F. Laruelle
Abstract:
Introduced in Europe in the seventies, R. philippinarum had spread along the Atlantic coast, due to transfer of individuals for aquaculture. Firstly restrained to aquaculture beds, the clams naturally recruited in various sites. In Brittany, this natural settlement has been low in the Bay of Brest while clams proliferated in the Morbihan Gulf so that a fishery developed in this site. R. decussatus, a native species phenotypically close to R. philippinarum, shares somehow the same biotope, but does not show the same ability to colonize the environment. We therefore decided to study the reproduction of these species at both the individual and population level.
As the gonad is diffused among other tissues such as muscles, digestive gland, and digestive ducts within the visceral mass, classical condition indices appear not to give an accurate picture of the reproductive pattern. The other tissues may vary concomitantly, thus inducing bias to the measure. For a better understanding of the reproduction in these two species, gonad filling ratios, oocyte diameters and gonad volume have been estimated using image analysis throughout a reproductive season.
The filling ratio culminates higher in R. philippinarum (45-50%) than in R. decussatus (35%) and the occupation by oocytes is superior in R. philippinarum throughout the breeding season. This ratio reveals the same pattern than the condition index concerning the spawning pattern in R. philippinarum whereas it does not in R. decussatus. In this species, it shows a regular decrease in the mean filling ratio corresponding to the time spread release of gametes, at the population and/or individual level.
The exotic species, R. philippinarum, known to have a higher metabolism than the native one, R. decussatus in term of filtration rate and growth, appear to invest much more energy in its reproduction, regarding to the volume of gonad, the gonad filling ratios, the fecundity and the length of the breeding season.
(UMR CNRS 6539, Institut Universitaire Europeen de la Mer, UBO-Place Nicolas Copernic, Technopole Brest-Iroise, 29280 Plouzane, France)