Stock structure analysis and species identification

There is a particular lack of data for exploited squid on fundamental aspects of taxonomy, genetic population structure, locations and patterns of spawning and migrations, and factors affecting population dynamics. This review considers currently available information on species identity and stock structure in the genus Illex (order Teuthoidea), using predominantly ecological, parasitological, molecular genetic and morphometric data. Molecular genetic and ecological (distribution and abundance of life-history stages) approaches have so far been most useful at the population level, though species identity has been aided also by studies on squid morphology and parasite distributions. The majority of relevant population studies have been undertaken on Illex argentinus in the southwest Atlantic, where data strongly suggest the presence of two or more cryptic species. Indeed, cryptic speciation is evident among other ommastrephids (e.g. Martialia hyadesi) and loliginids (e.g. Nototodarus sloani, Loligo edulis). Despite the extensive migrations of these animals, gene flow appears to be sufficiently restricted to allow the accumulation of genetic and presumably life-history differentiation likely to affect recruitment dynamics. Thus, the feeding groups captured by most fisheries may not necessarily represent interbreeding units. There is an urgent need to integrate molecular, morphological and demographic studies to review teuthoid systematics. Illex typically display low levels of genetic variability (P, proportion of polymorphic loci (P(O,95)) = 0.172–0.280; HL mean observed heterozygosity per locus = 0.000– 0.011), which together with the characteristic population differentiation at the allozyme level, indicates a particular susceptibility to over-exploitation of weakly represented stocks. Microgeographic differentiation in allozymes on the scale of tens of km has occasionally been observed in I. argentinus, perhaps representing movements of squid schools. Ecological studies based on larval and pre-spawning adult distributions support the notion of a complex stock structure in the southwest Atlantic. Several spawning groups based on water depth and season of maximal spawning activity have been proposed, though the genetic integrity of such groups remains unclear. Among other teuthoids, both population homogeneity (indicating freely interbreeding groups) and heterogeneity (indicating some restriction to gene flow) has been disclosed, indicating a complex relationship between dispersal capacity, gene flow and population differentiation in these animals.