Periodic increments in the jumbo squid (Dosidicus gigas) beak: a potential tool for determining age and investigating regional difference in growth rates

Age determination of the jumbo squid (Dosidicus gigas) has been successfully carried out over its geographic range based on the use of the rings present in the statoliths. In this study, we propose an additional method to identify ages of D. gigas off the Ecuadorian and Chilean Exclusive Economic Zones (EEZs) waters by using upper beak rostrum sagittal section (RSS) analysis. Our results show periodic growth increments and occasional checks located in the RSS surface. Growth increments meet at the internal rostral axis in the shape of chevron, and the marks in the hood are more discernable and clearer than in the crest. As has been suggested for octopi, checks are thought to represent temperature oscillation and reproductive events. Growth increments are laid down in the RSS daily, and numbers were similar to the numbers of increments used to determine age in statoliths. There was a significant regional difference in age but not in increment width when mantle length effects were considered, where squid off the warm Ecuadorian EEZ waters had a shorter lifespan and higher somatic growth in mantle length than those in the temperate Chilean EEZ waters.