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The concentrations of Cd, Cu, Fe, Pb and Zn were measured in holothuroids Holothuria tubulosa collected during different seasons and at several depths in three N W Mediterranean Posidonia oceanica meadows (Calvi, Corsica; Ischia, Italy; Marseille, France). Metals were found to be selectively distributed among the holothuroid body compartments (i.e. body wall, gut, and haemal system). Among the sources of variation taken into account, the major part of the variability observed in metal concentrations was associated to the body compartment factor, then to the seasonal one and, finally, to the geographical and bathymetric factors. Our results suggest that, if one uses H. tubulosa as a bioindicator species for surveying and monitoring metal contamination in the Mediterranean P. oceanica meadows, attention should be paid to compare only concentrations measured in the same body compartments of individuals collected during the same period of the year. Body compartments that should be preferably considered for metal analyses are the haemal system and, secondarily, the gut (i.e. the compartments concentrating metals most efficiently). Due to the peculiar ecological characteristics of H. tubulosa (i.e. a deposit feeder living in close relationships with the sediments), this species could be effectively used to complement the small set of bioindicators available so far for surveying metal contamination in the P. oceanica ecosystem (viz. the phanerogam P. oceanica itself and the grazing echinoid Paracentrotus lividus).

ABSTRACT-Asteroids Asterias r u b e r~s [Linneaus, 1758) were exposed In the field to various concentrations of waterborne or dietary Cd (from contam~nated mussels). Cadmium uptake, and subsequent loss, kinetics were studied for 45 d for each Dietary (110 pg Cd g-l dry ivt of the prey) and waterborne (5 and 20 py C d I-') C d was significantly accumulated in the 3 measured body compartments: pyloric caeca, body wall, and skeleton. Waterborne C d (1 p g C d I-') was also significantly accumulated in the body wall but not in the pyloric caeca or the skeleton. Dietary C d accumulation occurred in all body compartments. In the pyloric caeca, a steady state equilibrium was reached within 20 d, the concentration a t steady state ranging from 3 to 9 pg Cd g-' dry wt according to the exposure mode. T h e body wall and the skeleton accumulated Cd linearly during the exposure perlod at a rate of 0.07 to 0.25 p g C d g.' dry wt d-' according to the exposure mode. C a d m~u i n loss klnetics were fltted by Inverse exponent~al functions to all body compartments, except to the skeleton where the loss was generdlly nonsignificant. Results indicate that there is a Cd flux through the asteroid body from the digestive system to the body wall where Cd IS slo~vly incorporated to the skeleton. Concentration factors calculated for the pyloric caeca fit the range of values previously reported from laboratory experiments and thus validate them in field conditions. Thus A. rubens may be considered a valuable bioindicator of C d contamination.

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Echinoids Paracentrotus lividus (Lamarck, 1816) were contaminated with Cd (as CdC12) in field or laboratory conditions through 3 modes of exposure: (1) exposure via the sea water, ( 2 ) exposure via the food chain (Posidonia oceamca leaves), and (3) 'mixed-source' exposure via both sea water and food. Results indicate that Cd accumulation by P. lividusvaries with both the body compartment of the echinoid and the exposure mode to the metal. The digestive wall shows the highest Cd uptake rates regardless of exposure mode. Bioaccumulation from sea water is prevalent in all compartments. Cd accumulation in the digestive wall and body wall of echinoids directly exposed to Cd in sea water is probably a dose-dependent process. Echinoids take up Cd with a lower efficiency (up to a 65% decrease) when they are contaminated via both sea water and food than when they are contaminated via sea water only. We suggest that this phenomenon is due to an increase in both excretion and mucus production (impeding Cd absorption from sea water) tnggered by high Cd levels in the food. Cd loss rates are low in companson with uptake rates and are dependent upon the body compartment and exposure mode. The data indicate that Cd has a relatively long biological half-life (215 d) once mcorporated in echinoid tissues. It is concluded that P. lividus -particularly its digestive wall -could serve as a good bioindicator of Cd contamination in the environment.