AGE GROUP DETERMINATION BY ANALYSIS OF THE CUTTLEBONE OF THE CUTTLEFISH SEPIA-OFFICINALIS L. IN REPRODUCTION IN THE BAY OF BISCAY

Very few hard pieces of evidence can be used to determine the age of t he cuttlefish Sepia officinalis. We have studied the cuttlebone of cut tlefish living in Southern Brittany over a complete life cycle (from 1 988 to 1990) in order to separate clearly the two age groups existing in this area. Cuttlebone is formed with successive calcium carbonate l amellae formation during growth. The rhythm of lamellae deposition is not constant and appears to be closely linked with temperature. It rea ches a maximum in summer (18.75 lamellae per month) and falls drastica lly in winter (1.6 lamellae per month). The lamellar thickness measure d on the mid-sagittal surface of the hypostracum shows characteristic zones of narrow lamellae. The first series of narrow lamellae appears at the 9th lamella and is formed just after hatching. The second one, seen only in group II breeders, is located between the 45th and 60th l amellae. This takes place in May when this group begins its first spri ng migration to the coast. This second set of narrow lamellae is also visible on the siphuncular surface of the phragmocone by an inflexion point. A final series of narrow lamellae is formed during the season o f reproduction and appears in both groups of breeders. During the bree ding season, the separation of the two year-classes of breeders is als o possible by counting the total number of lamellae formed. Group I ha s a minimum of 80 lamellae whereas group II has abour 120.