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

This study aimed to differentiate local and migratory spawninggroups of the shortfin squid Illex argentinus caught in Brazilianwaters using geometric and traditional morphometric methodsand size-selective processes. The back-calculated length distributionsreconstructed from daily growth increments deposited in thegladius allowed the identification of size-selective processes thatmay be related to different life history strategies. Landmark analysison body shape (geometric morphometric) revealed thatspawning groups presented significant ontogenetic variations interms of body outline. In addition, traditional morphometric methods,based on multivariate analysis, associated juveniles of theexpected migratory group (large size) and differentiated themfrom the local group individuals (small size). The changes inform, probably linked to the environmental gradients experiencedby individuals throughout ontogeny, were interpreted as adaptationsto improve swimming capacity. Migrant individuals havebeing differentiated from the smaller sizes group by body characteristics(broad fins, elongated and thicker mantle), which mayincrease the ability to perform long migrations.