Dwarf Wedge Mussel

Alasmidonta (Pressodonta) heterodon (Lea, 1830)

Unio heterodon Lea, 1830:428, pl. 8:fig. 11 [Type-locality: “Schuylkill [River] and Derby [sic] Creek, P[ennsylvani]a.” Figured specimen and other original material not in Smithsonian Institution. Syntypes in mollusk collection of Academy of Natural Sciences of Philadelphia, catalog numbers 41004 and 41005 (Johnson, 1970:347; Johnson and Baker 1973:157).]

THE SHELL

DESCRIPTION.—Shell small, subtrapezoidal or “hump backed,” with unusual hinge teeth, up to 56 1/2 mm long, 31 mm high, and 18 1/2 mm wide, slightly thickened anteriorly (up to about 2 mm thick) and thinner, but not fragile, posteriorly. Anterior margin sharply curved; ventral margin broadly and flatly curved, predominantly straight, or slightly concave centrally; posterior margin roundly pointed near the base and broadly and obliquely truncated above; and dorsal margin of medium length and slightly curved. Maximum inflation a little above the center of the shell. Beaks low, rounded, located slightly more than 1/4 the distance from anterior to posterior, and projecting only a little above the hinge line. Posterior ridge rounded, somewhat inflated, and rather prominent. Posterior slope of medium width, concave above, and in some specimens traversed below by a low, rounded, radial ridge above the posterior ridge. Growth increments marked by concentric wrinkles and lines, especially in northern specimens. Except for lines of growth, additional post-juvenile sculpturing is lacking. Periostracum yellowish, olive brown, or blackish brown and, in some specimens, with narrow and wide reddish brown rays.

Hinge teeth small but well developed and unusual. Pseudocardinal teeth elevated, compressed, serrated, and numbering 1 or 2 in the right valve and 2 in the left. Interdental projection in left valve well-marked and articulating with a corresponding depression in the right valve. Lateral teeth narrow but strong, extending posteriorly to the end of the ligament or a little beyond, and characteristically 2 in the right valve and 1 in the left, the reverse of all other North American species having lateral teeth. Beak cavity rather narrow and not very deep. Anterior muscle scars impressed, pallial line well marked anteriorly but becoming somewhat obscure posteriorly, posterior muscle scars shallow but clearly marked, and scars within beak cavity consisting of a small ovate, elongate pit on the back of the hinge plate. Nacre bluish or silvery white, iridescent posteriorly, and with pinkish or salmon suffusions near the beak cavity.

Beak sculpture strong and composed of about 4 or 5 moderately heavy, blunt bars, the first 2 concentric and simple, the following ones broadly double-looped and forming sharp angles on the posterior ridge. Ridges visible only in well-preserved young specimens.

VARIATION.—Ortmann (1919:174) observed sexual differences in the shell, i.e.: “The male as a rule has a more compressed, and more ovate and elongate shell, with the lower margin gently and uniformly curved, the posterior ridge less sharp, and the posterior slope not truncated; while in the female the shell is more swollen in the region of the posterior ridge and just in front of it; has a more distinct posterior ridge, which renders the posterior slope more truncate, and makes the whole outline of the shell shorter and more trapezoidal. Furthermore in the female the lower posterior angle is more produced downward, so that the lower margin of the shell becomes rather straight, in some cases even concave.” Sexual differences in the shells of young specimens are not apparent.

Aside from this somewhat obscure sexual dimorphism the species is rather uniform in shell features (Table 7). There is some variation in the development of the hinge teeth. Specimens from some canals also tend to become larger and paler than those from other localities. The largest specimen seen, 56.5 mm long, is from a medium-sized river (Ashuelot River) near Keene, New Hampshire. Some of the largest specimens are also more conspicuously rayed than small, dark-colored specimens. The latter appear to be more characteristic of fine substrates containing organic material. This condition often occurs near river banks and coincides in part with the general trend seen in other unionids: that at a given river site small, dark-colored specimens tend to occur in mud near river banks whereas larger, paler specimens of the same species tend to occur in sandy substrates farther from the river banks when the current is more rapid.

