Anatomy and reproduction of viviparous Pisidium (Parapisidium) reticulatum Kuiper, 1966: Implications for the phylogeny of Sphaeriidae (Mollusca: Bivalvia: Heterodonta)

The pea clams Sphaeriidae represent a major molluscan freshwater radiation with cosmopolitan distribution in all kinds of lotic and lentic habitats. Their phylogenetic relationships are still controversial, with comprehensive taxonomic sampling and examination of morphological characters still challenging. Here, based on rare and rediscovered original material, we study in detail the anatomy and aspects of brood protection of the African Pisidium reticulatum Kuiper, 1966. Representing the monotypic subgenus Parapisidium Kuiper, 1966, this species is characterized by its peculiar combination of shell and anatomical features of potentially high phylogenetic relevance. While similar to other congeners in several anatomical characters (e.g. reduction of inhalant siphon and descending lamella of outer demibranch, simplified structure of intestine coil and nephridium), P. reticulatum differs from other Pisidium species in retaining both pairs of retractor muscles of the inhalant siphon, and particularly in its peculiar mode of brooding. The yolky eggs are relatively large (160–170 μm in diameter) and are incubated in the gill, albeit in the absence of the formation of brood pouches. During later stages of incubation the larvae are surrounded by large cells similar to nourishing cells in other sphaeriids and probably with similar function. This unique combination of reproductive features is hypothesized to represent an intermediate stage between the typical ovoviviparity of Euperidae and euviviparity (i.e. nourishment by the parent animal) as found exclusively in Sphaeriidae, the latter being characterized by the possession of closed brood pouches. Phylogenetic analyses based on a comprehensive set of morphological characters reveal Parapisidium as the most basal lineage within a clade Pisidium. Evaluating the phylogenetic reconstructions based also on available molecular data for Sphaeriidae, we discuss alternative scenaria of (parallel) evolution of brood pouches and viviparity in this group.