Northern Blue Mussel

How to Distinguish from Similar Species: Mytilus californianus is similarly shaped but is usually even more elongated, has coarse radiating ribs, often has an eroded surface, grows much larger, and is most common on the outer coast. Horse mussels such as Modiolus modiolus usually have dark brown , hairy periostracum. This species is very difficult to distinguish morphologically from M. galloprovincialis, which is found from San Francisco south, and from M. edulis, which is found in the Atlantic Ocean.

Mostly quiet bays. Said to be found also on open coast in mixed populations with M. californianus.

Depth Range: Intertidal to 40 m; wharves, docks, pilings

This mussel has a relatively smooth shell with smooth growth lines, little erosion, and no major radiating ridges. The periostracum is a smooth, shiny black and without hairs. The shell is longer than high and the umbones are at the pointed anterior end. Length usually less than 7 cm; up to 11 cm.

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 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, 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 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. 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. McCartney and Lima state that there are at least some barriers to hybridization between M. trossulus and M. edulis. M. trossulus is now known to also occur in several populations in northern Europe, including Norway and Russia.On the open coast this species is more susceptible to predation by the snails Nucella emarginata and N. canaliculata than is the larger Mytilus californianusA study done on the related species Mytilus edulus found that the movements of individual mussels correspond to a L�vy walk pattern rather than a ballistic or Brownian pattern. This resulted in creating clusters with small-scale crowding, along with larger-scale dispersion. In this pattern mussels maximized the number of mussels immediately close to them while at the same time minimized the number of mussels in the general area.

Geographical Range: North of San Francisco to Arctic Ocean. Populations of very similar mussels are also found in Japan and Australia, but I do not know whether these are of this species or of one of the very similar-appearing species. Likely also on the Russian Pacific coast. Some populations occur in the North Atlantic from Cape Hatteras to Hudson Bay and from northern Labrador to the northern Gulf of Maine.

Mytilus trossulus, the bay mussel or foolish mussel, is a medium-sized edible marine bivalve mollusc in the family Mytilidae.

Mytilus trossulus is one of the three principal, closely related taxa in the Mytilus edulis complex of blue mussels, which collectively are widely distributed on the temperate to subarctic coasts the Northern Hemisphere, and often are dominant inhabitants on hard substrates of the intertidal and nearshore habitats.

plankton

Gulf of St. Lawrence (unspecified region), southern Gaspe waters (Baie des Chaleurs, Gaspe Bay to American, Orphan and Bradelle banks; eastern boundary: Eastern Bradelle Valley), lower St. Lawrence estuary; Cobscook Bay; south to Little Machias Bay

intertidal and infralittoral of Gulf and estuary; common in rocky intertidal to 40 m, also found on docks, pilings, floats, and gravel