Phylum: Mollusca Mytilus trossulus Class: Bivalvia, Pteriomorphia Order: Mytilida, The bay mussel Family: Mytiloidea, Mytillidae, Mytillinae Taxonomy: Confusion has surrounded the particularly around ventral (posterior) shell taxonomy of Mytilus species because the margin. Tissues are orangish-tan. genus has historically been based on mor- General Morphology: Bivalve mollusks are phological shell characters, which have bilaterally symmetrical with two lateral valves been shown to be plastic and varies with or shells that are hinged dorsally and sur- habitat (e.g. see Growth, Gosling 1992a round a mantle, head, foot and viscera (see and b). Mytilus trossulus is the species na- Plate 393B, Coan and Valentich-Scott 2007). tive to the west coast of North America, and Mytilids have roughly cylindrical shells and was previously confused with M. edulis. two adductor muscles, with associated scars Thus, in many intertidal guides of the past, that are unequal in size (see Plate 395, Coan (e.g., Kozloff 1993; Ricketts and Calvin and Valentich-Scott 2007). Mytilids often use 1952; Kabat and O’Foighil 1987; Haderlie byssal threads to connect them to the sub- and Abbott 1980) M. edulis is actually M. stratum (Kozloff 1993). trossulus. Many of the references to which Body: we refer are for M. edulis (and we call M. Color: trossulus, for clarity). Mytilus trossulus is a Interior: Mytilus trossulus as well as member of the Mytilus edulis species com- other bivalves can develop hemic neoplasia, a plex, a group of three sibling species (M. blood cell disorder that is often linked to envi- trossulus, M. edulis, M. galloprovincialis), ronmental contaminants (e.g. polycyclic aro- recently differentiated using molecular meth- matic hydrocarbons, chlorinated hydrocar- ods (McDonald and Koehn 1988; Gosling bons). Up to 30% of M. trossulus in Puget 1992a and b; Seed 1992; Geller 2007). The Sound, WA were infected. (Krishnakumar et three species can be defined by both molec- al. 1999). A widely prevelant genus, the ular and, less easily by, morphological char- physiology of Mytilus has been the subject of acters (McDonald et al. 1991) (see Range). much research (e.g., Smith 1982). Additional north Pacific and Arctic synonyms Exterior: for M. trossulus include (but are not limited Byssus: to): M. glumeratus, M. pedroanus, M. edulis Gills: latissimus, M. edulis kussakini, M. edulis de- Shell: Pointed shell, anteriorly, with very clinis, M. septentrionalis, M. ficus, McDonald broad posterior (Haderlie and Abbott 1980). and Koehn 1988; Kafanov 1999). Individuals that are exposed to more wave action have shells that are thicker and grow Description more slowly (Haderlie and Abbott 1980). Size: Individual size is about 70–110 mm Interior: Large posterior muscle scar, (Coe 1945; Kozloff 1974; Haderlie and small anterior scar that is near the beak on Abbott 1980). Mean dry body weight is 7 the anterior ventral margin (Fig. 2). Pit-like grams (Harger 1968). byssal gland is present at the base of foot and Color: Blue, violet and white shell with shiny produces liquid that hardens into byssal brown-black periostracum. Blue-black color A publication of the University of Oregon Libraries and the Oregon Institute of Marine Biology Individual species: https://oimb.uoregon.edu/oregon-estuarine-invertebrates and full 3rd edition: http://hdl.handle.net/1794/18839 Email corrections to: [email protected] Hiebert, T.C. 2016. Mytilus trossulus. In: Oregon Estuarine Invertebrates: Rudys' Illustrated Guide to Common Species, 3rd ed. T.C. Hiebert, B.A. Butler and A.L. Shanks (eds.). University of Oregon Libraries and Oregon Institute of Marine Biology, Charleston, OR. threads which are visible on the ventral shell drical shells that are dark brown or black that margin (Fig. 1). Digestion is both intra and sometimes taper anteriorly, and the two shell extra cellular, and is aided by the crystalline valves are of similar morphology. They lack style and associated enzymes (Haderlie and both a chondrophore (e.g., compare to Mya Abbott 1980). arenaria, this guide) and dorsal margin ears, Exterior: Valves of similar morpholo- and the shell is not cemented to the substra- gy, wedge-shaped, and longer than high. tum (Coan and Valentich-Scott 2007). There Shell regular, smooth, and with concentric are nearly 20 local species in the family Myti- growth lines, but no radial ribs (Fig. 1, lidae comprising the genera Septifer (S. compare to Clinocardium nuttallii, this bifurcatus), Lithophaga (L. plumula), Geu- guide). Fine byssal threads attach to kensia (G. demissa), Musculista (M. substrate, and beaks (the most prominent senhousia), Modiolus (six species) and point on the shell, or umbo) are anterior and Mytilus (one to four species). Other genera terminal. The shell is made of calcium, possibly present in the area, but not included which is absorbed from the surrounding sea- in current keys, may include Crenella (C. water, and precipitated at the shell edge un- decussata), Gregariella (G. coarctata), and der the periostracum (Haderlie and Abbott Solamen (S. columbianum) (Coan