Phylum: Mollusca Mytilus trossulus Class: Bivalvia, Pteriomorpha Order: Mytiloida The bay mussel Family: Mytilidae Taxonomy: Confusion has surrounded the 2007). Mytilids have roughly cylindrical shells taxonomy of Mytilus species because the and two adductor muscles, with associated genus has historically been based on scars that are unequal in size (see Plate 395, morphological shell characters, which have Coan and Valentich-Scott 2007). Mytilids been shown to be plastic and varies with often use byssal threads to connect them to habitat (e.g. see Growth, Gosling 1992a and the substratum (Kozloff 1993). b). Mytilus trossulus is the species native to Body: the west coast of North America, and was Color: previously confused with M. edulis. Thus, in Interior: Mytilus trossulus as well as many intertidal guides of the past, (e.g., other bivalves can develop hemic neoplasia, Kozloff 1993; Ricketts and Calvin 1952; Kabat a blood cell disorder that is often linked to and O’Foighil 1987; Haderlie and Abbott environmental contaminants (e.g. polycyclic 1980) M. edulis is actually M. trossulus. Many aromatic hydrocarbons, chlorinated of the references to which we refer are for M. hydrocarbons). Up to 30% of M. trossulus in edulis (and we call M. trossulus, for clarity). Puget Sound, WA were infected. Mytilus trossulus is a member of the Mytilus (Krishnakumar et al. 1999). A widely edulis species complex, a group of three prevelant genus, the physiology of Mytilus sibling species (M. trossulus, M. edulis, M. has been the subject of much research (e.g., galloprovincialis), recently differentiated using Smith 1982). molecular methods (McDonald and Koehn Exterior: 1988; Gosling 1992a and b; Seed 1992; Byssus: Geller 2007). The three species can be Gills: defined by both molecular and, less easily by, Shell: Pointed shell, anteriorly, with very morphological characters (McDonald et al. broad posterior (Haderlie and Abbott 1980). 1991) (see Range). Additional north Pacific Individuals that are exposed to more wave and Arctic synonyms for M. trossulus include action have shells that are thicker and grow (but are not limited to): M. glumeratus, M. more slowly (Haderlie and Abbott 1980). pedroanus, M. edulis latissimus, M. edulis Interior: Large posterior muscle scar, kussakini, M. edulis declinis, M. small anterior scar that is near the beak on septentrionalis, M. ficus, McDonald and the anterior ventral margin (Fig. 2). Pit-like Koehn 1988; Kafanov 1999). byssal gland is present at the base of foot and produces liquid that hardens into byssal Description threads which are visible on the ventral shell Size: Individual size is about 70–110 mm margin (Fig. 1). Digestion is both intra and (Coe 1945; Kozloff 1974; Haderlie and Abbott extra cellular, and is aided by the crystalline 1980). Mean dry body weight is 7 grams style and associated enzymes (Haderlie and (Harger 1968). Abbott 1980). Color: Blue, violet and white shell with shiny Exterior: Valves of similar brown-black periostracum. Blue-black color morphology, wedge-shaped, and longer than particularly around ventral (posterior) shell high. Shell regular, smooth, and with margin. Tissues are orangish-tan. concentric growth lines, but no radial ribs (Fig. General Morphology: Bivalve mollusks are 1, compare to Clinocardium nuttallii, this bilaterally symmetrical with two lateral valves guide). Fine byssal threads attach to or shells that are hinged dorsally and substrate, and beaks (the most prominent surround a mantle, head, foot and viscera point on the shell, or umbo) are anterior and (see Plate 393B, Coan and Valentich-Scott terminal. The shell is made of calcium, which 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. A publication of the University of Oregon Libraries and the Oregon Institute of Marine Biology Individual species: http://hdl.handle.net/1794/12912 and full 3rd edition: http://hdl.handle.net/1794/18839 Email corrections to: [email protected] is absorbed from the surrounding seawater, Septifer species possess an internal septum and precipitated at the shell edge under the at their anterior end. All other genera have periostracum (Haderlie and Abbott 1980). shells with beaks that are anterior, but not Hinge: No hinge teeth or terminal. Of those, Lithophaga plumula chondrophore but small denticles are present individuals are with cylindrical shells and a near the beak. No shell-like septum (or shelf) posterodorsal slope that is rough and with at anterior end (Fig. 4). chalky encrustations, while Adula species Eyes: have a posterodorsal slope without chalky Foot: Foot is reduced and internal. Although encrustations and, instead, have a thick mat individuals are attached to substrate by and are sometimes covered with mud or byssal threads, they are capable of moving. debris. The genera Geukensia, Modiolus, By extending the foot and