Upon their arrival in the water column, coral larvae use physical and chemical cues to navigate toward a suitable habitat and begin their settlement process. To engineer substrates that influence settlement, it is important to have quantitative data about the types and concentrations of chemicals that elicit desired behavioral responses before and after contact with the substrate. Here, we quantified the behavioral and morphological responses of coral larvae (Colpophyllia natans and Orbicella faveolata) to crustose coralline algae exudates (CCA) and ions found in coral skeletons using chemotactic assays in microfluidic channels. Multiple larvae in each channel were tracked over 30 min to quantify their overall attraction or repulsion to the presence of various dissolved chemical cues. Larvae showed repulsion to \({\text{MgC}}{\text{l}}_{2}\) , attraction to both \({\text{CaC}}{\text{l}}_{2}\) and CCA exudates, and both attraction and repulsion to \({\text{SrC}}{\text{l}}_{2}\) , depending on the concentration. The behavioral and morphological changes exhibited by individual larvae were investigated as well. Using particle tracking methods to quantify larval behavior, we found that the typically straight swimming larvae of C. natans increased turning behavior in regions with high concentrations of CCA exudates and \({\text{CaC}}{\text{l}}_{2}\) , a behavior associated with local searching, while they decreased turning behavior near high concentrations of \({\text{MgC}}{\text{l}}_{2}\) . We also found that larvae shrink in length when exposed to 50× the seawater concentration of calcium, a potential stress or escape response, while these larvae elongated when exposed to CCA exudates, a morphological response associated with benthic contact and crawling. These results highlight the value of direct observation in understanding the interplay between coral larvae and their chemical environment. Incorporating cues such as calcium or CCA exudates into artificial substrates can elicit specific behavioral and physical changes in coral larvae, thereby enhancing settlement and contributing to reef restoration efforts.