and 1980). Valentich-Scott 2007). Hinge: No hinge teeth or chondro- Genera can be differentiated as fol- phore but small denticles are present near lows: Mytilus is a cosmopolitan genus with the beak. No shell-like septum (or shelf) at at least four species (Koehn 1991), has anterior end (Fig. 4). shells with beaks at the terminal portion of Eyes: the anterior end and lack internal septa, Foot: Foot is reduced and internal. Alt- while Septifer species possess an internal hough individuals are attached to substrate septum at their anterior end. All other gene- by byssal threads, they are capable of mov- ra have shells with beaks that are anterior, ing. By extending the foot and attaching it to but not terminal. Of those, Lithophaga plu- the substrate, they are able to break byssal mula individuals are with cylindrical shells threads a few at a time and, eventually, de- and a posterodorsal slope that is rough and tach and move to another location where with chalky encrustations, while Adula spe- they use their foot to reattach (Ricketts and cies have a posterodorsal slope without Calvin 1952). chalky encrustations and, instead, have a Siphons: No siphons, instead there are un- thick mat and are sometimes covered with dulating openings between mantle edges mud or debris. The genera Geukensia, Mo- (especially conspicuous along the posterior diolus, and Musculista have shells that are margin). not cylindrical, G. demissa shells have promi- Burrow: nent ribs externally and are dark brown or black in color. Modiolus and Musculista spe- Possible Misidentifications cies have shells without external ribbing and The family Mytilidae is characterized members of the former genus have periostra- by two adductor muscles and associated cial hairs while Musculista do not. scars that are unequal in size; the anterior Species in the genus Mytilus are diffi- scars are smaller and near the shell beak cult to differentiate morphologically. The (see Plate 395, Coan and Valentich-Scott three local species are M. californianus, M. 2007). Members of this family have cylin- trossulus, and M. galloprovincialis (Evans et A publication of the University of Oregon Libraries and the Oregon Institute of Marine Biology Individual species: https://oimb.uoregon.edu/oregon-estuarine-invertebrates and full 3rd edition: http://hdl.handle.net/1794/18839 Email corrections to: [email protected] al. 2010). Mytilus californianus has a shell to reliably differentiate M. edulis and M. with radial ribs that are conspicuous, espe- trossulus (McDonald et al. 1991; Seed 1992; cially near the shell posterior. On the other Sarver and Foltz 1993). Fortunately, they do hand, M. trossulus, M. galloprovincialis and not co-occur in Oregon, as they do in the M. edulis all have smooth shells and are north Atlantic (Varvio et al. 1988; Mallet and lumped into the M. edulis species complex. Carver 1995; Kafanov 1999; Liu et al. 2011), Recent genetic research has shown that where less hybridization has been observed the smooth species that is present locally is between sympatric populations (Toro et al. M. trossulus. However, this species over- 2002; Vainola and Strelkov 2011; Tam and laps and forms a hybrid with M. galloprovin- Scrosati 2014). Larval characters were cialis to the south in central California. The proposed for higher-level classification in the latter species has also been observed in Mytilinae by Evseev et al. (2011). BC Canada, but their current range in- Other rarer mussels include Modiolus cludes only central California south to Baja sp., the horse mussel, which has external California. Of these three species, only M. subterminal beaks and is brown and hairy. trossulus is native to the north Pacific, while This species is found in clumps in the mud M. galloprovincialis is native to Europe and subtidally. Septifer bifurcatus is found under M. edulis to the north Atlantic. Mytilus rocks, is black outside, purple within, and trossulus is often found with Mytilus califor- with definite radiating ribs and shell-like nianus, the larger, coarser "common mus- septum across the anterior end. sel" of the West Coast. Internally M. califor- Ecological Information nianus is orange, but the most dependable Range: Mytilus trossulus was originally de- distinguishing characteristic is the presence scribed from Tillamook, OR and Puget Sound, of radial ridges in M. californianus. When Washington (= “Killimook and Puget Sound, small, the two are more difficult to distin- Oregon” McDonald and Koehn 1988; Kafanov guish, however, M. trossulus has sharper 1999), but due to recent understanding of the edges, a thinner profile (Fig. 3), finer byssal M. edulis species complex, a neotype desig- threads, and more delicate concentric rings nation was suggested from molecularly identi- than does M. californianus. It also can be fied shells by Kafanov (1999). Mytilus trossu- found higher in the intertidal zone, in more lus is the native smooth blue mussel from the protected spots, not on exposed rocks with northeast Pacific and ranges from the Arctic heavy surf and turbulence.