Predator-specific deployment of defensive behaviors in the marine isopod Cleantiella isopus (Isopoda, Idoteidae): Arched posture against fish and escape against crabs

Many prey species are exposed to multiple predators that use contrasting hunting modes. To enhance survival under such conditions, the prey may discriminate among predator types and deploy predator-specific defenses. Secondary defenses expressed following contact with predators are likely to require an accurate assessment of predation risk, as inappropriate responses can prove fatal. However, the selective deployment of secondary defenses against multiple predators remains poorly understood. Tonic immobility (thanatosis or death-feigning) is one of the most widely reported secondary defenses of diverse taxa. The marine isopod Cleantiella isopus exhibits tonic immobility in response to predator attacks, adopting a rigid, arched posture while remaining motionless. Here, we tested whether tonic immobility was selectively deployed against different predator types by comparing isopod responses to three fish species and a crab. Tonic immobility was exclusively expressed in response to fish predators, whereas crabs elicited escape responses. Further experiments revealed that the arched posture reduced the probability of swallowing, whereas immobile individuals were attacked less frequently and experienced longer delays before the attack. These results suggest that tonic immobility in C. isopus enhances survival against fish predators through multiple mechanisms. Our findings demonstrate that tonic immobility in C. isopus is not a generalized response to external stimulation, but rather a predator-specific defense against fish predators, supporting the hypothesis that tonic immobility represents an adaptation to particular predator types. The ability to discriminate between predator types and selectively deploy appropriate defensive behaviors may enable C. isopus to enhance its survival in environments with multiple predators.