TOPOGRAPHIC ANATOMY

SPECIMEN DESCRIBED.—From Petitcodiac River, Riverglade, 19 mi (30.4 km) WSW of Moncton, N.B., collected 22 June 1960 (NMC 46074, A. H. Clarke station C50); preserved directly in 70% ethyl alcohol; shell length 31.6 mm, presumed sex male.

DESCRIPTION.—Mantle pale brown but covered in the central portion with a network of fine, short, white, filamentous muscle fibers coalesced into spots and strands and in the posterior region by beautiful silvery radial muscle strands. Adductor muscles orange and contrasting with the mantle. A brown pigmented band runs just within the edge of the mantle from the posterior part of the ventral region to the posterior region and dorsally (touching the mantle edge) to a point above the posterior adductor. The incurrent opening is 5 mm long and surrounded just within the edge by about 3 ranks of small, narrow, reddish brown, pyramidal papillae, the inner papillae the longer (about 1.0 mm). Separation of incurrent and anal openings achieved by the diaphragm and by a narrow (0.5 mm) zone on each mantle edge that is held in contact with its opposite member during life. Anal opening 4 mm long and with crenulate edges and with a dark, reddish brown color band just within the opening. Mantle connection between anal and supra-anal opening of medium length (3.5 mm). Supra-anal opening narrow, slit-like, bordered within by a brown band, and 3 mm in length.

Demibranchs pale brown in preserved specimen. Outer demibranch 19.7 mm long, 5.9 mm high, with tapering ends and convex, ventral edge and with wavy radial wrinkles. Surface tissue with about 12 double radial filaments per mm and about 4 cross filaments; water tubes about 2.0 per mm. Inner demibranch about 26 mm long, 13 mm high, also with rounded ends and convex ventral edge, and extending well beyond the outer demibranch anteriorly and ventrally but only slightly overlapping it posteriorly. Surface tissue as in outer demibranch but cross filaments more obscure; water tubes also about 2.0 per mm. Inner lamina of inner demibranch not attached to visceral mass. Diaphragm split near the anal opening and perforated by the ends of the water tubes.

Labial palps narrow; slightly overlapping the inner demibranch; with long convex ventral edge; shorter, flatly sigmoid dorsal edge; sharp, centrally located posterior point; and with dorsal edges rolled outward. The outer surfaces are smooth and the inner opposing surfaces of each member are radially furrowed (about 7 furrows per mm at the margin); the furrows are also visible through the outer palp surfaces. The outer palpus is fused to the mantle and, for about 2/3 of its length, to the inner palpus.

Ortmann (1912:295) states: “Color of soft parts whitish; charged marsupium brown.” In live specimens collected from Ashuelot River near Keene, New Hampshire, the foot was orange.

VARIATION.—Table 8 shows the extent of variation of most of the features observed in available material. Mantle pigmentation is in a continuous band and is restricted to the posterior region in all specimens, although it differs in intensity between individuals. The incurrent opening is slightly longer than the anal or supra-anal openings (which are about equal in length) and the mantle connection between the anal and supra-anal openings varies from being approximately equal to them in length (6 specimens) to being much shorter (2 specimens). The labial palps overlap or touch the inner demibranch in 7 of the 8 specimens examined. These observations are in only partial agreement with those of Ortmann (1912:295) who reported that in about 30 specimens seen from Pennsylvania, A-SA is shorter than A in all of them, and the labial palps do not touch the inner demibranch.

Other features examined were the presence or absence of crenulation at the margin of the anal opening and relative dimensions of the demibranchs. The Petitcodiac River specimens are all finely crenulated and the Tar River specimen is strongly crenulated. The length and height of the demibranchs were measured and are given below expressed as a percentage of the shell length. Petitcodiac River specimens: outer demibranch, length 57%–63%, height 16%–19% (females) and 23% (male); inner demibranch: length 62%–70%, height 26%–30% (females) and 31% (male). Tar River specimen (gravid): outer demibranch, length 62%, height 34%, inner demibranch, length 68%, height 34%. The inner lamina of the inner demibranch was free from the visceral mass in all specimens.