attaching it to the and Musculista have shells that are not substrate, they are able to break byssal cylindrical, G. demissa shells have prominent threads a few at a time and, eventually, ribs externally and are dark brown or black in detach and move to another location where color. Modiolus and Musculista species have they use their foot to reattach (Ricketts and shells without external ribbing and members Calvin 1952). of the former genus have periostracial hairs Siphons: No siphons, instead there are while Musculista do not. undulating openings between mantle edges Species in the genus Mytilus are (especially conspicuous along the posterior difficult to differentiate morphologically. The margin). three local species are M. californianus, M. Burrow: trossulus, and M. galloprovincialis (Evans et al. 2010). Mytilus californianus has a shell Possible Misidentifications with radial ribs that are conspicuous, The family Mytilidae is characterized by two especially near the shell posterior. On the adductor muscles and associated scars that other hand, M. trossulus, M. galloprovincialis are unequal in size; the anterior scars are and M. edulis all have smooth shells and are smaller and near the shell beak (see Plate lumped into the M. edulis species complex. 395, Coan and Valentich-Scott 2007). Recent genetic research has shown that the Members of this family have cylindrical shells smooth species that is present locally is M. that are dark brown or black that sometimes trossulus. However, this species overlaps taper anteriorly, and the two shell valves are and forms a hybrid with M. galloprovincialis to of similar morphology. They lack both a the south in central California. The latter chondrophore (e.g., compare to Mya species has also been observed in BC arenaria, this guide) and dorsal margin ears, Canada, but their current range includes only and the shell is not cemented to the central California south to Baja California. Of substratum (Coan and Valentich-Scott 2007). these three species, only M. trossulus is There are nearly 20 local species in the native to the north Pacific, while M. family Mytilidae comprising the genera galloprovincialis is native to Europe and M. Septifer (S. bifurcatus), Lithophaga (L. edulis to the north Atlantic. Mytilus trossulus plumula), Geukensia (G. demissa), is often found with Mytilus californianus, the Musculista (M. senhousia), Modiolus (six larger, coarser "common mussel" of the West species) and Mytilus (one to four species). Coast. Internally M. californianus is orange, Other genera possibly present in the area, but the most dependable distinguishing but not included in current keys, may include characteristic is the presence of radial ridges Crenella (C. decussata), Gregariella (G. in M. californianus. When small, the two are coarctata), and Solamen (S. columbianum) more difficult to distinguish, however, M. (Coan and Valentich-Scott 2007). trossulus has sharper edges, a thinner profile Genera can be differentiated as (Fig. 3), finer byssal threads, and more follows: Mytilus is a cosmopolitan genus with delicate concentric rings than does M. at least four species (Koehn 1991), has shells californianus. It also can be found higher in with beaks at the terminal portion of the the intertidal zone, in more protected spots, anterior end and lack internal septa, while not on exposed rocks with heavy surf and 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. turbulence. In profile, M. galloprovincialis to invade new regions, Braby and Somero shells are higher and flatter than M. trossulus 2006), is native to Europe but was introduced and the anterior end is beaked in the former to the west coast of North America and now species, while it is blunt in the latter. has a distribution that overlaps with M. Adductor muscles scars are also different trossulus in central California, and extends between the two species. The anterior scars south to Baja, California. Where the two are small and round in M. galloprovincialis species meet, between Monterey and Cape and narrow to elongate in M. trossulus (see Mendocino, they form a hybrid zone (Braby Fig. 1.2 Gosling 1992b). Mytilus trossulus is and Somero 2006). Additional hybrid zones more elongate than M. edulis, with which it noted between the two species include co-occurs in eastern Newfoundland (Innes Whidbey Island, Washington, San Francisco and Bates 1999). Few, if any, morphological and San Diego Bay, California (Suchanek et characters (e.g., shell shape) can be used to al. 1997). The final member of this species reliably differentiate M. edulis and M. complex, M. edulis, is native to the north trossulus (McDonald et al. 1991; Seed 1992; Atlantic (Coan and Valentich-Scott 2007). Sarver and Foltz 1993). Fortunately, they do Mytilus trossulus was previously mistaken for not co-occur in Oregon, as they do in the M.