Analysis of shape variability and life history strategies of Illex argentinus in the northern extreme of species distribution as a tool to differentiate spawning groups

Abstract

ABSTRACTThis study aimed to differentiate local and migratory spawning groups of the shortfin squid Illex argentinus caught in Brazilian waters using geometric and traditional morphometric methods and size-selective processes. The back-calculated length distributions reconstructed from daily growth increments deposited in the gladius allowed the identification of size-selective processes that may be related to different life history strategies. Landmark analysis on body shape (geometric morphometric) revealed that spawning groups presented significant ontogenetic variations in terms of body outline. In addition, traditional morphometric methods, based on multivariate analysis, associated juveniles of the expected migratory group (large size) and differentiated them from the local group individuals (small size). The changes in form, probably linked to the environmental gradients experienced by individuals throughout ontogeny, were interpreted as adaptations to improve swimming capacity. Migrant individuals have being differentiated from the smaller sizes group by body characteristics (broad fins, elongated and thicker mantle), which may increase the ability to perform long migrations.