Supercoil of collagen fibrils in the integument of Alvinella, an abyssal annelid

The polychaete annelid Alvinella pompejana was discovered near the hydrothermal vents, recently explored in the Eastern Pacific Ocean. This worm is protected by a cuticle deeply transformed over certain areas of the body and some changes are due to the presence of a very special bacterial flora. The present work however deals mainly with the supercoiled collagen fibrils, which are well visualized in thin sections observed by transmission electron microscopy. This character strongly differentiates this species from other annelids and worms in general, the cuticle of which includes straight and apparently non-coiled collagen fibrils. This indicates that fibrils are extensible in Alvinella, possibly under physiological conditions, and that internal pressure and local volume variations are regulated according to principles which depart from what is recognized in other worms, where cuticular fibrils are considered as inextensible. Possible models of this cuticle are discussed and particularly aspects which show a relationship with certain liquid crystals. Very different factors may be involved in morphogenesis of such cuticles: microvilli distribution, self-assembly of collagen fibrils, mechanical constraints. An appendix recalls some classical data on worm cuticle geometry and presents an estimate of volume variations resulting from coiling of fibrils.