Comparative analysis of the mastax musculature of the rotifer species Pleurotrocha petromyzon (Notommatidae) and Proales tillyensis (Proalidae) with notes on the virgate mastax type

One defining rotiferan character is the mastax, a muscular pharynx that contains a set of cuticularized jaw elements, referred to as trophi. According to different arrangements and functions of the trophi, different basic mastax types are distinguished in literature. There have always been cases in which a distinct classification of a certain species' mastax has turned out to be difficult, since its trophi display characteristics of different mastax types. In our study we analyze the mastaxes of Pleurotrocha petromyzon (Ehrenberg, 1830) and the recently described species Proales tillyensis (Wilts and Ahlrichs, 2010) by means of TEM, CLSM and SEM to reconstruct their mastax musculature. Due to the strong agreement in its trophi morphology with Proales fallaciosa, whose mastax was designated as both, modified malleate and virgate, the mastax musculature of P. tillyensis was compared to the one of P. petromyzon. In total, we found eight muscles being associated with the pharyngeal jaw elements in P. petromyzon and nine such muscles in P. tillyensis. Both species share several muscles that previously have been delineated for other rotifer species. Among these muscles, P. petromyzon shows mastax receptor retractors that present the most important characteristic of the virgate mastax type and have been described for different species in earlier literature; by evoking a negative pressure in the mastax cavity when contracting, the muscles bring about the pumping function of this mastax type. Contrary to our expectations, P. tillyensis lacks mastax receptor retractor musculature, what let us draw the conclusion that this species' mastax cannot be categorized as distinct virgate, since the mastax receptor retractors present the most significant character of the virgate mastax.