Bald disease in a natural population of the purple sea urchin Paracentrotus lividus of the Mediterranean Sea: From spines to tissues

Recently, unusual mortality outbreaks have been reported in the echinoderm populations over broad geographic regions. The present work used different diagnostic approaches to unravel the Bald Sea Urchin Disease (BSUD) causes in a natural population of Paracentrotus lividus from the Gulf of Naples sampled in 2021. Symptomatic individuals displayed the typical signs such as test discoloration and ulceration, loss of spines and pedicellariae and visceral hyperpigmentation. Scanning electron microscopy of diseased individuals (stage 2 and stage 3) revealed a bare exoskeleton with multiplying bacteria penetrating the damaged test, and histopathology revealed inflammatory lesions and phagocytosis only in stages 2 and 3, with Gram negative and positive bacteria at stage 3. Metagenomics revealed an increase in DNA virus and Proteobacteria during disease progression. Microbial community analyses failed to reveal a single putative pathogen associated with symptoms, but microbiome analysis showed higher diversity in asymptomatic individuals compared to the symptomatic. Different Vibrio spp. belonging to the Splendidus clade were also isolated, with V. crassostreae as the most represented in advanced stages of disease. We cannot confirm that the observed microorganisms were associated with tissue damage and their contribution to the disease outcome remains unclear as they could be just opportunistic in the lesions. This preliminary study on a wild population highlights the importance of morphological analysis (histopathology and SEM) coupled with microbiome and metagenome analyses in sea urchin disease investigations. Moreover, we suggest also performing transmission electron microscopy (TEM), experimental challenges and in situ hybridization methods (ISH) to provide morphological evidence of potential infective agents. Future studies should also include histopathology of the test.