PREDATORY INTERACTIONS BETWEEN ANEMONES AND BRITTLE-STARS DURING OXYGEN DEFICIENCY EVENTS IN THE NORTHERN ADRIATIC SEA

The brittle star Ophiothrix quinquemaculata is one of the designating species of a peculiar, epifaunal suspension-feeding community on the northern Adriatic shelf, which may serve as a modern equivalent to typical Palaeozoic and Mesozoic epeiric seas. Richard Aronson has shown that such dense populations of brittle stars are rare on most modern shelves, and it has been suggested that massive predation, mostly by fish and crabs, led to their gradual displacement into low-predation refuges. Investigations of the benthic fauna in the northern Adriatic Sea, however, indicate that predation intensities on ophiuroids are not necessarily low: flight reactions and the specific distribution of Ophiothrix quinquemaculata point to the decapod Pilumnus hirtellus, the hermit crab Paguristes eremita and sea stars as important potential predators.

The present study reports a new and unexpected predatory interaction between this ophiuroid and two species of anemones during conditions of hypoxia. During oxygen crises, benthic faunas show a series of behavioral patterns that reflect the duration and severity of the event. During artificially induced oxygen deficiencies in 24 m depth in the North Adriatic Sea, we photographically documented for the first time predatory interactions involving the sea anemones Cereus pedunculatus and Calliactis parasitica, and the brittle star Ophiothrix quinquemaculata. Five predatory events were recorded in 4 anemones (from a total of 10 experiments, 918 hours observation). Under near-anoxic conditions, individuals of both anemones made contact with, pulled in, and consumed the brittle stars. The predatory events took between about 1.5 and 7.5 hours. In three of these events, brittle star remains were regurgitated between 2 and 12.5 hours later. Our time-lapse sequences demonstrate that oxygen deficiency, beyond eliciting a series of specific behaviours in individual species, also promotes previously unobserved interspecific interactions. Anemones have not yet been reported as predators on brittle stars. Our results show that they are not only highly resistant to anoxia, but may also actually benefit by opportunistically taking advantage of more vulnerable prey. This adds a new facet to anemone feeding behavior and to benthic community interactions under hypoxic conditions.