Induction of metamorphosis in the sea urchin Holopneustes purpurascens by a metabolite complex from the algal host Delisea pulchra

Most benthic invertebrates have complex life cycles with planktonic larvae that return to the substratum to settle and metamorphose into a benthic sta ge. Although naturally produced chemical cues have long been thought to be important for the settlement or metamorphosis of invertebrate larvae, few e cologically relevant chemical cues have been clearly identified. The marine echinoid Holopneustes purpurascens has a complex life cycle, with a plankt onic, nonfeeding dispersive larva that metamorphoses into a benthic stage t hat lives in the canopy of subtidal benthic algae such as the red alga Deli sea pulchra and the kelp Ecklonia radiata. Recently recruited juveniles are found primarily on D. pulchra, and we hypothesized that this was in respon se to a chemical cue produced by this alga. Competent larvae metamorphosed in the presence of D. pulchra, or seawater surrounding this alga, but not i n response to the presence of E. radiata or its extracts. A cue for metamor phosis was isolated and characterized from D. pulchra and found to be a wat er-soluble complex of the sugar floridoside and isethionic acid in a 1:1 mo lar ratio. The floridoside-isethionic acid complex also triggered settlemen t in H. purpurascens; however, this response was less specific than metamor phosis and was reversible. Larvae of H. purpurascens also metamorphosed in the presence of several other species of red, but not brown or green, algae from their habitat. Floridoside is found only in red algae, suggesting tha t the floridoside-isethionic acid complex may be acting as a cue for metamo rphosis in other red algae as well as in D. pulchra.