Some study has been done on interbreeding in Mytilus.  Some populations seem more prone to interbreed than others.

As you suspected, the levels of genetic variability and apparent associated barriers vary considerably.  Simply finding a certain level of genetic difference between two individuals does not establish their specific status.  Rather, you must establish levels of variation within and between populations.  For example, several species have notable genetic difference between populations on the U.S. Atlantic and Gulf Coasts.  However, you cannot simply analyze an individual from New Jersey and one from Texas and name a new species.  Unless you sample multiple indidivuals at several points along the coast, you do not know whether this is continuous variation, or a distinct break with Atlantic populations all uniform and Gulf all uniform (which you must then decide whether it merits recognition as subspecific, specific, etc.), or perhaps a variable species in which every population has many forms.  Dr. Laura Adamkewicz did a study of this sort on Donax.

    Dr. David Campbell
    "Old Seashells"
    46860 Hilton Dr #1113
    Lexington Park MD 20653 USA
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"That is Uncle Joe, taken in the masonic regalia of a Grand Exalted Periwinkle of the Mystic Order of Whelks"-P.G. Wodehouse, Romance at Droigate Spa




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