Tonic immobility in a marine isopod: the effects of body size, sex, and colour morph

Tonic immobility is considered an anti-predator defence, wherein prey adopts a motionless state in a characteristic posture elicited by external stimuli. The marine isopod Cleantiella isopus exhibits tonic immobility with an arch-like posture and motionless state lasting several seconds or minutes in response to external stimuli such as predatory attacks by fish. In this study, we describe tonic immobility by wild-caught C. isopus and examine the influence of body size, sex, and colour morph on the frequency and duration of tonic immobility. All individuals exhibited tonic immobility regardless of body size, sex, or colour morph, suggesting that the behaviour plays a major role in predator avoidance following detection by a predator. In males, smaller individuals exhibited more prolonged tonic immobility than larger individuals, whereas the relationship between the duration of tonic immobility and body size was unclear in females. Colour morph had no effect on the duration of tonic immobility. These findings provide a detailed documentation of tonic immobility in C. isopus and suggest that the factors affecting tonic immobility differ between males and females.