Proposed revision of ‘crystal cells’ as phagocytised metabolic by-products in holothuroid echinoderms

Abstract Crystal cells in echinoderms have been described in the literature as a type of coelomocyte (immune cell) containing a crystalline structure. Their putative function has been widely ascribed to osmoregulation but this assertion was never robustly tested. In the present paper, a review of crystal cells and crystalline structures provides evidence supporting a different identity and function. First, the same microcrystals can be found either free or encapsulated by a cell membrane (with or without a structure resembling a nucleus). Specifically, they are typically non-encapsulated when found inside tissues of internal organs and encapsulated when free floating in the hydrovascular or perivisceral fluids. Although usually individually packaged, microcrystals were also observed encapsulated in groups of up to four, with or without other particles. Their morphological features, coupled with their chemical and optical properties, match that of microcrystals of uric acid, described in other phyla, including chordates. Two pathways of excretion of these crystallised by-products were evidenced: rejected with sea water out of the respiratory tree and expelled via transrectal coelomoducts among coelomocyte aggregates. Overall, the present synthesis strongly supports that ‘crystal cells’ historically described as a distinct type of coelomocyte in holothuroids are phagocytes that have engulfed uric acid microcrystals generated as waste by metabolic activities.