Most of each valve was then snapped off, leaving only a small area of shell surrounding the chondrophore. A thin section of the chondrophore was removed by cutting into the umbo along the axis of growth with a pair of circular blades spaced 2 mm apart and removing the slice between the blades for analysis (Almeida & Sheehan 1997).
Looking at a surfclam chondrophore cross section, the shell material deposited consists of opaque zones and narrower hyaline zones which appear as light and dark bands, respectively, under reflected light.
For each of the 1,613 shells with an acceptable chondrophore image, the straight-line distance from the umbo to the distal edge of each annulus was measured using ImagePro imaging software (Fig.
It has been shown that growth increments within a chondrophore section correspond to those in the entire valve in number and relative position (Wagner 1984, Ropes 1985, Picariello 2006), but SL is the conventional measure of clam size used for growth analysis.
These methods were: (1) reading the external rings on the shell; (2) counting the growth rings on thin sections of the hinge plate (chondrophore): and (3) counting growth rings of whole shell thin sections.
The number of translucent annuli was counted in the whole shell section and the chondrophore.
The three methods for age determination were compared: counting external rings on the shell; internal rings in whole shell thin sections: and internal rings in thin sections of the chondrophore.
To examine precision, band counts on the chondrophore thin sections were made independently by two readers for each specimen without prior knowledge of the cockle's length or of previous counts.
The MI is defined here as the distance between the chondrophore margin and the distal edge of the last-completed opaque increment, and the PI is the distance between the distal edges of the outermost two opaque increments (Fig.