Alasmidonta arcula (I. Lea 1838)

Alasmidonta (Alasmidonta) arcula (Lea, 1838)

Margarita (Margaritana) arcula Lea, 1838:71, pl. 22: fig. 69. [Type locality: Altamaha [River], Liberty [now Long] Co., Georgia. The lectotype herein selected, Lea's figured specimen, is in the Smithsonian Institution (USNM 86170).]

THE SHELL

DESCRIPTION.—Shell triangulate, greatly inflated, up to 80 mm long, 60 mm high, and 53 mm wide, somewhat thickened anteriorly (up to about 3 mm thick) and thinner and rather fragile posteriorly. Anterior margin sharply curved, especially above; ventral margin flatly rounded; posterior margin bluntly pointed basally and obliquely truncated above in adults (squarely truncated in juveniles); and dorsal margin oblique, sigmoid, and rather short. Maximum inflation at or in front of center in adults, at posterior ridge in juveniles. Beaks greatly enlarged, triangular, located about ⅓ the distance from anterior to posterior or a little more, and projecting far above the hinge line. Posterior ridge elevated and sharply angular except becoming rounded near the margin in adults. A second, low ridge above the first is also present. Posterior slope broad and concave near the shell margin. Growth increments marked by irregular ridges, threads, and grooves. Additional post-juvenile sculpturing consisting of a few, poorly defined, oblique threads on the posterior slope. Periostracum yellowish brown with numerous narrow and wide green rays in juveniles; rays partly obscure in adults but visible with transmitted light. Ligament short, thick, and strong.

Hinge teeth peculiar and incomplete. Pseudocardinal teeth greatly compressed and flattened, elongated, deeply serrated, buttressed below by a strong ridge, and numbering 1 in the right valve and 1 or 2 in the left. The pseudocardinal in the right valve and the posterior pseudocardinal in the left are both irregularly produced posteriorly and in some specimens are confluent with the one or two irregular interdental projections that occur in each valve. Lateral teeth vestigial or absent. Beak cavity broad, inflated, and deep. Anterior muscle scars partly impressed and well defined, pallial line clearly defined anteriorly but obsolete posteriorly, posterior muscle scars shallow and almost imperceptible, and scars within beak cavity consisting of one or two radial ridges and grooves on the back of the hinge plate. External features of sculpturing are also visible within the shell. Nacre bluish white and shiny, iridescent posteriorly and toward the beak cavity.

Beak sculpture very heavy and composed of six or seven short, broad, swollen, irregular, concentric ridges in the center of the shell and extending about 20 mm from the umbonal apices. Ridges very prominent both in young specimens and in mature specimens.

VARIATION.—Table 10 demonstrates that there is considerable variation, especially in the relative thickness of the hinge plate and in relative weight, and that this variation is not attributable to relative maturity. The exceedingly inflated form, sharp posterior ridge, large size, incurved umbones, and exaggerated beak sculpturing are characteristic features and serve to readily distinguish A. arcula from its most closely related congener, A. undulata. Anatomical and glochidial differences are also diagnostic.

TOPOGRAPHIC ANATOMY

SPECIMEN DESCRIBED.—From Ohoopee River, Ga. Rt. 147 bridge, 5.7 mi (9.1 km) SSW of Reidsville, Tattnall Co., Georgia, collected 22 August 1976 (E. P. Keferl, B. Marzluf, OSUM 40165.4); preserved in 70% ethyl alcohol, body length 75.7 mm, sex male (inferred).

DESCRIPTION.—Mantle of preserved specimen whitish, somewhat translucent, slightly darkened by branchial color showing through. An interrupted brown band of pigment borders the mantle edge throughout and is especially prominent posteriorly where it resembles a thick dashed line. This pigmented band is adjacent to the edges of the posterior mantle openings, i.e. no separating lip is present. The incurrent opening is 10.9 mm long and surrounded just within the edge by a single (partly double) row of small, very short (up to 0.6 mm), blunt papillae. Papillae pale orange-tan and gray in preserved specimen (but reddish brown according to Lea, 1863). Portions of mantle edges, which are appressed in life and separate the incurrent and anal openings, very short (0.5 mm). Anal opening with thickened, smooth edges, without papillae, and 7.2 mm long. Mantle connection between anal and supra-anal opening long (13.0 mm). Supra-anal opening narrow and slit-like, with an internal, narrow gray band, and 10.0 mm long.

Demibranchs of preserved specimen tan-brown, the inner somewhat darker than the outer. Outer demibranch 50 mm long, 30 mm high, with anterior and ventral margins curved and evenly convex, the posterior margin broadly concave below (the demibranch becoming narrow posteriorly), with 2 or 3 prominent radial wrinkles in its central portion, several wrinkles in the posterior portion, and with about 11 double filaments on the surface and about 1.0 underlying water tube per mm. Inner demibranch about 55 mm long and 30 mm high, with free margin rounded, and extending beyond the outer demibranch anteriorly (5 mm beyond) and posterior-ventrally (2.5 mm) but centrally slightly overlapped (1 mm) by the outer demibranch. Inner demibranch with about 11 double filaments per mm and 1.0 underlying water tube per mm. The inner lamina of the inner demibranch of the specimen examined is not completely attached to the visceral mass but exhibits definite breaks in that attachment.

