Adhesion Mechanisms Developed by Sea Stars: A Review of the Ultrastructure and Composition of Tube Feet and Their Secretion

Abstract

Like all animals belonging to the phylum Echinodermata, sea stars are characterized by a water-vascular system. This sophisticated hydraulic system consists of a series of interconnected canals: a central ring canal that encircles the gut of the animal and from which arise a single axial canal, the stone canal, communicating with the external seawater through a perforated plate (the madreporite), and five radial canals which extend into each arm of the sea star. The radial canals lead to a multitude of specialized external appendages, the tube feet (Nichols, 1966). According to the sea star species, tube feet may be involved in one or several of the following functions: locomotion, fixation to the substratum, feeding and burrowing. These different functions are allowed by the mobility of the proximal part of the tube foot (the so-called stem) as well as by the attachment of the distal part of the tube foot to the substratum (Flammang, 1996). Tube foot attachment is temporary. Indeed, although tube feet can adhere very strongly to the substratum, they are also able to detach easily and voluntarily before reinitiating another attachment-detachment cycle (Thomas and Hermans, 1985; Flammang, 1996). Suction has long been regarded as a major mean of tube foot attachment in sea stars (Paine, 1926; Smith, 1937; Nichols, 1966). However, a number of more recent observations argue for an adhesive process principally, if not exclusively, mediated by the secretion of an adhesive material (Chaet, 1965; Thomas and Hermans, 1985; Flammang et al., 1994, 1998). For instance, sea star tube feet can adhere very strongly to meshed or perforated substrata, or with only the margin of their distal part, two situations which prevent the use of suction by tube feet (Thomas and Hermans, 1985; Flammang et al., unpubl. obs.).