Size-related habitat shifts facilitated by positive preference induction in a marine kelp crab

Individual variation in habitat use has important ecological and evolutionary consequences. Here, we combine developmental and functional approaches to study habitat use in Pugettia producta, a marine crab that sequesters pigments from the algae it inhabits and consumes to camouflage with different colored algal habitats. Pugettia from different habitats differ in size and color; individuals living in red intertidal algae are small and red, whereas crabs in amber-colored kelp forests are larger and more amber. We examine the developmental causes (effects of size and experience) and the functional consequences (growth and mortality rates) of size-specific habitat use in Pugettia. We demonstrate that positive preference induction may be a mechanism facilitating size-specific habitat use: prior feeding experience with kelp (in the field or laboratory) increases habitat preferences for kelp but only for larger crabs. Habitat use also has important fitness consequences; small red crabs grow equally fast on both kelp and red algae but suffer much higher mortality in kelp; large red crabs grow faster on kelp diets relative to red algae but experience only marginally higher mortality in kelp. Thus, habitat-specific mortality rates may explain the use of intertidal red algal habitats by small crabs, whereas reduced growth of large crabs on red algae may help trigger shifts to subtidal kelp habitats. Size-specific functional consequences of habitat use may select for developmental processes—such as positive preference induction—that facilitate shifts in habitat use in Pugettia and other phenotypically plastic organisms that shift habitats through ontogeny.