Labial palpi pale yellowish brown, wide and somewhat irregular, bluntly expanded distally, with everted edges, with outer surfaces of each pair smooth, inner surfaces radially furrowed (about 4 furrows per mm at margin). Each palpus is joined below its dorsal margin to its corresponding member for more than half its length.

VARIATION.—Table 11 contains measurements, derived from all of the available preserved specimens, of the same soft-part characteristics listed for other species. The material is not sufficient to reveal very much about intraspecific variability. Some contrasts with Alasmidonta undulata are apparent, however, viz. in A. arcula A-SA tends to be longer than the anal opening (6 out of 6 specimens) and longer than the supra-anal opening (5 out of 6) but that tendency is reversed in A. undulata, the papillae within the incurrent opening are fewer and much smaller in A. arcula than in A. undulata.

Other characters examined were the number and extent of anal papillae (absent in all specimens) and the extent of the connection between the inner lamina of the inner demibranch and the visceral mass. Specimens 2, 4, 5, and 6 were completely connected but specimens 1 and 3 were not. The outer demibranch is radially wrinkled in this species (in contrast to A. undulata) even, as in specimens 3, 4, and 5, when thickened and charged with eggs or glochidia. The latter character, i.e. and incomplete connection between the inner demibranch and the visceral mass (in 2 of 6 specimens examined), and the “crimpled” appearance of the outer demibranch, were also observed by Lea (1863:83).

GLOCHIDIUM

DESCRIPTION.—Glochidium pyriform, asymmetrical, about 0.339 mm high, 0.270 mm long, and 0.181 mm in width. The posterior margin is much more convex and protuberant than the anterior, the anterior portion is more inflated than the posterior, and the apex is located about 37% of the distance from anterior to posterior. The surface is malleated (depressions shallow, subcircular or polygonal, and about 6–14 μm wide) and pitted. The pits are about 2 μm wide and number about 1–6 in each depression. The apical and marginal areas are without pits or depressions but there are fine, concentric wrinkles near the apex and low, obscure ridges perpendicular to the margin on the marginal areas. The hinge is flatly sinuate, about 0.200 mm long, and the ligament is narrow.

Each apical stylet is recurved, about 0.112 mm long, 0.043 mm wide at the base, subtriangular in cross-section, and gradually narrowing to a pointed, lancehead-shaped terminal flange. Each stylet is also supported on each side, for about ⅖ of its length, by a membrane that is connected to the ventral edge of the valve. Except for lateral and terminal parts of the flange, the flatly rounded and exposed surface of each stylet is covered with about 110 microstylets, each about 15μm long, and arranged in about 8 rows (about 4 rows near the apex), the rows approximately parallel with the midline of the stylet. The microstylets are directed toward the distal end of the stylet and are lanceolate or spatulate, compressed, with 2 to 6 sharp, lateral, cutting edges and a roundly-pointed apex. Other smaller (1–2 μm long) pyramidal microstylets occur on the edges and on the base of the stylet and continue as tiny (0.5–1 μm), rounded, micropoints along the ventral edges of the valves.

This description has been drawn from views of three glochidia taken from an adult specimen collected in Little Ocmulgee River near Lumber City, Wheeler/Telfair County, Georgia on 28 May 1979, by A. H. Clarke and J. J. Clarke. There is some variation among the numerous specimens mounted for examination with the SEM, especially in that some of the stylets were without laterally-expanded apices and in that the cross-sectioned shapes of the microstylets were diverse. In general, however, they resemble the glochidia of Alasmidonta undulata except that they are much more asymmetrical and are somewhat smaller.

LIFE HISTORY

GRAVID PERIOD.—Two of the specimens from the Ohoopee River (see Table 11), collected on 22 August 1976 by E. P. Keferl and R. Marzluf, contained eggs, each about 0.2 mm in diameter. This date is probably close to the beginning of the gravid period for this species. One of the specimens collected in Little Ocmulgee River on 28 May 1979 by me had mature glochidia remaining only in the anterior portion of the outer demibranch. That date, then, is probably very close to the end of the gravid period for A. arcula.

NATURAL HOST.—Not determined.

HABITAT.—According to Johnson (1970:353) A. arcula “lives in sandy mud below sand bars in sluggish water and eddies.” This is confirmed by our observations. In Little Ocmulgee River we collected four specimens in mid-river, on a sand bar, and in sand below the bar, in water depths of 0.2 to 0.8 M. The river is about 25 M wide at that point, up to about 1.5 M deep and with sandy bottom throughout. All of the other records for this species are also from riverine habitats in the Altamaha River System.

GEOGRAPHICAL RECORDS

ALTAMAHA RIVER SYSTEM, GEORGIA,—Ocmulgee River Drainage: Limestone Creek, near Hartford, Pulaski Co. (Johnson, 1970). Ocmulgee River at Hawkinsville, Pulaski Co., and below Lumber City, Telfair Co. (Johnson, 1970); and 4 mi (6.4 km) E of Lumber City, Wheeler Co.-Jeff Davis Co. (1962, H. D. Athearn and M. A. Athearn! (NMC, OSUM)).

Little Ocmulgee River Drainage: Gum Swamp Creek at bridge, Scotland, Wheeler Co.-Telfair Co. boundary and below Ga. Highway 134 at Towns, Wheeler Co.-Telfair Co. boundary (both 1965, C. B. Stein and J. Scofield! (OSUM)). Little Ocmulgee River at Ga. Highway No. 134 bridge, Towns, Wheeler Co.-Telfair Co. boundary (1976, K. