Phylum: Mollusca Tresus capax Class: Bivalvia, Heterodonta Order: Veneroida The gaper clam, horseneck clam, or fat gaper Family: Mactridae Taxonomy: Originally described as Lutraria Shell: Overall shell shape is oval, with larger capax (in 1850 by Gould), few synonyms are specimens quadrate (Fig. 1). The posterior end known and are rarely used (e.g., L. maxima is truncate, and gapes widely (Figs. 1–2). and Schizothaerus capax). Individuals 1.5 times long as tall (Kozloff 1993). Description Interior: Porcelaneous and white, with Size: Individuals up to 20 cm in length, with two adductor muscle scars of similar shape, average size 10–12 cm. Tresus capax are and a long pallial sinus (Fig. 2). some of the largest northwest clams (Kozloff Exterior: Right and left valves are 1993), some specimens weigh up to 1.8 kg similar and with smooth sculpture, with only (Ricketts and Calvin 1971). concentric rings. Some periostracum, but not Color: Shell chalky white and occasionally necessarily covering the entire shell. Beaks are with patches of black due to the presence of one third from anterior end. sulfides (Kozloff 1993). The periostracum is Hinge: One small cardinal tooth on dull brown in color, and often flaking. each valve, and a J-shaped, socket-like General Morphology: Bivalve mollusks are chondrophore (Fig. 2). The left valve is with bilaterally symmetrical with two lateral valves A-shaped tooth (Fig. 2a). or shells that are hinged dorsally and Eyes: surround a mantle, head, foot and viscera Foot: (see Plate 393B, Coan and Valentich-Scott Siphons: Large, fused siphons are 2007). The Veneroida is a large and diverse retractible (almost completely), and with bivalve heterodont order that is characterized rather leathery tips, and small, inconspicuous, by well-developed hinge teeth. There are 22 leathery plates at the tip (Kozloff 1993). local families, and members of the family Burrow: Burrow depth ranges from 30–50 Mactridae are characterized by a A-shaped cm and depends on substrate type (e.g., cardinal tooth (see Plate 396F, Coan and those in stiff clay have more shallow burrows Valentich-Scott 2007; Fig. 2a). than those in soft mud) (Kozloff 1993). Body: (Fig. 295, Kozloff 1993). Tresus capax individuals apparently lose the Color: ability to re-burrow themselves once they Interior: Biphasic action of the heart reach 60–75 mm in length (Pohlo 1964 in is facilitated by acetylcholine in Mytilus Zhang and Campbell 2002). planulatus, M. edulis, Spisula solidissima, and T. capax (Greenberg 1970). Stomach Possible Misidentifications described as Type V (for definition, see Reid The bivalve family Mactridae is 1977). A visceral “skirt", or a fold along the monophyletic based on both molecular and edge of the mantle tissue, often gives a home morphological characters and appears in the to commensal pea crabs in the genus fossil record beginning in the Cretaceous Pinnixa. Maximal systolic pressure was (Rice and Roddick 1993); there are seven recorded for T. capax to be 13 cm H20, which species reported locally. This heterodont is higher than the 11 cm H20 recorded for family is characterized by an internal Saxidomus giganteus (Florey and Cahill ligament, large shells (greater than 25 mm in 1977) (see description in this guide). length) that are internally porcelaneaus and a Exterior: pallial line with conspicuous sinus (Coan and Byssus: Valentich-Scott 2007). The anterior cardinal Gills: tooth on the left valve has an inverted A- Hiebert, T.C. 2015. Tresus capax. 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/1794/20649 and full 3rd edition: http://hdl.handle.net/1794/18839 Email corrections to: [email protected] shape in mactrids (see Fig. 2a), unlike the Francisco. Populations uncommon south of family Semelidae. Only those in the genus Humboldt Bay, where the congener Tresus Tresus have a shell with a conspicuous gape nuttallii becomes the more common species posteriorly (Fig. 3). In T. nuttallii, the shells (Coan and Valentich-Scott 2007). are ovate or elongate with a posterior end that Local Distribution: Local distribution in is longer than the anterior. Sometimes called many of Oregon's larger estuaries (e.g., Coos, the southern or Pacific gaper clam, Tresus Netarts, Tillamook, Yaquina). However, this nuttallii occasionally occurs in our area, but is species is not yet found in Siletz, or Nestucca, common only from Tomales Bay, California which is potentially due to the stronger south. It is more elongate than T. capax, and currents in the latter estuaries. has prominent horny plates on its siphon, not Habitat: Occurs in sheltered intertidal flats, just leathery tips. Its periostracum is more e.g., in sand, mud, mud with gravel and shell extensive and its beaks closer to