Abstract
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 large amount of comparative data on them has been accumulated since that time. 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 morphological and ecological differences are of a local character, or whether they have simply been overlooked in other contact zones.
Highlights
Cryptic or sibling species are species that are difficult or impossible to distinguish based on morphological characters [1]
The allelic composition of the two parental populations estimated by STRUCTURE was almost identical to that of the external reference Mytilus trossulus Gould (MT) and M. edulis L. (ME) populations
In all three geographical populations where subsamples from different substrates were treated separately, mussels from the algal substrates were dominated by MT alleles and those from the bottom by ME alleles
Summary
Cryptic or sibling species are species that are difficult or impossible to distinguish based on morphological characters [1]. The existence of such taxa may reflect either inadequate exploration of the morphology, or differences in habitats, life histories or chemical recognition systems that have evolved without parallel divergence in morphology [2]. With time and further research, diagnostic morphological characters will be revealed, and the species would be characterized as pseudo-sibling species [2]. The Pacific mussel Mytilus trossulus Gould (MT) was one of the first examples of marine cryptic taxa revealed by molecular genetic methods. A bulk of comparative biological and molecular data on these two taxa has been accumulated since (see [7,8,9])
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