GLOCHIDIUM

DESCRIPTION.—Glochidium depressed pyriform, with hinge straight, apex protuberant, and valves convex and rounded. Shell 0.255 mm high, 0.325 mm long, and about 0.100 mm in single valve convexity. The posterior margin is longer and more convex than the anterior and the apex is located about 42% of the distance from anterior to posterior (measured parallel to the hinge axis). Except for a band near the margin and near the apex the shell is pockmarked with shallow depressions (about 5–10 μm wide) and tiny pits (1–2 μm wide) within many of the depressions. The hinge is about 0.267 mm in length.

Each apical stylet is flatly recurved, about 0.080 mm long, about 0.028 mm wide at the base, tapering rapidly in the proximal third of its length to about half that width, and tapering gradually in the distal portion. Each stylet is connected on each side, and for most of its length, by a wide membrane which is attached to the valve margin. The exposed surface of each stylet is covered with major microstylets, each about 10 μm long and 5 μm wide, and bluntly pyramidal in shape. On the distal two-thirds of the stylet they are arranged alternately in transverse rows of 2 or 1 microstylets, or in alternate positions in successive rows each with 2 microstylets. The microstylets become more numerous and smaller on the base of the stylet and extend as small micropoints (<0.5 μm) onto the shell surface near the apex and onto the supporting membrane.

The glochidium described is from an adult collected from Canoe River, 2.45 mi (3.9 km) NNE of Norton, Bristol Co., Massachusetts by H. D. Athearn on 2 June 1969 (OSUM 25106). Other glochidia from the same adult were similar although minor variations in valve shape and convexity were apparent.

LIFE HISTORY

GRAVID PERIOD.—Ortmann (1919:174) gives the month of February and 24 April as gravid dates for A. heterodon. Additional dates provided now are 2 June 1969 for a specimen from Canoe River near Norton, Massachusetts (mature glochidia) and 27 August 1977 for a specimen from Tar River near Creedmore, North Carolina (immature glochidia). In the species as a whole the gravid period therefore spans at least the period from late August to the following early part of June.

NATURAL HOST.—Not recorded.

HABITAT.—”This is a rather rare and inconspicuous species…[It] occurs in medium or rather slow-flowing rivers of varying size on gravel, sand, or muddy sand bottoms and sometimes among submersed aquatic plants” (Clarke and Berg, 1959:25). In the Ashuelot River 4 mi (6.4 km) west of Keene, New Hampshire, on 8 August 1978, I found A. heterodon in muddy sand in 12 to 18 inches of water depth near the river bank under overhanging tree limbs. It shows markedly clumped distribution there and in the Petitcodiac River, the only localities from which I have collected it in abundance. Ortmann (1919:173) has also reported it as abundant in a canal. Its usual associate is Elliptio complanata and, occasionally, Strophitus undulatus or other riverine species.

GEOGRAPHICAL RECORDS

PETITCODIAC RIVER SYSTEM, NEW BRUNSWICK.—North River, 5 mi (8.0 km) NW of Salisbury, Westmoreland Co. (Athearn, 1953). Petitcodiac River at River Glade, 19 mi (30.0 km) WSW of Moncton, Westmoreland Co., (1960, A. H., A. R., and Louise R. Clarke! (NMC)).

MERRIMAC RIVER SYSTEM, MASSACHUSETTS.—Merrimac River, Andover, Essex Co. (MCZ).

TAUNTON RIVER SYSTEM, MASSACHUSETTS.—Canoe River, 2.45 mi (3.9 km) NNE of Norton, Bristol Co. (1969, H. D. Athearn! (OSUM)).

AGAWAM RIVER SYSTEM, MASSACHUSETTS.—Agawam River (MCZ).

