Tube Feet

Being exclusively benthic animals, echinoderms have activities and adaptations that are correlated with a benthic existence. Most of these activities, such as attachment to the substratum, locomotion, handling of food and burrow-building, rely on adhesive secretions allowing the animal to stick to or to manipulate a substratum. In post-metamorphic echinoderms, these adhesive secretions are always produced by specialized organs, the podia or tube feet. These are the external appendages of the ambulacral system and are also probably the most advanced hydraulic organs in the animal kingdom. Tube foot attachment is typically temporary adhesion. Indeed, although tube feet can adhere very strongly to the substratum, they are also able to detach easily and voluntarily from the substratum before reinitiating another attachment-detachment cycle (Thomas and Hermans 1985; Flam-mang 1996)

Tube feet have diversified into a wide variety of morphotypes, which were classified by Flammang (1996) into disc-ending, penicillate, knob-ending, lamellate, ramified and digitate. In terms of adhesion, however, for practical considerations only disc-ending tube feet involved in attachment to the substratum and locomotion have been studied in detail. These tube feet consist of a basal extensible cylinder, the stem, and an enlarged and flattened apical extremity, the disc (Fig. 1A). Tube foot adhesive strength was evaluated by measuring their tenacity, which is the adhesion force per unit area and is expressed in Pascals (Pa). Tenacity of a single tube foot has been quantified in several species of asteroids and echinoids. The mean normal tenacities measured on a glass substratum were 170 kPa in Asterias vulgaris (Paine 1926), 198 kPa in Asterias rubens (Flammang and Walker 1997) and 59, 120 and 290 kPa in Arbacia lixula, Sphaerechinus granularis and Paracentrotus lividus, respectively (Santos 2003). Tenacities of whole individuals were also measured in the same echinoid species and respectively average 190, 260 and 330 kPa (Santos 2003). All these values are in the same range as those observed in other marine invertebrates known to adhere very strongly to the substratum (e.g. 230 kPa in limpets, 520 kPa in barnacles, 750 kPa in mussels; see Walker 1987 for review). Tube foot adhesive secretions therefore appear to be well tailored to provide an efficient attachment to the substratum, allowing echinoderms to resist hydrodynamically generated forces.

The histological structure of the tube feet is remarkably constant for all echinoderm species. Their tissue stratification consists of four layers: an inner myomesothelium surrounding the ambulacral lumen, a connective tissue layer, a nerve plexus and an outer epidermis covered externally by a cuticle (Flammang 1996). At the level of the tube foot tip, these tissue layers are specialized in adhesion and sensory perception: the connective tissue layer and the nerve plexus are thickened, and the epidermis is differentiated into a well-developed sensory-secretory epithelium. The latter comprises two types of secretory cells: non-ciliated secretory cells (NCS cells) enclosing large heterogeneous granules and ciliated secretory cells (CS cells) enclosing small homogeneous electron-dense granules (see Flammang 1996 for review). In some species, two types of NCS cells co-occur in the sensory-secretory epidermis. The study of the ultrastructure of these different types of secretory cells during a complete cycle of attachment-detachment of the tube foot in A. rubens (Fig. 2) demonstrated that they function as a duo-gland adhesive system as originally proposed by Hermans (1983), and in which NCS cells release an adhesive secretion and CS cells a de-adhesive secretion (Flammang et al. 1994; Flammang 1996). The adhesive is present as a thin film between the tube foot cuticle and the substratum and, when detachment occurs, it takes place at the level of the outermost layer of the cuticle, the fuzzy coat, leaving the adhesive material strongly attached to the substratum as a footprint (Fig. 1B,C) (Flammang 1996). In A. rubens, polyclonal antibodies have been raised against footprint material and were used to locate the origin of footprint constituents in

Fig. 1A-C. Tube foot adhesion and adhesive in the echinoid Paracentrotus lividus (originals). SEM photograph of a disc-ending tube foot (A), and 3-D and 2-D topographical views of adhesive footprints deposited on a glass substratum by this tube foot (B and C, respectively). D Disc; S stem; SSE sensory-secretory epidermis

