Patiriella exigua: grazing by a starfish in an overgrazed intertidal system

Abstract

Intertidal rocky shores in south-eastern Australia are dominated by a diverse assem- blage of grazing invertebrates that feed on micro-algal biofilms. This resource is spatially variable and frequently over-grazed, causing strong inter- and intra-specific competition among grazers. Most studies on intertidal grazing are about gastropod molluscs. We observed, however, damaged patches in intertidal biofilms that appeared to be associated with the herbivorous asterinid starfish Patiriella exigua (Lamarck). In contrast with predatory starfish, there have been few ecological studies about herbivorous starfish, even though they are often abundant. We demonstrated that these patches were caused by grazing by this starfish. We then used field-based remote-sensing methods to demonstrate that amounts of chlorophyll were reduced inside grazing marks, quantified these changes and measured their longevity. In experiments, starfish could graze up to 60% of the epilithic micro-algae beneath their everted stomach during a single feeding event lasting on average 22 min. Over 5 d, 2 caged starfish could remove nearly half of the available micro-algae from areas of 144 cm 2 . Changes to the amounts of chlorophyll in grazing marks were persistent, remaining visible on sandstone substrata for several weeks. Amounts of chlorophyll on grazed areas of substratum were significantly different from ungrazed areas up to 28 d after being grazed. Recovering areas were demonstrated to be bio-equivalent to ungrazed areas after a similar period. Thus, P. exigua can have a large effect on the temporal and spatial distribution of micro-algal food, with numerous possible consequences for intertidal assemblages. The ecological importance of herbivorous starfish may be greater than previously perceived.