Some aspects of the functional morphology and biology of Pseudopythina subsinuata (Bivalvia: Leptonacea) commensal on stomatopod crustaceans

Pseudopythina subsinuata (Lischke) has been found in Hong Kong waters to be commensally associated with the following stomatopod crustaceans: Oratosquilla perpensa (Kemp), Oratosquilla interrupta (Kemp), Oratosquilla oratorio (De Haan), Harpiosquilla japonica Manning. The location on the host is highly specific. Studies on the functional morphology and biology of this bivalve have shown that it is relatively unspecialized except with regard to its sexual cycle. It has been shown to be a protandric hermaphrodite in which a smaller male normally occurs in close association with a larger female. Fertilization occurs in the supra‐branchial chamber of the female and the young are subsequently incubated in the same position. Solitary females also occur. It is suggested that a complex sexual cycle was a prerequisite for the successful exploitation of the commensal mode of life by the Leptonacea in general and that subsequent morphological specializations have occurred in only some. Summary: Pseudopythina subsinuata has been found in Hong Kong waters living commensally on the following species of stomatopod crustaceans: Oratosquilla perpensa (Kemp), Oratosquilla interrupta (Kemp), Oratosquilla oratorio(DeHaan), HarpiosquiliajaponicaManning. [ts location on the host is extremely specific and is occasioned by the requirements of the commensal and the limitations imposed by the behaviour of the host. Pseudopythina subsinuata possesses many characters which suggest that it has only comparatively recently adopted a commensal mode of life: (1) It is not host specific. (2) It possesses a large shell with distinct hinge teeth. (3) The mantle edges are not reflected over the shell, and there is no inhalant siphon. (4) There is a well developed byssal apparatus. (5) The stomach is large, not reduced and non‐specialized. (6) The style sac still retains a connection with the mid‐gut along half its length. Some of these primitive characters, e.g. the large byssus, have been retained because they possess an obvious survival value. Other characters, e.g. the large labial palps, and the absence of interlamellar junctions in the ctenidia are probably specializations. The sexual cycle of Pseudopythina subsinuata is complex, the animal being a protandrous consecutive hermaphrodite. Solitary females also occur in the population. Fertilization occurs in the supra‐branchial chamber and the young are incubated there at least until the late veliger stage. It is suggested that a highly specialized sexual cycle was developed early in the evolution of the Leptonacea and was a prerequisite for the successful adoption of a commensal mode of life. Anatomical specializations evolved later, and to different degrees in different phylogenies.