Stargrazing: Trophic ecology of sea stars in the Galápagos rocky subtidal zone

Abstract Sea stars (class Asteroidea) can play powerful and wide-ranging roles as consumers of algae and prey items in benthic ecosystems worldwide. In the Galápagos rocky subtidal zone, sea stars are abundant and diverse but their distribution, feeding habits and ecological impacts have received little attention. We compared diets and distributions across the six most abundant sea star species to examine functional roles of this important group. Bi-annual censuses carried out between 2006–2014 at two depths (6-8m, 12-15m) at 12 sites in Galápagos identified two abundance “hotspots” and revealed higher densities at locations with more heterogeneous benthic topographies. Field surveys revealed a high incidence of feeding (35–68% of individuals across species) and distinct diets were evident for each species in terms of food items and dietary breadth, suggesting niche partitioning. Most species can be classified as facultative herbivores, with diets dominated by crustose and turf algae supplemented by a small proportion of sessile invertebrates. The two most abundant species (Pentaceraster cumingi and Nidorellia armata) had the narrowest diets. Field prey selectivity experiments identified P. cumingi as a size-selective predator of the pencil urchin Eucidaris galapagensis. In field caging experiments, N. armata reduced biomass of unbleached crustose coralline algae and macroalgae by 72% and 52%, respectively. In the context of emerging threats such as disease, ocean acidification and climate change, a deeper understanding of distinct functional roles can inform ecological models and management plans.