Particularit�s microstructurales du squelette de Paracentrotus lividus et Arbacia lixula: Rapports avec l'�cologie et l'�thologie de ces �chino�des

Abstract

Scanning-electron-microscope investigations on the test microstructure of two regular sea urchins, Paracentrotus lividus (Lamarck) and Arbacia lixula Linne have been especially devoted to the sutures between test plates, and the spines. Some features of both these components may be related to ecological and ethological differences between the two species. It is well documented that the structure of the sea urchin's test plates consists of a meshwork of calcareous trabecules embedded within the mesenchyme. The suture between the two lines of plates of each radial or intertradial zone exhibits a gap which plays some part in the process of the plates' growth. However, the gap may also constitute a complementary “stress-breaker” of mechanical forces (such as waves) which are exerted upon the test in the natural environment. In A. lixula this gap is so wide and the height: diameter ratio so low, that this species is particularly well fitted to bear physical stress and force on its apical region: since A. lixula almost exclusively inhabits vertical or subvertical rocky substrates at 2 to 15 m depth, where wave action is mainly exerted perpendicular to the substrate, these particular features of sutures and test shape may be a morphofunctional adaptation to this habitat. The sutures of P. lividus exhibit a narrower gap, making this species less able to bear strong apical pressure; consequently, P. lividus usually occurs on exposed horizontal or gently inclined substrates, but also inhabits sea-grass beds as deep as 15 to 20 m. The spines of P. lividus bear deep longitudinal grooves with lateral teeth, which seem especially fitted for collecting and transporting organic particles from the top of the spines to the apical region of the test where they are digested and assimilated by coelomocytes and epithelial cells. In the natural environment, the slightest water motion provides the spines with suspended particles; in extremely sheltered places or in aquaria however, the spines actively collect particles. Therefore, P. lividus populations can thrive in places where food resources other than suspended particles are scarce. The grooves on the spines of A. lixula are less marked, and thus unsuitable for collection of suspended particles. This species can therefore fulfil its energy requirements only by grazing and absorption of dissolved material. It appears that although belonging to the same biocoenosis, P. lividus and A. lixula do not occupy the same ecological niche. Their potential utilization in either urban or chemical pollution monitoring studies is discussed.