Evidence for multiple year classes of the giant Australian cuttlefish Sepia apama in northern Spencer Gulf, South Australia

摘要

Giant Australian cuttlefish form a mass spawning aggregation at a single site in northern Spencer Gulf (NSG) in South Australia every austral winter. Samples of cuttlefish were collected from this region over three consecutive years. Analysis of regular growth increments in the cuttlebones of these individuals, revealed a polymorphism in growth pattern for both sexes. Three distinct “bone patterns” were identified based on the variation in increment widths over the lengths of the bones. All bones analysed conformed to one of the three bone patterns, and the increment width patterns were consistent between years. Interpretation of the patterns, suggested that Sepia apama have two alternative life cycles. The first involves rapid juvenile growth during the first summer after hatching, with maturity reached within 7–8 months. These individuals return to spawn in their first year as small individuals. The second life cycle involves much slower juvenile growth during the first summer, with maturity deferred until their second year, when they return to spawn as much larger individuals. Thus, the age compositions of populations of S. apama in the NSG appear to consist of two year classes for both sexes.