Prorhynchella minuta Ruebush, 1939

Fig. 10

Material examined

USA • 9 specs, live observations, one of which sagittally sectioned and two horizontally sectioned; Alabama, Ponderosa Lake; 32°54′53″ N, 87°19′28″ W; 27 May 2009; Niels Van Steenkiste leg.; submerged vegetation in shallow, clear water in the southeast corner of the lake; XIV.3.01–XIV.3.03; HU .

Description and discussion

Specimens are about 1 mm long (Fig. 10E) and can be placed within ‘Protoplanellinae’ due to the ventral position of the testes relative to the vitellaria and the separate protonephridiopores (see Reisinger 1924). Additionally, sectioned specimens clearly show ciliated pits (Fig. 10B: cp) at the anterior body end. Within ʻProtoplanellinae’, the only genus containing species with such pits is Prorhynchella (Van Steenkiste et al. 2010). Moreover, the studied specimens also possess the typical proboscis-like organ (Fig. 10B: plo) associated with a glandular complex (Fig. 10B: gl) that is unique for this monospecific genus (described as ‘rüssel-like organ’ by Ruebush 1939).

The only difference between our specimens and the original description of P. minuta is the position of the protonephridiopores (Fig. 10A: pp), which are situated somewhat more posteriorly compared to the original description (Ruebush 1939).

The studied specimens show some intraspecific variation in the structure of the bursa (Fig. 10A, C–F: bu). In one specimen, this is merely an evagination of the genital atrium while in another it is an oval, thin-walled structure, filled with sperm (compare Fig. 10C–D, F). It is connected to the genital atrium (Fig. 10A, D: ga) by a long bursal stalk (Fig. 10C, F: bs), which is surrounded by very strong sphincters (Fig. 10C, F: sph). Only two sphincters were visible on live specimens (Fig. 10F), but at least six were counted in one sectioned specimen (Fig. 10C). In the original description of P. minuta, Ruebush (1939) describes a bursa with a stalk, similar to the situation in most of our specimens (Fig. 10C, F). Whether or not the specimen drawn in Fig. 10D is an aberration is not clear: The specimen appears fully adult, hence the poorly developed bursal structure is unlikely a result of a different degree of development. With the data available to us, it is difficult to speculate on the biological significance of this reduced organ and we hence refrain from drawing conclusions on the matter.

Previously known distribution

Freshwater ponds near Pine Orchard, Connecticut, USA (Ruebush 1939).