Reevaluating the dynamics of intertidal rocky shores: The foraging and behavioural ecology of an understudied grazer and its effects on benthic assemblages

Abstract

The strongest grazing effect among all marine benthic systems is on intertidal rocky shores. Among intertidal grazers, limpets are well-known structuring agents altering the distribution of rocky shore assemblages. Globally, there are other abundant herbivores whose ecological importance in the dynamics of rocky shores is poorly studied as is the case for herbivorous starfish. Parvulastra exigua is a small starfish that occurs at great densities in rock pools on intertidal shores. This research aimed to understand the behavioural ecology of P. exigua by investigating the feeding and foraging habits of the starfish, competitive interactions between P. exigua and limpets, and the consequent effects on algal assemblages. Parvulastra exigua showed a narrow diet breadth with selection for microalgal food. This species fed during all periods of the tidal cycle. During immersion, the starfish foraged within and around rock pools and only foraged in pools whilst the tide was out. The starfish foraged near their rock pools, potentially creating a halo effect due to localized grazing. Rock pool fidelity in P. exigua varied among locations suggesting that the impact of this starfish on algal assemblages might be contingent on the availability of refugia at low tide and that the extent of the halo effect by P. exigua will vary from place to place. Parvulastra exigua revealed to be an equal competitor to a key limpet on the shore and the starfish was as efficient in controlling macroalgal growth as the limpet, with the strongest effect inside rock pools. These findings demonstrate that P. exigua is an important grazer in intertidal rocky shores. Herbivorous starfish occur on other rocky shores around the world and in sand flats, seagrass and kelp forests. Thus, future work is relevant to examine the effects of starfish herbivory on other shores and ecosystems.