Little-known and phylogenetically obscure South African estuarine microgastropods (Mollusca: Truncatelloidea) as living animals

Areas of the Knysna estuarine bay in the Western Cape are dominated by three endemic South African truncatelloid microgastropods, temporarily known as `Hydrobia' knysnaensis (Krauss), `Assiminea' capensis (Sowerby) and `Assiminea' globulus Connolly. Although first described 80-170 years ago and present in abundance (up to 100,000 m(-2)), they remain surrounded by confusion and still await taxonomic assignment, largely because they appear most atypical members of their groups by virtue of anatomy and/or biogeography and/or habitat. This study contributes in-life perspectives to morphological and phylogenetic analyses known to be on-going. At Knysna, they are syntopic: at least two occurring in > 85% and all three in > 40% of individual 0.0026 m(2) samples from their region of dominance. Nevertheless, they tend to greater abundance in divergent microhabitats; `A.' globulus dominating higher tidal levels, and `A.' capensis and `Hydrobia' lower ones; the former especially unvegetated sediment, the latter, if anything, seagrass. Interspecific feeding interactions appear unlikely to be responsible for these patterns, other evidence suggesting that all are maintained below carrying capacity. Field biology of `H.' knysnaensis generally appears equivalent to that of northern-hemisphere intertidal hydrobiids and that of `A.' globulus is typically assimineid, albeit at atypically low shore height. Unlike assimineids, however, `A.' capensis is truly aquatic. The success of these truncatelloids in unusual circumstancesmay be consequent on the absence from South Africa of other microgastropod groups that fill their niches elsewhere in the southern hemisphere.