Mytilus galloprovincialis and M. edulis on the coasts of the Iberian Peninsula

Abstract

Mytilus edulis L. and M. galloprovincialis Lmk. are 2 forms of mussels that live on the European coasts, and where they coexist, hybridize in varylng proportions. While the Atlantic coast of the Iberian Peninsula is a key area for understanding the distribution of the genus Mytilus in Europe, the available evidence on the distribution from thls area is scarce and contradictory. Five partially diagnostic allozyme loci (aminopeptidase-l, esterase-D, leucine aminopeptidase-l, mannose phosphate isomerase and octopine dehydrogenase) and the best diagnostic morphological characters (length of the anterior adductor muscle scar and internal radius of the hinge plate and shell length and height) between M. edulis and M. galloprovincialis were investigated in 37 and 39 samples respectively taken from the Iberian coasts. Concordance between morphological variation and enzyme polymorphlsms is virtually complete and in&cates that M. galloprovincialis occurs all along the Iberian Peninsula (Mediterranean, Lusitanian, Gahcian Rias and Cantabrian coasts). M. galloprovincialis follows, therefore, a continuous distribution all along the Atlantic coast of Europe, from the Mediterranean Sea to the English Channel and the British Isles. Moreover, M. galloprovincialis exhibits an abrupt discontinuity in allozyme frequencies of about 40% for Odh'100 and 25% for Ap-1'100 in the southeastern Mediterranean Iberian coasts. This strong genetic differentiation appears to be associated with an oceanographic current barrier (the Almeria-Oran front). On the other hand, evidence for the presence of M. edulis and hybrids of M edulis and M. galloprovincialis was obtained only on the Atlant~c coast near the Hispano-French frontier. Therefore, it seems that the meridional h i t of the geographical distribution of M. edulis in Europe, and the concomitant h u t of the hybrid zone, are between the mouths of the Bidasoa and Nervion rivers in Spain. These results clarify some evolutionary aspects of M. edulis and M. galloprovincialis.

Chromosome differences between European mussel populations (genusMytilus)

“…We think that it could be due to i) the different chromosome condensation degree, ii) the temperature employed to spread gill chromosome metaphase, and/or iii) the method employed for fixation of larvae and gill tissue chromosomes. In this sense we must point out that M. galloprovincialis, from de the southern Mediterranean exhibits an abrupt discontinuity in allozyme frequencies of the loci octopine dehydrogenase (Odh), phosphglucose isomerase (Pgi) and aminopeptidase (Ap) (SANJUAN et al 1994;QuESADA et al 1995). We suppose that may exist any relationship between this abrupt discointinuity and the chromosome polymorphism.…”

The stock genetic structure of two Sparidae species, Diplodus vulgaris and Lithognathus mormyrus, in the Mediterranean Sea

“…On the other hand, the Wahlund effect (Hartl and Clark, 1989), by which the presence of different genetic stocks in a single sample can cause an excess of homozygotes, could be a plausible explanation. The mixture of different populations with different allozyme frequencies (Wahlund effect) is common in marine species (Sanjuan et al, 1994;Mamuris et al, 1998) due to difficulties in identifying the boundaries of different demes. The other possible causes of this deficiency may be inbreeding, assortative mating or null allele.…”

No evidence for genetic differentiation of the mussel Mytilus galloprovincialis between lagoons and the seaside

“…2). The sampled area does not correspond to a hybrid zone between different mussel taxa (Sanjuan et al 1994), which could have partly helped to interpret our results. Samples were also large enough to prevent type I1 errors.…”