Geographic variation of body morphology of the Atlantic bluefin tuna, (Thunnus thynnus, Linnaeus, 1758)

Abstract

Geometric morphometric methods were used to explore body shape morphology in 260 Atlantic bluefin tuna, Thunnus thynnus, collected in Sardinia (Western Mediterranean) during the breeding phase and in the Bay of Biscay (North Eastern Atlantic) during the feeding phase. The shape of each specimen was captured by high resolution digital images and recording the 2-D coordinates of seven morphological landmarks. A general procruste analysis (GPA) was applied in order to eliminate any morphological variations resulting from size, position or orientation of specimens. A thin plate-spline (TPS) method was then used to provide a graphical representation of the shape conformation between two sets of data. Results of the regression model between the direct and indirect measurements accounted for a R-2 = 0.98. The Principal Components Analysis shows differences linked to the two sampling areas, accounting for 37\% and 19.97\% of the body shape variation in the first (PC1) and second (PC2) principal component, respectively. Specifically, the deformation grid projection highlights the major differences regarding the anterior-ventral part of the body (landmark 5-6-7). These differences might not necessarily be linked to an actual population substructure. Instead, it was hypothesized that such body shape differences were due to the diverse life phases during which specimens were collected, since the reproductive specimens show a `pot-bellied' shape, which was larger than for the feeding specimens that showed a `slimmer' shape. Analyses of likely sexual dimorphism conducted on Sardinian specimens did not reveal any significant differences; whereas body shape differences related to the pre- and post-reproductive sizes were detected.