Causes of rarity and range restriction of an endangered, endemic limpet, Siphonaria compressa

The pulmonate limpet Siphonaria compressa is South Africa's most endangered marine mollusc. It is endemic to just two localities: Langebaan Lagoon on the west coast, and Knysna Estuary on the south coast, and occurs only on the eelgrass Zostera capensis. In Langebaan Lagoon, eelgrass has fluctuated substantially over the last 34 years, and S. compressa has twice approached extinction. S. compressa is largely confined to the lower edge of the eelgrass beds there, being replaced higher up by another small gastropod, Assiminea globulus. We explored the physical and biological factors underlying the limpet's narrow habitat, using field observations, translocations, caging and transplant experiments. Abundance of S. compressa was positively correlated with Z. capensis cover and negatively correlated with shore height. When moved to the upper portions of the eelgrass bed, S. compressa had lower rates of persistence and survival than in the lowest zone. The lower limit of zonation for S. compressa was set indirectly by bioturbation by the sandprawn Callianassa kraussi, which excluded eelgrass from intertidal sandbanks. Transplants of eelgrass into the sandbanks proliferated provided C. kraussi was experimentally eliminated, and supported densities of S. compressa 20-fold greater than in control eelgrass beds, suggesting that high-shore eelgrass beds to which S. compressa is normally confined are suboptimal for the limpet. A. globulus showed patterns opposite to those of S. compressa: its persistence and survival were greatest in the upper zone and it actively avoided the lower sections of these beds and never colonised eelgrass transplanted into sandflats lower on the shore. There was no evidence that competition between S. compressa and A. globulus influenced the zonation or abundance of either species. Rarity of S. compressa and its endangered status seem dictated by its extremely narrow and temporally changeable habitat-range, which is defined by physical stress in the high-shore and bioturbation by C. kraussi in the low-shore. Fluctuations in eelgrass abundance and limitation of S. compressa to just two localities add substantially to the risks of extinction for this embattled stenotypic limpet.