Morphometric variation in Loligo forbesi and Loligo vulgaris: regional, seasonal, sex, maturity and worker differences

Morphometric variation in the squid Loligo forbesi was investigated to quantify effects of region, season, sex and maturity. Practical exercises were carried out using this species and the congeneric Loligo vulgaris to evaluate differences in measurements by different workers. The utility of meristic characters was also examined. Consistent significant differences were found between measurements by different workers, although this problem may be reduced with experience and many of the differences were small compared with observed differences between areas, particularly differences between Loligo forbesi from the Azores and elsewhere (Scotland, Spain, Portugal). Body shape in Loligo forbesi varied significantly in relation to maturity stage and, to a lesser extent, with season. Multivariate analysis of morphometric characters for samples collected in Scottish waters over 12 months revealed no consistent differences between localities. In contrast, there were marked differences between Loligo forbesi from the Azores and those from UK waters. If future studies on geographic variation in loliginids are to make use of morphometric characters, they should be based on simultaneous sampling to minimise effects of season and maturity, and all measurements on a single character should be made by a single worker.