Diversity and distribution of extracellular microcrystals in holothuroid echinoderms

Biomineralization research in echinoderms has been focused on skeletal structures, which provide strength and protection. Other minerals, specifically extracellular microcrystals, are described here for the first time in echinoderms. Six morphotypes were isolated from a holothuroid and classified into three chemical compounds. Uric acid crystals were associated with fluids from the hydrovascular system, perivisceral coelom and respiratory tree, while calcium carbonate crystals were detected in the epithelium of the respiratory tree and calcium oxalate dihydrate crystals were found in tissues of the cloaca, integument, Polian vesicle and tentacle. Uric acid crystals were mostly non-encapsulated in tissues, whereas, in fluids, they were often encapsulated by phagocytes, alone or in groups, suggesting that they are waste products. Calcium carbonate crystals in the respiratory tree and calcium oxalate dihydrate crystals in the cloaca suggest that they are being expelled from the body. Insofar as calcium oxalate dihydrate crystals could be involved in calcium regulation, we speculate that they might be crystallized and retained in ossicle-associated integumentary tissues and tentacles, which are susceptible to damage and could use them as a calcium reserve to synthesize and repair ossicles. Observations of microcrystals in different holothuroid species examined suggest their ubiquity in the class Holothuroidea and more generally, deuterostomes.