Changes in podial skeletons during growth in the echinoid Hemicentrotus pulcherrimus

A feature of the Echinodermata is the presence of a water vascular system; the tube feet, or podia, are the terminal structure of this system and are an organ used for movement, attachment to a substrate, and feeding. The tube foot possesses calcite skeletons in the sucker (the disk skeleton consisting of rosettes and frames) and basal connective tissue (ossicles); however, details of the skeletal elements have not yet been studied in detail. We analyzed the relationship between morphology and the amount of skeleton in the tube feet and other regions by investigating growth in the echinoid Hemicentrotus pulcherrimus (family Strongylocentrotidae). The tube foot disk skeleton consisted of three or four three skeletal elements in a rosette with some distal projections. Approximately 98% of tube feet ossicles showed a ‘C’ shape, while the remainder exhibited an ‘S’, ‘bow’ or ‘triradiate’ shape ( n = 24 sea urchins). The tube feet in the oral region possessed bigger distal skeletons and approximately twice the number of ossicles than those of the lateral and aboral regions. With growth, the diameter of the distal skeleton increased to four skeletal elements in a rosette. The number of ossicles in the tube feet increased significantly with a test diameter up to 20–30 mm, and thereafter decreased, but beyond a test diameter of 35 mm the number of ossicles again increased. These results suggest that test growth may cause the observed fluctuation in the number of tube feet ossicles, and that sea urchins accumulate calcium carbonate in the tube feet skeleton as the test grows.