Types and quantities of coelomocytes in three common tropical sea cucumber species

We examined the types of coelomocytes and their relative abundances in three common tropical sea cucumbers: Holothuria atra, Holothuria leucospilota, and Holothuria edulis (n=5 per species). The results showed that these species share the same coelomocyte types, totaling seven categories: phagocytes (subdivided into filopodial and petaloid forms), spindle cells, morula cells, crystal cells, lymphocytes (two morphotypes), hemocytes, and progenitor cells. However, significant interspecific variations were observed in the relative abundances of these cells, reflecting diverse immune strategies and ecological adaptations. Specifically, H. edulis was characterized by higher proportions of filopodial phagocytes (34.63%±0.38%) and morula cells (19.77%±0.60%); H. atra was relatively enriched in petaloid phagocytes (27.04%±0.29%), spindle cells, and lymphocytes; in contrast, H. leucospilota exhibited significantly higher proportions of crystal cells (34.00%±0.33%) and hemocytes (16.30%±0.57%). Progenitor cell abundance also varied among species, being highest in H. edulis (10.98%±0.23%), followed by H. atra (4.96%±0.23%), and lowest in H. leucospilota (3.32%±0.47%). Ancestral state reconstruction based on the COI gene suggests that while the core coelomocyte lineages are evolutionarily conserved within the genus Holothuria, specific branches exhibit lineage-specific evolution characterized by cell type heterogeneity.