Use of refuges by the ophiuroid Ophiopholis aculeata: contrasting effects of substratum complexity on predation risk from two predators

We investigated the effect of substratum complexity on the vulnerability of the ophiuroid Ophiopholis aculeata to its 2 main predators, the sea star Asterias vulgaris (possibly synonymous with A. rubens) and the urchin Strongylocentrotus droebachiensis, and further examined behavioral responses of ophiuroids to these predators. Field and laboratory experiments showed that the use of crevices by ophiuroids tended to reduce predation by urchins. The ophiuroids’ response to contact with the urchin was to retract deeper into crevices. In contrast, in the laboratory, the ophiuroids’ vulnerability to the sea star was higher on complex substrata. In the laboratory, ophiuroids reacted weakly to contact with the arm tip or ambulacral groove of A. vulgaris, but immediately fled refuges when there was contact with the sea star’s stomach. In the field, the density of ophiuroids in 20 cm diameter circular plots placed around sea stars (both feeding and non-feeding) was similar to that in areas without sea stars. However, the estimated density of ophiuroids directly under sea stars was less than in areas without sea stars, and exposed ophiuroids (with the disk outside a crevice) were more abundant near A. vulgaris than in areas without sea stars: both differences were greater when the sea star was feeding. Ophiuroids display risk sensitivity adjusted to the 2 predators. They mainly flee crevices under a sea star when there is contact with the sea star’s stomach, but only move a few centimeters. A last-moment departure should increase the probability of capture by sea stars, but reduces time spent on open surfaces where they are vulnerable to urchin attacks.