MALACOLOGIA, 1984, 25(2): 551-568 THE SYSTEMATIC STATUS OF MYTHUS GALLOPROVINCIALIS IN WESTERN EUROPE: A REVIEW E. M. Gosling Department of Zoology, University College. Galway and Regional Technical College. Galway, Ireland ABSTRACT The controversy concerning the systerлatic status of the Mediterranean mussel Mytilus galloprovincialis Lmk. dates back to the 1860s. While some authonties regard M. galloprovlnci-alis as a distinct species, others consider it a variety of Mytilus edulis L. Separation of M. edulis and M. galloprovincialis has traditionally been based primarily on morphological shell character-istics. The first part of this paper discusses the reliability of these characters in separating mixed samples of the two types of mussel in W Europe. The conclusion is that with few exceptions — e.g. Rock in SW England, where there is some evidence of premating isolating mechanisms — separation based solely on shell morphology is unreliable, especially in areas where extensive hybridization and introgression is taking place. In recent years, techniques such as gel electrophoresis, cytology, immunology and artificial hybridization have been used in conjunction with shell morphology in an attempt to resolve the problem of the taxonomic status of M. galloprovincialis. The second and major part of this paper reviews the results of such studies. Data from electrophoretic surveys has shown that the two forms of mussel are closely related; genetic identity and genetic distance values are similar to those observed between subspecies of other invertebrates. Cytological and immunological studies on M. edulis and M. galloprovincialis indicate that the differences between them are small and hardly sufficient to warrant them being classified as separate species. Results from hybridization studies have probably contributed most to a resolution of the problem; there is no evidence of genetic incompatibility between the two forms since the hybrids are fertile and viable backcross F2 individuals have been produced. Thus, while the differences between the two forms of mussel are greater than between geographically isolated populations of most species, they are hardly large enough to justify M. galloprovincialis being considered a distinct species. The conclusion from this review is that M. galloprovincialis cannot be regarded as more than a race or subspecies of M. edulis L. and it is suggested that the name Mytilus edulis var. galloprovincialis be readopted, the ranking sug-gested by Jeffreys (1863). Key words; mussels; Mytilus: morphological variations: population genetics; cytogenetics: immunology: hybridization; systematics. INTRODUCTION The systematic status of Mytilus gallopro-vincialis Lmk. has been the subject of con-siderable discussion since the 1860s. While some authorities regard it as a distinct spe-cies of Mytilus others regard it merely as a variety of the larger Mytilus edulis L. complex (review, Lubet, 1973). M. galloprovincialis is believed to have orig-inated in the Mediterranean area (Barsotti & Meluzzi, 1968) but evidence now indicates that it has extended its range northwards onto the Atlantic coasts of W Europe, where it is found intermixed with M. edulis in varying proportions (Hepper, 1957; Seed, 1972; Lubet, 1973; Seed, 1974). The two forms of mussel have been tradi-tionally separated on the basis of shell mor-phology but in some areas, e.g. the Atlantic coasts of Ireland and NW France where there is evidence of extensive hybridization (Seed, 1972, 1974; Gosling & Wilkins, 1977; Skibin-ski & Beardmore, 1979; Gosling & Wilkins, 1981), identification on shell characters alone is difficult and even impossible. In recent years the technique of gel electrophoresis (in conjunction with morphological analyses) has been used in an attempt to quantify the ge-netic differences between the two forms (Ahmad & Beardmore, 1976; Gosling & Wilkins, 1977: Skibinski et al.. 1978b; Skibin-ski et al.. 1980: Gosling & Wilkins. 1981). The first part of this paper discusses the (551 ;
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