Genetic, Ecological and Morphological Distinctness of the Blue Mussels Mytilus trossulus Gould and M. edulis L. in the White Sea

The data file presents the initial genetic data on 4 allozyme loci (Pgm, Odh, Gpi, Est-D) for 31 populations of mussels Mytilus from the Kandalaksha Bay of the White Sea (3 of the samples contain subsamples from different substrates:A-algae, B-bottom). File format is .str which is an input file for the genetic STRUCTURE Software (Prichard et al. 2000)*. Alleles grouping is following: Pgm1 (90-105), Pgm2 (110-115), Pgm3 (117-122), Pgm4 (125-135), Odh1 (90), Odh2 (94), Odh3 (100), Odh4 (105), Odh5 (108-110), Gpi1 ( 86-96), Gpi2 (98), Gpi3 (100-102), Gpi4 (105), Gpi5 (107-110), Est1 (80), Est2 (90), Est3 (95), Est4 (100), Est5 (110). *Pritchard JK, Stephens M, Donnelly P. Inference of population structure using multilocus genotype data. Genetics. 2000;155:945–59.Two blue mussel lineages of Pliocene origin, Mytilus edulis (ME) and M. trossulus (MT), co-occur and hybridize in several regions on the shores of the North Atlantic. The two species were distinguished from each other by molecular methods in the 1980s, and a bulk of comparative data on them has been accumulated since that times. However, while ME and MT are now routinely distinguished by various genetic markers, they tend to be overlooked in ecological studies since morphological characters for taxonomic identification have been lacking, and no consistent habitat differences between lineages have been reported. Surveying a recently discovered area of ME and MT co-occurrence in the White Sea and employing a set of allozyme markers for identification, we address the issue whether ME and MT are true biological species with distinct ecological characteristics or just virtual genetic entities with no matching morphological and ecological identities. We find that: (1) in the White Sea, the occurrence of MT is largely concentrated in harbors, in line with observations from other subarctic regions of Europe; (2) mixed populations of ME and MT are always dominated by purebred individuals, animals classified as hybrids constituting only ca. 18%; (3) in terms of shell morphology, 80% of MT bear a distinct uninterrupted dark prismatic strip under the ligament while 97% of ME lack this character; (4) at sites of sympatry MT is more common on algal substrates while ME mostly lives directly on the bottom. This segregation by the substrate may contribute to maintaining reproductive isolation and decreasing competition between taxa. We conclude that while ME and MT are not fully reproductively isolated, they do represent clearly distinguishable biological, ecological and morphological entities in the White Sea. It remains to be documented whether the observed differences are of a local character, or whether the basic morphological and ecological differences just have been so far overlooked in other contact zones.Russian Fund for Basic Research (grants NN 08-04-01315-а, 13-04-00394-a), St. Petersburg State University (grants NN 1.38.253.2014, 1.42.1493.201