Fish kairomones induce spine elongation and reduce predation in marine crab larvae

Specialized defense strategies are induced in zooplankton upon detection of predator chemical cues or kairomones. These defenses are well‐described for freshwater zooplankters, with morphological defenses being particularly striking, but few studies have reported kairomone‐induced morphological defenses in marine zooplankton. Here, we compare morphological responses to kairomones in the larvae of two marine crab species, estuarine mud crabs ( Rhithropanopeus harrisii ) and Asian shore crabs ( Hemigrapsus sanguineus ). When reared in the presence of fish kairomones, spine length increased by 2–3% in larval R. harrisii , while no morphological changes were identified in H. sanguineus . In subsequent feeding assays with a co‐occurring fish predator (Atlantic silversides, Menidia menidia ), consumption of R. harrisii was lower on larvae that had been reared with kairomones. In addition, we found that broods with smaller larvae are more likely to exhibit increases in spine length after kairomone exposure. Hence, the observed morphological response is likely influenced by larval size.