Oysters as habitat for limpets in a temperate mangrove forest

Abstract

In temperate mangrove forests in New South Wales, Australia, the limpet Patelloida mimula Iredale lives on the oyster Saccostrea commercialis Iredale and Roughley, which, along with mangrove trees, provide the only hard substrata in a habitat otherwise dominated by soft‐sediment. The objective of this study was to ascertain the degree of association between the limpet and the oyster by examining their patterns of co‐occurrence in the forest and the relationship between individual pairs of limpets and oysters. Sampling of the distribution and abundance of limpets and oysters throughout the mangrove forest revealed that limpets were rarely present on substrata other than oysters. Patterns of abundance of limpets were, however, not directly related to the amount of habitat provided by the oysters. For example, there was a dramatic decline in the abundance of oysters from seaward to landward in the mangrove forest which was not reflected in the densities of limpets. Consequently, oysters appear to set the limits of distribution of limpets, but other factors modify their broad‐scale patterns of distribution and abundance within these bounds. Limpets leave scars on oysters which might be home sites. About 98% of oysters with limpets had only one limpet per surface of oyster, and the distribution of limpets was overdispersed or repulsed. Moreover, in some areas of the forest, the lengths of limpets were directly related to lengths of oysters. Tracking of individual limpets for 13 days revealed that > 70% remained on the same scar of an oyster. This evidence suggests that for some limpets there is a strong association with particular oysters. Factors determining the distribution, abundance, and sizes of oysters are likely to be important sources of variation to the structure of populations of P. mimula in temperate mangrove forests.