The shape of sagitta otoliths was used to compare individuals of little tunny (Euthynnus alleteratus) harvested on board commercial fishing vessels from the coastal areas along the Eastern Atlantic, including the Mediterranean Sea. Fish sampling and selection was designed to cover possible seasonal changes and tuna size. The research encompassed both morphometric and shape analyses of left sagittal otoliths extracted of 504 fish specimens. Four shape indices (Circularity, Roundness, Rectangularity, and Form-Factor) were significantly different between two groups, showing a statistical differentiation between two clear spatial units. The degree of divergence was even more pronounced along the rostrum, postrostrum, and excisura of the generated otolith outlines between these two groups. One group corresponds to the samples from the coastal areas in the Northeast Temperate Atlantic and Mediterranean Sea (NETAM Area) and a second group from the coastal areas off the Eastern Tropical Atlantic coast of Africa (ETA Area). This study is the first to use otolith shape to differentiate tunas from separate spatial units. These results could be used to re-classify previously collected samples and to correct time series of data collected.

Euthynnus(family Scombridae) is a genus of marine pelagic fish species with a worldwide distribution that comprises three allopatric species:E. alletteratus,E. affinisandE. lineatus. All of them targeted by artisanal and commercial fisheries. We analyzed 263 individuals from Atlantic and Pacific Oceans using two genetic markers, the mtDNA Control Region (350 bp) and nuclear calmodulin (341 bp). The results obtained challenge the phylogeny of this group. We found a deep genetic divergence, probably at species level, withinE. alletteratus, between the North Atlantic-Mediterranean and the Tropical East Atlantic. This deep genetic divergence was tested with several species delimitation methods. This complete phylogeographic association between the North Atlantic and the Tropical East Atlantic support the hypothesis of two cryptic species. In addition, population genetic heterogeneity was detected between the North East Atlantic–Mediterranean and North West Atlantic regions. Our results indicate two scales of differentiation in what is currently considered a single population. Accordingly, for management purposes, the populations ofE. alletteratus, should be divided into a minimum of three management units. On the other hand, the high level of differentiation found inE. alletteratuscontrasts with the shallow genetic divergence ofE. affinisandE. lineatus.