the anterior hash. Individuals also occur in stiff clay down end than those of T. capax. The shell of T. to 30 cm. Occurs in eelgrass (e.g., Zostera capax the shell is ovate to rhombiodal in marina) beds, where digging for their harvest shape (Coan and Valentich-Scott 2007). (see Predators) does not seem to effect Tresus nuttallii also lacks the visceral “skirt" of eelgrass production (Yaquina Bay, Oregon, mantle tissue found in T. capax (Coan and Boese 2002), as well as non-native seaweed Valentich-Scott 2007). This skirt is an species (e.g., Sargassum muticum, White and elongated inner lamellar palp that hangs Orr 2011). However, they are rarely found dorsally and covers a portion of the posterior within mudflats dominated by ghost shrimp, N. visceral mass. Tresus capax also hosts up to californiensis (see description in this guide), three species of pea crabs (see Associates), presumably because of the unstable which are not found in T. nuttallii. Young substrate these create (Hancock 1979). individuals of both Tresus species can be Tresus capax is occasionally used as a easily confused with Mya arenaria, the bioindicator species in toxicity testing (e.g., softshell clam. Mya arenaria, however, has a tributyltin, Horiguchi et al. 2003). chondrophore on only one valve, its posterior Salinity: is rounded, not truncate and its siphons lack Temperature: A temperate northwest the leather-like flaps found in Tresus local species, where temperatures range 9–15° species. Tresus allomyax, an offshore (Matchell et al. 1971). species, is also reported from central Tidal Level: Occurs from 25–60 cm below California to Oregon (Coan and Valentich- the surface and mid- to subtidal to 50-meter Scott 2007). depths (Zhang and Campbell 2002). All other mactrid species have shells Associates: Hosts at least three pea crab with a narrowly gaping or completely closed species within the mantle cavity (Pearce posterior (Coan and Valentich-Scott 2007). 1965; Campos-Gonzalez 1986) including Mactromeris species have a chondrophore Pinnixa faba (see Fig. 296, Kozloff 1993; that projects ventrally. Mactromeris description in this guide), P. littoralis, Fabia catilliformis has a shorter anterior end than subquadrata (Kozloff 1993). They ingest posterior, and the opposite is true for M. material (e.g., diatoms) brought in by the clam hemphillii. Mactromeris californica and and their presence make the mantle tissue Simomactra species lack a chondrophore and irritated or blistered (Kozloff 1993). Only one only members of the latter genus have a beak adult pair is found per clam, however, several without undulations. In S. planulata, the shell juveniles can be present (Kozloff 1993). valves are subequilateral however the shell in Occasionally inhabited by nemertean worm S. falcata is longer anteriorly. Malacobdella grossa (Haderlie 1980). Coexists with the congener, T. nutallii, in Ecological Information Humbolt Bay and occasionally in Oregon and Washington and British Columbia, Canada Range: Type locality is Puget Sound, (Zhang and Campbell 2002). However, T. Washington (Orr et al. 2013). Known eastern capax is much more commonly found (Kozloff Pacific range from Kodiak, Alaska, to San 1993). Hiebert, T.C. 2015. Tresus capax. 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. Abundance: Very abundant and prodissoconch I and is followed by a commonly dug in local estuaries. prodissoconch II, or shell that is subsequently Constituted nearly all of Coos Bay's added to the initial shell zone (see Fig. 1, commercial clam catch, and up to 60% of Caddy 1969). Finally, shell secreted following Oregon's total commercial catch (Hancock et metamorphosis is simply referred to as the al. 1979). Individuals can be found at dissoconch (see Fig. 2, Brink 2001). Once densities of over 108 individuals/m2 (Yaquina the larva develops a foot, usually just before Bay, Hancock et al. 1979). However, T. metamorphosis and loss of the velum, it is capax is less common south of Humboldt Bay called a pediveliger (see Fig. 1, Caddy 1969; (see Range); high mean densities of 24.8 Kabat and O’Foighil 1987; Brink 2001). (For individuals/m2 were reported for populations generalized life cycle see Fig. 1, Brink 2001). in Humboldt Bay, California (Wendell 1976). The straight-hinge veliger larvae of T. capax Individuals more abundant in shallow than are 75–80 x 60 µm, when umbones are deeper water (Campbell and Bourne 2000). present they are 140–150 µm, pediveligers are 230–250 µm and at settlement individuals Life-History Information are 250 x 240 µm. Straight hinge veligers Reproduction: Dioecious. Individuals free- have an anterior end that is longer and more spawn in Jan–March (northern California, pointed than the posterior (see Fig.