Optimal Foraging Theory Explains Feeding Preferences in the Western Pacific Crown-of-Thorns Sea Star Acanthaster sp.

AbstractThe selectivity of crown-of-thorns sea stars (Acanthaster sp.) for different coral prey types was quantified in the field and laboratory and compared with a range of nutritional and food quality parameters as well as the growth performance of sea stars fed on different types of coral. Growth rates of small juvenile Acanthaster sp. without previous exposure to coral fed for 6.6 months on 15 individual species of corals showed that the highest rates of growth were achieved on the same types of corals for which adult sea stars show the strongest preference, both in the field and in controlled aquarium conditions. Small Acanthaster sp. (ca. 20 mm, 0.5 g) fed on Acropora formosa, Stylophora pistillata, Seriatopora hystrix, and Pocillopora damicornis increased in size by an average of 9.2-10.7 mm (4.2-5.6 g) per month, compared with 0.1-0.4 mm (0.004-0.028 g) per month on coralline algae fed controls and species such as Porites lutea, Porites lichen, Lobophyllia hemprichii, and Turbinaria mesenterina. Field studies on the same reef where the parents of these juvenile sea stars were collected demonstrated a strong sequential preference for acroporid and then pocilloporid corals, with faviid, merulinid, and poritid corals selected significantly less frequently than other corals when their relative abundance was taken into account. This order of preference by adult field-collected sea stars was confirmed and exhibited even more emphatically in aquarium experiments, where the relative abundance of prey species could be controlled. The growth experiments and measurements of comparative food value between preferred and non-preferred coral prey suggest that feeding preferences in Acanthaster sp. for Acropora and pocilloporids arose consistent with optimal foraging theory and evolved in response to this species being able to feed successfully and efficiently. The high abundance and, therefore, encounter rate of Acropora and pocilloporids is not considered to be an important factor in the evolution of feeding preferences, although relative abundance of alternative prey does affect selectivity. Individual growth and population fitness and reproductive output of Acanthaster sp. will be enhanced by preferential feeding on acroporid and pocilloporid corals, reinforcing the importance of optimal foraging theory in the evolution of feeding preferences.