EVALUATION OF GLADIUS DORSAL SURFACE-STRUCTURE FOR AGE AND GROWTH-STUDIES OF THE SHORT-FINNED SQUID, ILLEX ILLECEBROSUS (TEUTHOIDEA, OMMASTREPHIDAE)

The potential use of growth increments deposited on the gladius of the short-finned squid (Illex illecebrosus) for age determination and gro wth studies was investigated. The gladius lies along the dorsal midlin e of the mantle and grows from the fins towards the head by the additi on of layers of chitin. Growth increments were observed on the dorsal surface of the gladius and could be consistently enumerated. The frequ ency of deposition approximated statolith incremental deposition rates and the daily nature of gladius growth increments during the adult ph ase was supported. Total age could not be obtained by gladius growth i ncrement counts on the dorsal surface because, as the gladius grows, t he posterior zone becomes progressively vague and the early growth inc rements are masked. However, gladius increments were highly correlated with mantle growth and provided accurate estimates of size-specific g rowth. Both group and individual growth trajectories were reconstructe d. The latter seemed to be sensitive to feeding events, temperature fl uctuations, and maturation processes in captive squids. The potential for reconstruction of individual growth histories and for the study of growth patterns in wild populations is discussed.