Exploring the use of righting time in predicting the immunological condition of echinoderms: A case study with the sea urchin Echinometra lucunter

Studies correlating behavior and health are common for vertebrates but are comparatively limited for invertebrates, particularly within the phylum Echinodermata. Behavioral studies in echinoderms are scarce, chiefly focusing on covering and righting behaviors. Given the lack of consensus on the meaning and potential use of the righting time in echinoderms, measured as the time necessary to return to a natural position after being overturned, the present work aimed to establish links between this behavior and reliable but invasive health indicators. Through an experimental infection with the yeast Saccharomyces cerevisiae, we assessed whether the righting time of the sea urchin Echinometra lucunter mirrored a distruption in cellular and humoral immune markers. Our hypothesis posited that for righting time to serve as a reliable health indicator, infected sea urchins should exhibit extended righting times to match any altered immunological responses, analyzed through cellular and humoral immune parameters. The results demonstrated clear alterations in the latter, including elevated total coelomocyte counts and increased proportions of phagocytes, alongside a decreased proportion of vibratile cells and reduced coelomocyte viability. Furthermore, all analyzed biochemical parameters—specifically total protein and echinochrome concentrations, and alkaline phosphatase activity—increased in yeast-infested animals. However, while the righting time of yeast-infected animals showed a tendency to be elevated compared to that of control animals, it did not significantly differ. Consequently, our findings indicate that righting time does not seem to be a sensitive proxy of the health status of E. lucunter, and its application as a stress marker or predictor of sublethal changes in health condition should be approached with caution in this and other echinoderms.