Fine structure of the integument, tentacles, podia, and papillae of Holothuria (Platyperona) sanctori (Holothuroidea, Echinodermata) from the west Algerian coast

The ultrastructure of sea cucumber organs has long interested researchers, as it provides insights into the behaviour, physiology, ecology and evolution of these organisms. In this study, we describe the fine structure of the integument, buccal tentacles, podia, and papillae of Holothuria (platyperona) sanctori, using both light and electron microscopy. A thin layer known as the cuticle covers all the organs. The cell composition of the epidermis varies between organs. The integument is primarily composed of support cells. The buccal tentacles feature a sensory-adhesive system that presumably allows them to detect and collect food particles, while the podia exhibit a duo-glandular adhesive/de-adhesive system that facilitates reversible attachment to the substrate. The papillae are highly innervated, suggesting a sensory role. The mesothelium of the ambulacral appendages consists of myoepithelial and peritoneal cells. Beneath the epidermis, the dermis comprises an inner dense and an outer loose connective tissue, both rich in collagen fibers; the latter also houses the endoskeleton. This endoskeleton composed of numerous tables underlying epidermal tubercles represents one of the most distinctive features of H. sanctori when compared to other holothuriid species.