What and when to eat? Investigating the feeding habits of an intertidal herbivorous starfish

Abstract

Although the ecology of molluscan grazers on intertidal rocky shores is very well documented, there are other abundant grazers, such as herbivorous starfish, that can modify the structure of benthic assemblages yet whose behaviour is poorly understood. The feeding habits of asteroids suggest that they are opportunistic feeders foraging on a broad range of available resources. The reasons for the variation in the food types consumed are not clear. We investigated the feeding habits of the intertidal herbivorous starfish Parvulastra exigua with respect to the temporal and spatial distribution of food and tidal cycles in two populations. We predicted that food choice by P. exigua would be a function of food abundance and that foraging would be independent of tidal cycle. The results showed that this starfish forages on a narrow range of algal food resources, with an apparent selection for microalgae. Foraging during high and low tide was variable across locations; however, most individuals from both populations foraged during any tidal period. These results indicate that P. exigua forages on the same food consumed by many other intertidal grazers, indicating the potential for competition for resources. Grazing on biofilm by P. exigua may play an important role in modifying the structure of algal assemblages on rocky shores by potentially preventing the growth of macroalgae.