No abstract

Bivalve closure responses to detect contaminants have often been studied in ecotoxicology as an aquatic pollution biosensor. We present a new laboratory procedure to estimate its potential and limits for various contaminants and animal susceptible to stress. The study was performed in the Asiatic clam Corbicula fluminea and applied to cadmium. To take into account the rate of spontaneous closures, we integrated stress problems associated with fixation by a valve in common apparatus and the spontaneous rhythm associated with circadian activity to focus on conditions with the lowest probability of spontaneous closing. Moreover, we developed an original system by impedance valvometry, using light-weight impedance electrodes, to study free-ranging animals in low-stress conditions and a new analytical approach to describe valve closure behavior as a function of response time and concentration of contaminant. In C. fluminea, we show that cadmium concentrations above 50 microg/L can be detected within less than 1 h, concentrations down to 16 microg/L require 5 h of integration time, and values lower than 16 microg/L cannot be distinguished from background noise. Our procedure improved by a factor of six the cadmium sensitivity threshold reported in the literature. Problems of field applications are discussed.

The life history of 15 giant clams, Hippopus hippopus, was studied in situ in the southern lagoon of New Caledonia; growth rate and animal behavior were studied both by sclerochronology and high-frequency noninvasive (HFNI) valvometry. Electrodes glued on each valve of each specimen recorded the shell-gaping behavior at 0.6-Hz frequency. A nonparametric regression model was used to model clam behavior. The daily increment thickness in the inner layer of five representative clams was measured. H. hippopus has its valves open during the day and partly closed during the night all year round, and shell growth is continuous. The cumulative growth using both techniques was similar, as was the mean daily thickness increment. The occurrence of one increment per day in H. hippopus shell was measured by valvometry. The five sclerochronological profiles were highly similar. Shell growth was significantly correlated to rising sea surface temperature (SST), up to 27uC. At the solar maximum, gaping behavior and increment thickness became erratic. SST-and solar irradiance-related stress could be related to physiological oxidative stress triggered by zooxanthellae symbionts. In the present context of globally increasing SST, our data indicate that the giant clams H. hippopus could live beyond their thermal comfort limits in summer in New Caledonia.