Testudinella crassa (Levander, 1894) sp. inq.

(Fig. 13)

T. clypeata after Rousselet (1898), Harring (1913), Koste (1978) and Segers (2007), T. clypeata var. crassa after Remane (1929) and Voigt (1957), T. clypeata crassa after Wiszniewski (1954) and Bērziņš (1960)

This poorly described taxon is characterized by a convex dorsal anterior margin, and a slightly undulate ventral anterior margin with small rounded median sinus bordered by two acute projections (dorsal and ventral was wrongly interpreted in the original description). The lorica outline is oval with shallow lateral indentations at the height of the foot. The sub-terminal foot opening is a wide, inverted U-shaped slit. A specimen with similarly shaped anterior margins was figured by Rousselet (1898) sub T. clypeata . Its outline is elliptical instead of oval and the lateral margins of the foot opening show an indentation; the lateral antennae are situated a little above the middle. However, similar variations in shape of the lorica and foot opening are evident in other species as well, and I consider the animals described by Levander (1894) and Rousselet (1898) most likely identical.

Measurements (values by Rousselet (1898) bracketed). Lorica length 157–162 µm (150 µm), lorica width 113–121 µm (102 µm).

The species was described from beach pools and seawater (Gulf of Finland), but freshwater ditches as well. The specimens studied by Rousselet (1898) originated from brackish tide pools of the Tay estuary, Scotland, UK. In her study on marine rotifers from beach pools in southern Finland Godske Björklund (1972) reported a population with forms corresponding to T. crassa, and clearly distinguished from populations of T. clypeata by lorica length and width as well. However, because of intermediate sizes of T. clypeata collected in western Norway, but without taking into consideration morphological aspects, she concluded that T. crassa does not seem to be a separate species.

Testudinella crassa has been synonymized with T. clypeata without convincing argumentation, but the characteristic anterior margins suggest a valid species, and I suggest that it be treated as species inquirenda until more information becomes available.