Gills of hydrothermal vent annelids: Structure, ultrastructure and functional implications in two alvinellid species

The anatomy, fine structure, diffusion distances and respiratory surface areas of the gills of two species of the polychaete family Alvinellidae have been investigated. Each gill consists of a stem on which are inserted two opposite rows of respiratory elements: flat sickle-shaped lamellae in Alvinella pompejana and cylindrical filaments in Paralvinella grasslei. Both lamellae and filaments have a ciliated mucous epidermis, a central layer of supporting and muscle cells and are devoid of coelomic cavity. Each respiratory element possesses one afferent and one efferent marginal vessel, united to each other distally, and connected proximally to separate longitudinal vessels running in the stem. Superficial parallel blood spaces connect the marginal vessels across the lamella or filament. Deeper, between the basal laminae of the epidermis and that of the central cell layer, a blood sinus is also present. The marginal vessels and the superficial blood spaces actually are intraepidermal extensions of this deep blood sinus. The diffusion distances are very small owing to the intraepidermal position of the respiratory blood spaces. The specific gill surface areas in A. pompejana and P. grasslei are the largest known today in polychaetes, respectively 12 and 47 cm² per g wet weight. The distinctive features of the gills are possibly related to a low oxygen content of the ambient seawater. Numerous crystalline granules scattered in the gill epidermis suggest that this epithelium has a detoxifying function.