Microstructure adaptative des radioles deParacentrotus lividus (Echinodermata: Echinoidea) en milieu eutrophis� par des eaux us�es

Abstract

Recent investigations (1984) on the biology and morphometry of populations of the edible sea-urchinParacentrotus lividus (Lamarck) exposed to massive domestic pollution from the Cortiou sewage outlet (Marseille, France), revealed a significant lengthening of sea-urchin spines at sites located at a distance of 500m (Station 0) and 1 200 m (Station 1) from the outlet. At Station 0, spine length (SL) is nearly half as long as the diameter (D) of the test, resulting in a higher SL:D ratio here-0.470 as opposed to 0.360 in unpolluted areas (e.g. Station 3, 5 200 m from the outlet). This spine lengthening appears to be a morphofunctional adaptation, enabling the sea-urchins to adopt sestonophagy in order to benefit better from the plentiful resources of both particulate and dissolved organic matter arising from the outfall; examination of pellets filling the stomachs and guts of the sea-urchins corroborated this. SEM examination revealed that spines of specimens collected at Station 0 are more “porous” than those from Station 3 and their microarchitecture is better adapted to capturing suspended particulate matter: the ridges are narrower, the grooves wider, and the latter display a dense and more coarsely-meshed network. The axial medulla exhibits true and regularly arranged channels with highly structured walls. The ratio dry wt of spine: dry wt of test revealed thatP. lividus spines from Station 0 (ratio= 1.2) are lighter than those from Station 3 (ratio=1.4), reflecting the more porous microstructure and lighter stereome of the Station 0 sea-urchins. Previous measurements to determine the degree of eutrophication in the area polluted by the Marseilles urban sewage outfall revealed carbon content, as % of total sediment, as high as 36% in newly deposited sediment (100 m from the outlet), 16.7% at Station 0 (500 m) and 2.6% near Station 3 (about 5 200 m). Further experiments using sediment traps indicated that the organic matter tends to accumulate chiefly along the shoreline, whereas the suspended mineral fraction tends to be deposited to the south, i.e., further offshore. Thus, theP. lividus population at Station 0 exhibits a very unusual ecological plasticity, having developed a very efficient adaptative microstructure in order to meet its energy requirements despite the presence of various chemical pollutants.