CONNECTICUT RIVER SYSTEM.—Connecticut River at Bloomfield, Essex Co., Vt. (MCZ); Northumberland, Coos Co., N.H. (ANSP and MCZ); Ryegate, Caledonia Co., Vt. (MCZ); 2 mi (3.2 km) N of Monroe, Grafton Co., N.H.; Hanover, Grafton Co.; Northfield, Franklin Co., Mass. (all MCZ); Canal, Westfield, Hampden Co., Mass. (ANSP, MCZ, USNM); Springfield, Hampden Co.; and Granby, Hartford Co., Conn. (both USNM). Ashuelot River, 2.5 mi (4.0 km) N of Keene, Cheshire Co. and 2 mi (3.2 km) NW of Keene (both 1954, A. H., A. R. and Louise R. Clarke! (NMC). Philo Brook, Suffield, Hartford Co., Conn. (1959, S.L.H. Fuller! (MCZ)). Scantic River, 0.5 mi (0.8 km) above Hampden, Hampden Co., Mass. (1951, H. D. Athearn! (MCZ)).

QUINNIPIAC RIVER SYSTEM, CONNECTICUT.—Ten Mile River, Mixville, New Haven Co. (ANSP, MCZ). Quinnipiac River, Meriden, New Haven Co. (MCZ). Wilmot Brook, New Haven, New Haven Co. (MCZ).

HACKENSACK RIVER SYSTEM.—Brook flowing W from Closter to Hackensack, Bergen Co., N.J. (ANSP).

DELAWARE RIVER SYSTEM, PENNSYLVANIA.—Delaware River Drainage: Delaware River, Shawnee, Monroe Co. (Ortmann, 1919). Princess Creek, Kunkleton, Monroe Co. (Ortmann, 1919). Pohopoco Creek, 6 mi (9.6 km) E of Leighton, Carbon Co. (USNM). Delaware River, Bucks Co. (MCZ).

Schuylkill River Drainage: Big Neshaminy Creek near Edderson, Bucks Co. (ANSP; Ortmann, 1919). Schuylkill River at Hazellea, Chester Co. (ANSP); also at junction with Darby Creek, Delaware Co. (Ortmann, 1919); Canal, Manayunk, Philadelphia Co. (ANSP; Ortmann, 1919); below Fairmont Dam, Philadelphia Co. (Ortmann, 1919).

SUSQUEHANNA RIVER SYSTEM.—Susquehanna River, Columbia, Lancaster Co., Pa. (ANSP).

POTOMAC RIVER SYSTEM.—Potomac River near Washington, D.C. (USNM). McIntosh Run, 4 mi (6.4 km) N Leonardstown, St. Mary's Co., Md. (Johnson, 1970).

RAPPAHANOCK RIVER SYSTEM, VIRGINIA.—Rappahanock River Drainage: Mountain Run, Culpeper Co. (Ortmann, 1919). Marsh Run, 3 mi (4.8 km) SE of Remington, Fauquier Co. (Ortmann, 1919).

Rapidan River Drainage: Blue River, Orange Co. (USNM; Johnson, 1970).

JAMES RIVER SYSTEM, VIRGINIA.—North River, Lexington, Rockbridge Co. (ANSP and USNM).

PAMLICO RIVER SYSTEM, NORTH CAROLINA.—Tar River, Hwy 15, 5 mi (8 km) N of Creedmore, Granville Co. (1977, A. H. and J. J. Clarke! (USNM)).

NEUSE RIVER SYSTEM, NORTH CAROLINA.—Neuse River at Poolec Bridge, Wake Co. (MCZ); Raleigh, Wake Co. (MCZ); 6 mi (9.6 km) E of Raleigh (Johnson, 1970); and NE of Wendell, Wake Co. (USNM). Little River at Tarpley's Mill, 2 mi (3.2 km) NE of Wendell, Wake Co., (1950–51, W. Walter! (MCZ); Walter, 1956).

TYPE-SPECIES.—Alasmidonta mccordi Athearn, here designated.

Alasmidonta (Alasmidens) mccordi is principally distinguished by its weak beak sculpturing which does not extend more than 6 mm beyond the umbonal apex, by the presence of a lunule, and by the pseudocardinal teeth in the right valve that are curved, placed one above the other, and oriented parallel with the lunule. Other members of Alasmidonta (sensu lato) have stronger beak sculpturing, do not possess a lunule, and the pseudocardinal teeth in the right valve, if well-developed, are placed beside each other and are approximately perpendicular with the dorsal margin. Alasmidonta (Alasmidens) mccordi is probably now extinct.