Mitochondrial DNA variation in northeast atlantic and mediterranean populations of norway lobster, Nephrops norvegicus
Analysis of the genetic structure of the Norway lobster (Nephrops norvegicus), a marine crustacean with high commercial value, was undertaken to gain information regarding the differentiation of Atlantic from Mediterranean populations of marine invertebrates. Restriction fragment length polymorphism analysis of two mitochondrial DNA segments, 3.6 kilobases in total, was performed. Twelve populations from the North Sea, Irish Sea, Portuguese coast and Aegean Sea were analysed. Low levels of differentiation were found among them (F-ST = 0.018, P < 0.001) and there were no signs of an Atlantic-Mediterranean divide or of an isolation-by-distance scheme of differentiation. Possible reasons for these low levels of differentiation can be found in the recent expansion of N. norvegicus populations. This is supported by the mismatch distribution of pairwise haplotype differences, as well as by the high mean haplotype diversity (h = 0.93) combined with medium nucleotide diversity (pi = 0.0057) (in comparison to values for marine crustaceans or teleosts) found in this study. This combination of high levels of haplotype diversity with moderate to low levels of nucleotide diversity has also been frequently attributed to a recent time of divergence for various marine species. No evidence was found for a Mediterranean refugium during glaciation periods, separate from the Atlantic, as has been reported for some marine species. The Irish Sea population was the most differentiated as a result of reduced levels of diversity. Results are also discussed in the light of future management of N. norvegicus stocks.