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Biology/Natural History: Mussels such as these attach by byssal threads and are often found in clusters. In the photo above the byssal threads secreted by this mussel can be seen on the ventral (lower) side, and several other byssal threads by which other mussels had attached to this one are visible on the side and the dorsal surface of the shell. Besides looking almost identical (they can be distinguished mainly by molecular means), the ecological requirements of this species seem to be very similar to that of Mytilus galloprovincialis, the Mediterranean or southern bay mussel, except for a few differences. Presumably M. trossulus favors cooler water, and my observation has been that M. trossulus is less tolerant of the strong wave action of the open coast than is M. galloprovincialis. Both these species are said to occur in mixed populations with M. californianus on the open coast, and I have observed this in S. California. However, it appears to me that very few M. trossulus grow on exposed sites on the open coast. I believe that the small, smoother mussels seen on the open coast of Washington along with the larger M. californianus are almost always simply young M. californianus rather than M. trossulus. I base this conclusion on the observation that M. californianus are usually more elongated with a more pointed umbo than is M. trossulus. This conclusion needs to be tested by more definitive means, such as by molecular analysis.
Bay mussels filter feed almost continuously, mainly on detritus particles but also on plankton down to 4-5 microns. Detrital particles sloughed off kelps such as Laminaria and Agarum seem to lead to the highest growth rates. They capture food on mucus sheets on the gills, but can sort and reject some particles. They can likely also absorb dissolved organic materials directly from the water. During blooms of the dinoflagellate Alexandrium catanella, which blooms at sea temperatures exceeding 13C, this species may accumulate toxic levels of the paralytic shellfish poisoning (PSP) toxin. Unlike the butter clam Saxidomus gigantea which also accumulates the toxin, after the bloom is finished this species rapidly purges its tissues of the toxin. Therefore it is one of the indicator species used to test for the presence of PSP. Pumping rate is up to 2.5 liters/h per gram of body weight. Predators include seastars such as Pisaster ochraceous, Leptasterias hexactis, and Pycnopodia helianthoides, anemones such as Urticina crassicornis and Anthopleura xanthogrammica, oyster drills such as the frilled dogwinkle Nucella lamellosa, large crabs, diving ducks, pile perch, shiner perch, surf scoters, black oystercatchers, Barrow's goldeneyes, gulls, crows, sea otters, and mink. It sometimes defends itself against oyster drill whelks by gluing a byssal thread to the whelk and to a nearby rock, thus tethering the snail to the rock until it starves. Byssal thread formation is inhibited by mercury and copper so this species does not live where these pollutants occur. The gut may contain the parasitic copepod Mytilicola orientalis. In Puget Sound they spawn in April and May. Larvae are planktonic veligers for 4-7 weeks. They settle into areas with filamentous algae, byssal threads, hydroids, Semibalanus cariosus, or bare rock. Adults live 2-3 years in California, probably longer farther north.
This species is probably referred to as the foolish mussel because of the large clusters which sometimes formed, attached weakly to one another by byssal threads. The cluster can become so large and the attachment to the rock so weak that the entire cluster breaks off and washes away, often into the waiting mouths of predators.
Note: Most references to bay mussels before 1990 refer to M. edulis. However, it has not been concluded from molecular analysis that M. edulis does not occur on our Pacific coast. Past references to M. edulis should usually be taken to refer to M. galloprovincialis if the location is south of San Francisco, and to M. trossulus if well north of San Francisco. The two species appear to by hybridizing just north of San Francisco, and M. galloprovincialis is extending its range northward. M. galloprovincialis is being commercially cultivated in Puget Sound, and a population of M. edulis has been introduced to Georgia Strait so further hybridization will likely occur. Elliott et al. (2008) report that by the time of their study, M. galloprovincialis and hybrids between M. galloprovincialis and M. trossulus occurred throughout the Puget Sound and Rosario area. Compared to M. trossulus, M. galloprovincialis and hybrids between the two were more common on docks rather than intertidally, and if intertidal were more common lower in the intertidal than was M. trossulus. The two species appeared to be mating randomly with one another.
M. trossulus is now known to also occur in several populations in northern Europe, including Norway and Russia (Väinölä and Strelkov, 2011).