The new Lessepsian entry Brachidontes pharaonis (Fischer P., 1870) (Bivalvia, Mytilidae) in the western Mediterranean: A physiological analysis under varying natural conditions

Abstract

The feeding behavior of Brachidontes pharaonis (Mollusca, Bivalvia), a new Lessepsian entry in the western Mediterranean, living in a cooling vat of a saltworks system in western Sicily, was assessed by estimating its physiological rates throughout a 6 month-long study (May 1998 to March 1999). Clearance, filtration, ingestion, and food absorption rates were estimated using the biodeposition method and the results correlated to variations in temperature, salinity, and quality and quantity of available food. Measured seston concentrations were on average 81.5 ± 95.5 mg L-1, its labile fraction (estimated as the sum of particulate lipids, carbohydrates and proteins) was on average 0.55 ± 0.07 μg L-1, representing only 15% of the total organics. Phytoplankton biomass, as suspended chlorophyll-a, was on average 0.88 ± 0.4 μg L-1. Mean weight standardised rates of Brachidontes pharaonis were clearance rate 1.64 ± 0.82 l h-1, filtration rate 110 ± 107 mg h-1 of total suspended material, and egestion rate 0.60 ± 0.16 mg material h-1. There was a mean selection efficiency of 0.50 ± 0.22. Ingested organic matter varied between about 2 mg h-1 and 270 mg h-1, and food absorption efficiency ranged between 0.1 and 0.99. The Brachidontes feeding process seems to be regulated at the initial filtration stage, and most of the control determines the quantity of absorbed ration. Clearance rate is maintained independent of changes in water temperature and salinity but reflects fluctuations in the quantity and quality of available food. Varying the rate of pseudofaeces production regulated ingestion rate, although this mechanism be fairly inefficient as a response to local environmental conditions. Absorption efficiencies may be sensitive to the balance of biochemical components. The physiological plasticity of B. pharaonis as expressed in this study is believed to have played a major role in its ability to reach the western Mediterranean.