Observations on shell form and its ecological significance in thaisid gastropods of the genus Lepsiella in New Zealand

Abstract

This paper is concerned with the ecological significance of variation in shell form within the thaisid gastropod genus Lepsiella in New Zealand. Shell form has been investigated by measurement of shell height and breadth, aperture length and width, the diameters of consecutive whorls, apical angle, shell weight, and shell capacity, although in many cases shell height and shell breadth could not be measured because of erosion. L.albomarginata has been studied intensively at 4 stations in the South Island, and L. scobina less intensively at 6 stations in the North and South Islands. Comparisons of pairs of characters between stations have been tested by regression analysis and analysis of covariance where appropriate. Shells of L. albomarginata are relatively taller and narrower, and have a thicker wall, at a very sheltered station (Hakahaka Bay) that at more wave-exposed stations. L. scobina (sensu stricto), characterised by the presence of spiral ribs on the shell, exhibits less striking but comparable differences in shell shape. In laboratory tests in a tidal tank the thicker-shelled L. albomarginata from a sheltered station (Hakahaka Bay in Port Underwood) was much better able to resist attack by the shore crab Hemigrapsus edwardsi than was L. albomarginata from a nearby wave-exposed station (Whites Bay, near Cape Campbell, South Island). L. scobina from both stations was resistant to attack. H. edwardsi abounds at sheltered stations, but is missing from wave-exposed rock reefs such as those at Whites Bay, so that the ability to survive encounters with shore crabs is ecologically important to L. albomarginata inhabiting sheltered stations. L. scobina occupies a lower zone on the shore, where it is probably liable to encounter other more powerful predators. Its spiral ribs probably strengthen the shell. We do not know to what extent differences in shell form and thickness depend on environmental factors, and to what extent they originate genetically. Thin shells are associated with an abundance of mussels (Mytilus edulis ssp. aoteanus or Modiolus neozelanicus). There is an interesting possibility that a scarcity of mussels or other food caused by superior nonspecific predators might result in the production of better-protected Lepsiella.