Sediment preference and burrowing behaviour in the sympatric brittlestars Ophiura albida Forbes, 1839 and Ophiura ophiura (Linnaeus, 1758) (Ophiuroidea, Echinodermata)

Ophiura albida and Ophiura ophiura are widespread and highly abundant brittlestar species occurring sympatrically on soft bottoms along the western European coasts. Laboratory choice experiments revealed that O. albida preferred staying on fine rather than on coarse sediments, whereas O. ophiura did not distinguish between these types of sediment. Sediment-specific burrowing behaviour of the two species was investigated under different stress and food conditions in order to evaluate relations of predator avoidance and feeding strategies with the observed sediment preference. In the presence of a predator, O. albida burrowed preferentially in fine sediment while coarse sediment did not seem to support quick burrowing for efficient escape. Conversely, O. ophiura tended to escape the predator by fleeing across the sediment surface rather than by burrowing, reflecting its unselectivity towards different sediment types. For O. albida, stationary burrowing behaviour suggests deposit feeding, predating and/or scavenging on infaunal organisms to be the predominant feeding behaviour rather than hunting for epibenthic prey organisms; more so, as this foraging strategy reduces the species' exposure to predators. In contrast, O. ophiura seems to be a true hunter and predator for epibenthic prey which was reflected in the experiments by low burrowing activities in presence of food enriched sediments. We suggest that in O. albida and O. ophiura the evolution of different mechanisms of predator avoidance has been associated with the evolution of contrasting foraging strategies, supporting the species' coexistence in broadly overlapping habitats and explaining their distribution on different types of sediment in the German Bight (North Sea).