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

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 physiol ogical rates throughout a 6 month-long study (May 1998 to March 1999). Clea rance, filtration, ingestion, and food absorption rates were estimated usin g the biodeposition method and the results correlated to variations in temp erature, salinity, and qualify and quantity of available food. Measured ses ton concentrations were on average 81.5 +/- 95.5 mg L-1, its labile fractio n (estimated as the sum of particulate lipids, carbohydrates and proteins) was on average 0.55 +/- 0.07 mug L-1, representing only 15% of the total or ganics. Phytoplankton biomass, as suspended chlorophyll-a, was on average 0 .88 +/- 0.4 mug L-1. Mean weight standardised rates of Brachidontes pharaon is were clearance rate 1.64 +/- 0.82 1 h(-1). filtration rate 110 +/- 107 m g h(-1) of total suspended material, and egestion rate 0.60 +/- 0.16 mg mat erial h(-1). There was a mean selection efficiency of 0.50 +/- 0.22. Ingest ed organic matter varied between about ? mg h(-1) and 270 mg h(-1), and foo d absorption efficiency ranged between 0.1 and 0.99. The Brachidontes feedi ng process seems to be regulated at the initial filtration stage, and most of the control determines the quantity of absorbed ration. Clearance rate i s maintained independent of changes in water temperature and salinity but r eflects fluctuations in the quantity and quality of available food. Varying the rate of pseudofaeces production regulated ingestion I atc, although th is: mechanism be fairly inefficient as a response to local environmental co nditions. Absorption efficiencies may be sensitive to the balance of bioche mical components. The physiological plasticity of B. pharaonis as expressed in this study is believed to have played a major role in its ability to re ach the western Mediterranean.