The distribution and habitat in the Knysna Estuary of the endemic South African mudsnail Hydrobia knysnaensis and the influence of intraspecific competition and ambient salinity on its abundance
The typical habitat of the rare, endemic South African mudsnail Hydrobia knysnaensis is the leaves of upper-shore Zostera capensis within high-salinity salt-marsh pools and channels. In the Knysna estuarine system, it is the numerically dominant member of a guild of six small microphytophagous gastropods; it is absent from lower level and more exposed Zostera meadows, where its place is taken by Rissoa pinna. H. knysnaensis is also here recorded from the nearby Swartvlei Estuary, in the same habitat type. This unusual habitat for a Hydrobia may in part account for the failure of earlier surveys to detect its presence, notwithstanding that it may well locally be the most numerous gastropod in each of these systems. Generally, however, it (and probably other small gastropods) seem to have been confused in estuarine fauna lists with Assiminea. Experiments show that the rate of feeding in H. knysnaensis is curtailed at population densities exceeding 2000–4000m−2 and in salinities below some 10psu. The proportion of non-feeding snails also increases at high population densities and in low salinities. The bearing of these results on whether H. knysnaensis is likely to be the 'Hydrobia sp.' recorded from some other South African localities and on the causes of its rarity are discussed.