Natural populations of widely distributed organisms often exhibit genetic clinal variation over their geographical ranges. The European anchovy, Engraulis encrasicolus, illustrates this by displaying a two-clade mitochondrial structure clinally arranged along the eastern Atlantic. One clade has low frequencies at higher latitudes, whereas the other has an anti-tropical distribution, with frequencies decreasing towards the tropics. The distribution pattern of these clades has been explained as a consequence of secondary contact after an ancient geographical isolation. However, it is not unlikely that selection acts on mitochondria whose genes are involved in relevant oxidative phosphorylation processes. In this study, we performed selection tests on a fragment of 1044 bp of the mitochondrial cytochrome b gene using 455 individuals from 18 locations. We also tested correlations of six environmental features: temperature, salinity, apparent oxygen utilization and nutrient concentrations of phosphate, nitrate and silicate, on a compilation of mitochondrial clade frequencies from 66 sampling sites comprising 2776 specimens from previously published studies. Positive selection in a single codon was detected predominantly (99%) in the anti-tropical clade and temperature was the most relevant environmental predictor, contributing with 59% of the variance in the geographical distribution of clade frequencies. These findings strongly suggest that temperature is shaping the contemporary distribution of mitochondrial DNA clade frequencies in the European anchovy.

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Aim As part of an emerging effort to understand the role played by climatic fluctuations in shaping the geographical distributions and abundances of marine organisms, we examined the genetic patterns of leading-edge populations in the European anchovy, Engraulis encrasicolus, and its American counterpart, the morphologically similar silver anchovy, Engraulis eurystole, in the North Atlantic Ocean.Location Adults were collected from the western Atlantic, eastern Atlantic (from Norway to Ghana) and western Mediterranean.Methods A 1045 bp fragment of the mtDNA cytochrome b gene was sequenced (n = 312) and nine microsatellite loci were genotyped (n = 462) for anchovies from 13 locations across the temperate North Atlantic. Populations were surveyed for diversity and differentiation with a range of summary statistics. Multivariate discriminant analysis of principal components was employed to detect the number of genetic clusters in the data and assign individuals to populations based on their microsatellite genotypes. Historical demographic inferencesmismatch distributions and Bayesian skyline plotswere used to observe population size changes relating to climatic oscillations.Results Two mitochondrial clades were recovered, consistent with previous studies of E. encrasicolus, in which the frequency of each clade varied by latitude. Four genetic clusters corresponding loosely to large geographical regions were identified with microsatellite data. The north-western Atlantic E. eurystole was not reciprocally monophyletic for either mtDNA or microsatellite analyses and is probably conspecific with E. encrasicolus. Genetic diversity peaked in Iberian populations, but differences in genetic diversity were only statistically significant for the least diverse population, Tangier. The indications of demographic expansion were more pronounced in the southern clade and both mtDNA clades exhibited genetic diversity and expansion imprints that are likely to be older than climatic oscillations of the recent Pleistocene. Main conclusionsThe highly mobile nature of anchovies has allowed them to track their optimal thermal physiological conditions during the extreme climate shifts of the Last Glacial Maximum and avoid wholesale population reductions and genetic bottlenecks. Both north-eastern and north-western Atlantic were probably rapidly recolonized after the Last Glacial Maximum by large numbers of anchovies, such that leading-edge populations retained the genetic diversity of parent populations. Long, longitude; Lat, latitude; n, number of individuals; n h , number of haplotypes; n P , number of private haplotypes; n P /n h , proportion of private haplotypes; h, haplotype diverstity; p, nucleotide diversity; Aavg, average number of alleles, Ar, allelic richness; Effnum, effective number of alleles; H O , observed mean heterozygozity; H E , expected mean heterozygozity; *putative Engraulis eurystole.

Genetic variation (mtDNA) of the European conger eel, Conger conger, was compared across five locations in the north-eastern Atlantic (Madeira, Azores, South Portugal, North Portugal and Ireland) and one location in the western Mediterranean (Mallorca). Genetic diversity of conger eel was high, and differentiation among regions was not significant. Additionally, comparisons of element:Ca ratios (Sr:Ca, Ba:Ca, Mn:Ca and Mg:Ca) in otolith cores (larval phase) and edges (3 months prior to capture) among the Azores, North Portugal, Madeira and Mallorca regions for 2 years indicated that variation among regions were greater for edges than cores. Therefore, while benthic conger may display residency at regional scales, recruitment may not necessarily be derived from local spawning and larval retention. Furthermore, data from otoliths suggest a separated replenishment source for western Mediterranean and NE Atlantic stocks. The combination of genetics and otolith chemistry suggests a population model for conger eel involving a broad-scale dispersal of larvae, with limited connectivity for benthic juvenile life stages at large spatial scales, although the existence of one or multiple spawning grounds for the species remains uncertain. Communicated by T. Reusch. Alberto T. Correia and Rita Castilho contributed equally to this work.

A total of 60 morphometric traits and nucleotide sequences of the entire mtDNA NADH dehydrogenase subunit 2 (ND2) gene [1047 base pair (bp)] in 23 individuals of blackmouth, Galeus melastomus, and 13 individuals of sawtail catsharks, Galeus atlanticus, caught in Southern Portugal, were examined to test the validity of these two taxa. These sharks closely resemble each other, have overlapping geographical ranges and are difficult to identify by morphological characters. Non-metric multidimensional scaling of morphometric variables indicates a clear separation between the two species, with 10 characters each contributing 2Á12-2Á45% of the total variability between species. Maximum likelihood, parsimony and neighbour-joining trees revealed two major mtDNA haplotype clades, corresponding to the two species, with an average corrected sequence divergence between them of 3Á39 AE 0Á56%. Within species divergences between haplotypes averaged 0Á27 AE 0Á18% in G. melastomus and 0Á12 AE 0Á08% in G. atlanticus. A total of 35 diagnostic nucleotide site differences and four restriction fragment length polymorphism recognition sites in the ND2 gene can be used to distinguish the two species. # 2007 The Authors Journal compilation # 2007 The Fisheries Society of the British Isles