Cue selection and ontogeny reveal larval settlement dynamics of the long-spined sea urchin Diadema antillarum, a keystone coral reef herbivore

Understanding early life histories of keystone marine herbivores can broaden insight into population dynamics and ecosystem functioning. Settlement, defined as the transitional process from planktonic larva to benthic juvenile, is not well understood for the long-spined sea urchin Diadema antillarum despite outsized herbivory effects on Caribbean coral reefs. Recent advances in aquaculture methods have enabled experimental investigation of larval ontogeny and cue-mediated settlement dynamics otherwise difficult to understand through field observations. In this study, the morphological attributes preceding metamorphosis of feeding larvae were characterized and factors influencing development of these attributes were examined. Simple access to the nutritious microalgae Rhodomonas lens led to metamorphic competence, defined here by presence of a large rudiment, or imaginal disc, and either internal or external podia with suction rings at the terminal tips. Subsequent exposure to a suite of individual cues revealed highest settlement to two calcareous algae, Halimeda sp. and crustose coralline algae, at 58.0% (+/- 3.7 SE) and 46.0% (+/- 5.1 SE), respectively, and zero settlement to a sterile seawater negative control. Significantly higher settlement to a ceramic tile overgrown with biofilm compared to separate sterile tile and isolated biofilm treatments revealed a synergistic relationship between a structural and biochemical cue. Microalgae exposure further enhanced settlement, but only when a strongly inductive primary cue was present. Together, these results provide insight into active habitat selection and factors likely influencing D. antillarum recruitment potential. Findings can be applied to improve both population dynamics models and aquaculture production methods.