Commensalism between an epizoic limpet, patelloida nigrosulcata, and its gastropod hosts, haliotis roei and patella laticostata, on intertidal platforms off perth, western australia

On intertidal limestone platforms off Perth and neighbouring islands, the limpet Patelloida nigrosulcata occurs only on the shells of living abalone (Haliotis roei) and other limpets (Patella laticostata). The incidence of commensalism varies among sites and between habitats within sites but is consistently high (>80%) among dense abalone populations along the seaward margin of platforms. There is usually only 1 limpet per shell, although 2 or 3 limpets occasionally co-occur. A strong positive relationship in size (shell length) between Patelloida nigrosulcata and H. roei indicates that limpets settle on juvenile abalone and grow at a rate proportional to the growth rate of their host. A similar size relationship exists between Patelloida nigrosulcata and Patella laticostata. Patelloida nigrosulcata forages diurnally from a home scar and markedly limits the growth of erect macroalgae on the host shell. Escape from intense interspecific competition with other molluscan grazers on the platforms may have been a strong selective force in the evolution of this commensal relationship.