Escape responses in four epibenthic brittle stars (Ophiuroidea : Echinodermata)

Escape reactions to simulated sub-lethal predation were investigated in fie ld and laboratory experiments in four epibenthic brittle stars, Ophiura alb ida (Forbes), Ophiura ophiura (L.), Ophiothrix fragilis (Abildgaard) and Op hiocomina nigra (Abildgaard) from the Swedish West Coast. Predation attacks were simulated in three ways: by (1) momentarily approaching an individual with the palm of the hand, simulating an attack attempt without contact, ( 2) pinching an arm with forceps simulating a failed attack, and (3) amputat ing an arm using scissors and thus simulating a sub-lethal attack causing a n injury. All brittle star species responded to predation attempts by escap ing in a direction away from the site of attack, and a more rapid response was elicited when an arm was amputated (3) than if the arm was pinched (2). However, O. albida showed little or no reaction at all when attacked. O. o phiura was the only species that responded when momentarily approached by t he palm of the hand (I). O. ophiura also showed a possibly intimidating (de imatic) behaviour by twisting up towards the point of attack, followed by a rapid escape response when pinched by forceps. In O. fragilis, observation s also indicated that the thrashing of amputated arms may act as an efficie nt decoy pre-occupying a predator such as O. nigra and allowing O. fragilis to escape. The escaping behaviour of O. fragilis and O. nigra in the field varied depending on the proximity to a potential shelter such as a holdfas t or aggregate of conspecifics. If a specimen was found close to a shelter it only withdrew its arms or took a single step. Crypsis was also the main defensive behaviour in aquarium experiments with O. nigra in the presence o f sustained predation by a fish, Labrus bergylta. The diverse reactions to predation encounter events found in this study suggest that the initial res ponse to predation events by active flight may be followed by a cryptic beh aviour to reduce the probability of detection and recognition if the predat ion pressure remains high.