Diapositive 1 - de l'Université libre de Bruxelles

Chapitre III2.6Skeletal Sr/Ca (mmol/mol)2.52.42.32.22.1y = 0.0183x + 1.8239R 2 = 0.808; p < 10-6y = 0.0173x + 1.7779R 2 = 0.730; p = 6 10-6220 25 30 35 40Salinity (psu)Salinity (psu)Figure 25: Sr/Ca ratio (mmol/mol) in the skeleton of Aterias rubens grown in aquarium according tosalinity and temperature of growth (———11°C;---------18°C); equations and parameters of thelinear regressions at each temperature (———11°C;---------18°C).DISCUSSIONMg/Ca ratio in different ossicle typesIn contrast to sea urchins, no strong differences of Mg/Ca ratio between ossicles weredetected in Asterias rubens. For example, the test of Echinometra mathaei contains 16.2%MgCO 3 (w/w) and its spines 7.5% MgCO 3 (w/w) (Weber 1969). In A. rubens, the highestmagnesium difference was found between adambulacral plates, 9.3% MgCO 3 (w/w), andambulacral plates, 8.0% MgCO 3 (w/w). Furthermore, no difference between oral andaboral faces and between more or less external ossicle positions were detected. Thisallows the use of the whole starfish skeleton for the Mg/Ca ratio analyses.Size (age) effect on the Mg/Ca and the Sr/Ca ratiosThe Mg/Ca ratio is independent of the starfish size (age), making comparisons betweendifferent populations robust.The Sr/Ca ratio was negatively related to the starfish size, as already shown in planktonicforaminifera, other organisms with a high-magnesium calcite skeleton (Elderfield et al2002). Growth rate is probably the mechanistic basis of this relation. Younger (smaller)starfish grow much faster than older ones and strontium distribution coefficients areknown to increase with increasing precipitation rate (Lorens 1981). On the contrary, nosize effect was detected on the Sr/Ca ratio in the skeleton of the experimental specimens,69