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oo tia 99 fere uln O on predation cues, including hiding (refuge use) and substratum avoidance (crawling to the water surface or out of the water). We assessed shell morphology using morphometric techniques that isolate geometri-cally independent components of size and shape variation corresponding to aperture width and apertural obstruction. Previous studies indicate that large size, narrow apertures and obstructed apertures reduce morphological vulnerability to the shell-entry predation tactics used by crayfish. In the present study, small, and thus more vulnerable, prey tended to show stronger antipredator behaviour than large prey (i.e. behavioural compensation for morphological vulnerability). In contrast, behavioural and shape-based defences were positively correlated; snails with narrow apertures showed strong antipredator responses. We refer to this ‘double defence ’ against predators as cospecialization. With either compen-sation or cospecialization, suites of correlated behavioural or morphological traits must be studied in tandem to understand the adaptive value of prey responses to predators. Ó 1999 The Association for the Study of Animal Behaviour Studies of antipredator behaviour typically find that prey show adaptive behavioural responses such as reducing their activity or increasing their use of refuge in the presence of predators (reviewed by Sih 1987, 1995; Lima & Dill 1990; Werner & Anholt 1993). The appropriate response, or the degree of response shown, often differs from one potential prey organism to the next. Differences in prey behaviour frequently reflect underlying connec-tions with other traits related to vulnerability or energetic reserves. For example, prey in good condition often use costly defences that prey in poor condition do not per-form (e.g. Stenzler & Atema 1977; Damsgård & Dill 1998). Vulnerable classes of prey (e.g. based on size, sex, repro-ductive status) may also be more likely to perform anti-predator behaviour than well-defended prey (Stein 1977