Comparative histological and immunohistochemical study of sea star tube feet (Echinodermata, Asteroidea)

Adhesion in sea stars is the function of specialized structures, the tube feet or podia, which are the external appendages of the water-vascular system. Adhesive secretions allow asteroid tube feet to perform multiple functions. Indeed, according to the sea star species considered, the tube feet may be involved in locomotion, fixation, or burrowing. Different tube foot shapes usually correspond to this variety of function. In this study, we investigated the variability of the morphology of sea star tube feet as well as the variability of the composition of their adhesive secretions. This second aspect was addressed by a comparative immunohistochemical study using antibodies raised against the adhesive material of the forcipulatid Asterias rubens. The tube feet from 14 sea star species representing five orders and 10 families of the Class Asteroidea were examined. The histological study revealed three main tube foot morphotypes, i.e., knob-ending, simple disc-ending, and reinforced disc-ending. Analysis of the results suggests that tube foot morphology is influenced by species habitat, but within limits imposed by the evolutionary lineage. In immunohistochemistry, on the other hand, the results were very homogeneous. In every species investigated there was a very strong immunolabeling of the adhesive cells, independently of the taxon considered, of the tube foot morphotype or function, or of the species habitat. This indicates that the adhesives in all the species considered are closely related, probably sharing many identical molecules or, at least, many identical epitopes on their constituents.