Pinna nobilis L., 1758 age determination by internal shell register
The age of the Mediterranean endemic Bivalve mollusc Pinna nobilis is estimated using the total size and the number and position of posterior adductor muscle scars (PAMS): a series of straight lines and rings observed in the dorsal nacre-lobe. Nevertheless, the position of PAMS of adult individuals inhabiting the same population and depth is highly variable. To assess the source of this variability, the shells of nine individuals were cut into radial sections along PAMS and the inner register was studied. The cuts showed clear marks of the inner register formed around the PAMS composed by a short nacre tongue introduced within the calcitic layer and two calcitic strips of differing colour (bright and dark). The three components are directed towards the posterior of the shell. The marks of the inner register coincide only with the straight lines of PAMS, but are observed in an anterior position further than the straight lines and rings, and in a posterior position further than the rings. Also, some of the most posterior marks can be observed one under the other in the oldest individuals. The existence of the inner register’s marks in a position further anterior than the straight lines and rings implies that a variable number of PAMS are obscured beneath nacreous deposits. Eventually, we advise against the utilization of PAMS to study P. nobilis age. We instead recommend the use of the inner register as it records more accurately the life history of the individuals, and data indicates that each mark is formed annually.
KeywordsCalcite Internal Register Adult Individual Empty Shell Nacreous Layer
We are grateful to J.G. Carter, N. Vicente, A.M. García-Carrascosa, A.L. Peña Cantero and M. Goodstadt for their support and constructive criticism of the manuscript and Araceli Terrones, for her help with the processing of the shells. The research was partially funded by the project KGL2005-015201BTE of the Spanish Science Education Ministry.
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