Growth increment patterns of Fulvia mutica were examined, and were used in age determination.
Fine growth increments are visible both in the cross-sections of the shell and chondrophore. Growth increments in the shell run approximately parallel to the inner surface, while those in the chondrophore shows a concentric ring pattern like tree rings or fish otolith. It is easier to measure the latter than the former, because growth lines in the shell are too long to trace, and because the outer layer of the shell in which the growth increments are visible is too thin in the umbo. The growth increment in the chondrophore is considered more reliable in age determination.
Chondrophore growth is allometric with respect to shell height, and a high correlation between growth line number and chondrophore length has been confirmed. Given that one increment is formed daily, the growth of the cockle was estimated as a following formula,
SHt=110.33[1-exp{-1.86×10-3(t+43.87)}]
where SHt represents shell height in mm at age t in days. This result corresponded with the growth estimated from size frequency analysis. It is clear that growth increments in the chondrophore are useful in age determination of the cockle.