Limecola Balthica

Phylum: Mollusca Macoma balthica Class: Bivalvia, Heterodonta, Euheterodonta Order: Imparidentia, Cardiida Family: Tellinoidea, Tellinidae, Macominae Taxonomy: Originally described as a mem- Description ber of the genus Tellina, Macoma balthica Size: Individuals averaging 30–35 mm in was the name of the Atlantic species. Our length (Oldroyd 1924), but usually under 30 west coast clam was originally called M. in- mm (Coan 1971) and rarely more than 45 mm conspicua (Broderip and Sowerby 1829), but (Coan 1971; Cardoso et al. 2003). Smallest they are now generally considered to be the adults are 2 mm (Caddy 1969). Body propor- same species (e.g., Vassallo, 1969, 1971; tions are generally 27 in length, 22 in height, Haderlie and Abbott 1980). An extensive and 11 mm in diameter (Oldroyd 1924). The taxonomic history has yielded many sy- illustrated specimen (from Coos Bay) is 17.5 nonyms for M. balthica. Some ambiguity mm long. exists whether individuals from the southern- Color: Distinct color is reddish, pale rose or most reaches of the distribution on east and white and is sometimes bluish or yellow west sides of the Atlantic should be conside- (Oldroyd 1924; see Plate 17, Kozloff 1993). red the same species (Beukema and Coos Bay specimens are usually pink inside Meehan 1985) and some researchers (e.g., and out, but individuals from British Columbia, Meehan 1985; Kamermans et al. 1990; Lut- Canada can have pink or yellow interiors tikhuizen et al. 2012; Sanier et al. 2015) (Quayle 1970). consider these allopatric populations to be General Morphology: Bivalve mollusks are subspecies (eastern Atlantic Macoma balthi- bilaterally symmetrical with two lateral valves ca balthica and western Atlantic Macoma or shells that are hinged dorsally and sur- balthica rubra) that have been reproductively round a mantle, head, foot and viscera (see isolated for 2–3.5 million years (Luttikhuizen Plate 393B, Coan and Valentich-Scott 2007). et al. 2012; Saunier et al. 2015). Macoma Among the bivalves, the Heterodonta are petalum populations in San Francisco Bay, characterized by ctenidia (or gills) that are once recognized as distinct species from M. eulamellibranchiate, fused mantle margins balthica (Vainola 2003), are now believed to and the presence of long siphons. Veneroid be genetically identical to Macoma balthica bivalves have well-developed hinge teeth and (the circum-Arctic species) (Brusati and members of the family Tellinidae have short Grosholz 2007). However, these may yet lateral hinge teeth (when present – see Pos- prove different species and their taxonomy sible Misidentifications), shells with external awaits further study (Coan and Valentich- striations or ribs, and deep pallial sinuses Scott 2007; Brusati and Grosholz 2007). (Coan and Valentich-Scott 2007). When Thus, the description below considers them holding closed shell in both hands with the together as is done in current local intertidal hinged area up and the ligaments toward you, guides (e.g. Coan and Valentich-Scott the right valve is in the right hand (Fig. 3) 2007). (see Vainola 2003 for molecular sys- (Keen and Coan 1974). tematics of M. balthica species complex.) Body: 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. and K. Meyers 2015. Macoma balthica. In: Oregon Estuarine Invertebrates: Rudys' Illustrated Guide to Common Spe- cies, 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. Interior: Ligament is short, but development, see Figs. 3, 6, Caddy 1969.) strong, partially sunken, seated on a stout Burrow: callus (Dunnill and Ellis 1969), but not on a Possible Misidentifications nymph (Tellinidae, Coan and Valentich-Scott Tellinids can be distinguished from 2007). other small or young bay clams (i.e., Exterior: Mactridae: Tresus; Veneridae: Protothaca, Byssus: Saxidomus; Myidae: Mya, Cryptomya) an ex- Gills: ternal ligament, an ovate shell, an inconspi- Shell: The shell shape is regularly oval, cuous nymph (or supporting projection for the round, thick, with equal valves, umbos low, external ligament), sometimes reddish hue almost central, and usually worn (Fig. 1). and lateral teeth as well as a shell with ribs or The dorsal margin is arched, the ventral striations (no radial pattern) and shells that margin is slightly contracted (Oldroyd 1924), never gape. Lateral teeth may or may not be and there is no posterior dorsal flange present in the Tellinidae (Coan 1971). Myids (posterior to ligament). Valves do not gape have a hinge with a spoon-shaped chondrop- (Tellinidae, Keen 1971) and the posterior hore (left valve) and a projecting tooth (right end is rounded. Shell usually heavy, but bay valve) (see Mya arenaria, this guide). Vene- specimens