Morphological and physiological impairments in algae-infested sea urchins of the species Echinometra lucunter (Echinodermata).

Epibiotic species, which may be characterized as those living attached to the outer surface of a host (the basibiont), have hardly been described as living on echinoderms, probably because the outer surface of these latter is covered by the epidermis. Studies analyzing epibiotic associations usually focus on taxonomical and ecological aspects, while a physiological approach - even being able to reveal the costs and/or benefits of such interactions - has been neglected. Here, we not only report an unprecedented algal epibiotic association for the sea urchin Echinometra lucunter, but we mainly investigate how such kind of interaction could affect spine morphology and basibiont physiology, and consequently its health. To achieve this, we compared the spine morphology of Echinometra lucunter with and without algal infestation using histology, microcomputed tomography, and SEM. Immunological parameters, such as the number, proportion, and viability of the coelomocytes were evaluated. Algal-infested individuals showed a higher number and lower viability of coelomocytes, as well as an altered proportion of phagocytes and red spherulocytes. Additionally, spine stroma and stereom were severely degraded in comparison to non-infested ones. Thus, our findings suggest that algal-infested E. lucunter may be less efficient in carrying out routine activities than non-infested individuals, such as physical protection, anchorage, or coping with immune challenges.