Fig. 2A-D. Ultrastructure of the tube foot sensory-secretory epidermis in the asteroid Asterias rubens during an attachment-detachment cycle (originals). TEM photographs showing non-ciliated secretory cells (A) and ciliated secretory cells (B) before attachment. During attachment to the substratum, non-ciliated secretory cells release some of their granules (C), while ciliated secretory cells remain unchanged (not illustrated). After voluntary detachment from the substratum, ciliated secretory cells have released their most apical granules (D). CS Ciliated secretory cell; CU cuticle; FC fuzzy coat; NCS1 type 1 non-ciliated secretory cell; NCS2 type 2 non-ciliated secretory cell; P pore; SCC subcuticular cilium; SG secretory granule

Fig. 2A-D. Ultrastructure of the tube foot sensory-secretory epidermis in the asteroid Asterias rubens during an attachment-detachment cycle (originals). TEM photographs showing non-ciliated secretory cells (A) and ciliated secretory cells (B) before attachment. During attachment to the substratum, non-ciliated secretory cells release some of their granules (C), while ciliated secretory cells remain unchanged (not illustrated). After voluntary detachment from the substratum, ciliated secretory cells have released their most apical granules (D). CS Ciliated secretory cell; CU cuticle; FC fuzzy coat; NCS1 type 1 non-ciliated secretory cell; NCS2 type 2 non-ciliated secretory cell; P pore; SCC subcuticular cilium; SG secretory granule the tube feet (Flammang et al. 1998a). Extensive immunoreactivity was detected in the secretory granules of both NCS1 and NCS2 cells, suggesting that their secretions make up together the bulk of the adhesive material. No immunoreactivity was detected in the secretory granules of CS cells and the only other structure strongly labelled was the fuzzy coat. This pattern of immunoreactivity suggests that secretions of CS cells are not incorporated into the footprints, but instead might function enzymatically to jettison the fuzzy coat, thereby allowing the tube foot to detach (Flammang 1996; Flam-mang et al. 1998a).

Footprints in echinoderms consist of a sponge-like material deposited as a thin layer on the substratum (Thomas and Hermans 1985; Flammang 1996; Flammang et al. 1998a). Although their diameter is easily measured after staining of the adhesive material (Flammang 1996), footprint thickness is difficult to estimate. Using an interference-optical profilometer, which generated three-dimensional images of the footprint surface, the mean maximum footprint thickness was found to be 100 nm in the echinoid P.lividus and 230 nm in the asteroid A. rubens (Figs. 1B,C; Santos, Gorb and Flammang, unpubl. data). The chemical composition of the footprint material was analysed in A. rubens. Leaving inorganic residue apart, this material is made up mainly of proteins and carbohydrates (Flammang et al. 1998a). The protein moiety contains significant amounts of both charged (especially acidic) and uncharged polar residues as well as cysteine. The carbohydrate moiety is also acidic, comprising both uronic acids and sulphate groups.Adhesive interactions with the substratum could be through ionic bonds, presumably involving the acidic residues of both carbohydrate and protein moieties (Waite 1987), whereas cohesive strength could be achieved by intermolecular disulphide bonds. So far, A. rubens is the only species in which the tube foot adhesive has been studied biochemically and nothing is known about other echinoderm species. Regarding the asteroids, however, a comparative immunohistochem-ical study of the tube feet from 14 species representing five orders and ten families revealed that the adhesives of all these species are closely related, and this independently of the taxon considered, of the species habitat and of the tube foot morphotype or function (Santos et al. 2005).

Comparison of the composition of the temporary adhesive of A. rubens with that of other marine invertebrates shows that it is closer to the transitory adhesive of limpets, also composed of an association of proteins and acidic glycans, than to the permanent adhesives of mussels and barnacles, made almost exclusively of proteins (Flammang et al. 1998a; Flammang 2003). A similar relationship between non-permanent adhesives is also observed when one compares the amino acid compositions of the temporary adhesive from asteroids with the temporary adhesive from monogenean flatworms and the transitory adhesive from limpets (see Flammang 2003). This relationship indicates convergence in composition because of common function (i.e. nonpermanent attachment to the substratum) and selective pressures.

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