sometimes thin (Coan 1971). rids have three cardinal teeth in each valve. Interior: Pallial line is narrow and Mactrids have an internal ligament, A-shaped faint. The pallial sinus is large (see Plate cardinal teeth, and gaping valves (Coan and 422, Coan and Valentich-Scott 2007) and Valentich-Scott 2007). The Tellinidae has equal among valves. The sinus ends ¾ of around 16 species distributed between two the way to anterior adductor muscle scar in genera locally – Tellina and Macoma. These both valves (Figs. 2a, 2b), and does not genera can be differentiated by the hinge reach the muscle scar (Fig. 2b) (Coan and teeth, Tellina species have a hinge with lateral Valentich-Scott 2007). teeth, while Macoma species do not. Maco- Exterior: Periostracum is thin, silky, ma species have shells that are also more not shiny (Coan 1971), and a trace is visible rounded and inflated thanTellina, and are only ventrally. Shell sculpture consists of smooth, white, often chalky. They are charac- fine concentric growth lines only (Figs. 1, 3) terized by having a ovate shell consisting of (Dunnill and Ellis 1969). two equal valves, a dark and deciduous pe- Hinge: The hinge area has no lateral riostracum, two cardinal teeth, the absence of teeth (Macoma, Coan and Valentich-Scott lateral teeth and a pallial sinus that is deeper 2007). Two cardinal teeth exist in each valve on the left valve (Scott and Blake 1998; Arru- (Figs. 4a, 4b), one stout, bifid, and the other da and Domaneschi 2005). Macoma species single and fragile (Dunnill and Ellis 1969). may also have a more northern geographic Eyes: distribution whileTellina are elongate, relati- Foot: vely compressed, conspicuously sculptured, Siphons: The siphons are long, separate, brightly colored, and usually warm water dwe- and mobile (Kozloff 1993). Inhalant siphons llers (Coan 1971). Eleven species in the in- are four times the shell length, when ex- faunal genus Macoma (Luttikhuizen et al. tended. Exhalant siphons held vertically 2012) are reported locally (although 30 have above surface are 1.5 cm. Siphons bear been identified in the northeastern Pacific, large palps, for sorting fine particles (Fig. 5) Dunnill and Ellis 1969), but only seven are (Yonge 1949). (For diagrams of siphon 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] described in local keys (e.g. Coan and Va- Macoma balthica has a pinkish hue and a pal- lentich-Scott 2007), the four most common lial sinus that ends ¾ of the way to anterior species of Macoma in our area are M. balthi- adductor muscle scar and is generally more ca, M. nasuta, M. inquinata, and M. secta oval than M. nasuta or M. inquinata (compare (Kabat and O’Foighil 1987). Fig. 1 M. balthica, with Fig. 1 M. nasuta, this Two species, M. secta and M. inden- guide) (Kozloff 1993). In M. inquinata, the tata have a posterior dorsal flange extending pallial sinus does not reach the ventral end of from posterior end to the external ligament, the anterior adductor muscle and the shell is this is absent in other Macoma species. The chalky white with a fibrous olive green former species is called the sand clam and periostracum. Macoma nasuta, on the other has a quadrate and truncate posterior. The hand, is not as round and heavy as M. latter is elongate, has a pointed posterior, inquinata and its pallial sinus reaches and unique muscle scars, is relatively rare and joins the anterior adductor scar above its base small (to 2.5 cm) and occurs from Trinidad, (left valve). (Its right valve may be more like California southward. Macoma secta, also M. inquinata’s). Furthermore, its siphons are has a white shell, with a yellowish epidermis. orange and its shell posterior is bent to the Its right valve is more inflated than the left, right. Macoma inquinata can also bend and it can be large (to 120 mm) and is more slightly posteriorly, and may be confused with common in clean sand, not in estuarine the thinner M. nasuta, without investigations mud. of the other aforementioned features. (see The morphology of the pallial sinus Plate 422 for diagrams of these distinguishing differentiates the other species. In species characteristics in Macoma). Macoma balthica without a posterior dorsal flange, M. acolas- and M. inquinata are generally smaller than ta and M. yoldiformis, the anterior ventral M. nasuta (up to 5 cm), with mature Macoma edge of the pallial sinus is detached for a balthica rarely exceeding 25 mm in length portion of the distance to the posterior ad- (Dunnill and Ellis 1969), but could be ductor muscle scar. Macoma acolasta also confused with the young of some of these has a rounded posterior, rather than pointed larger clams.

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