Larval desperation and histamine: how simple responses can lead to complex changes in larval behaviour

Some marine invertebrate larvae expand the range of settlement cues to which they will respond as they age. How do relatively simple larvae achieve such complex changes in behaviour? Larvae of the Australian sea urchin Holopneustes purpurascens settle and metamorphose specifically in response to a settlement cue, dissolved histamine, produced by the host alga Delisea pulchra. Older H. purpurascens larvae appear to accept a wider range of host algae, which contain far less histamine than D. pulchra, than newly competent larvae. We tested the hypothesis that older H. purpurascens larvae accept a greater range of host algae by metamorphosing in response to lower concentrations of histamine. We compared the response of newly competent and older larvae to a range of histamine concentrations in settlement assays. Larval age strongly affected the minimum concentration of histamine that induced metamorphosis in H. purpurascens, with older larvae responding to lower concentrations of histamine than newly competent larvae. Older larvae were more sensitive to lower concentrations of histamine yet still maintained a stringent requirement for exposure to histamine in order to metamorphose. In addition, older larvae metamorphosed after shorter exposure periods to histamine than did younger larvae. By using histamine concentration as a proxy for specific habitat cues, H. purpurascens larvae appear to expand their range of settlement preferences with age by simply changing their sensitivity to a single settlement cue. Overall, our results show that marine invertebrate larvae can exhibit surprisingly complex changes in behaviour via simple changes in their response to a single cue.