer resemblance to C. truncata (Abildgaard, 1789) Sekera, 1906, as indicated by the stylet, which has a proximal handle and a distal spiny section with one spine more developed than the others. Nevertheless, our phylogenetic analysis confirms that C. alba represents a distinct lineage from C. truncata (see section Molecular Phylogenetic Analyses; Fig. 16).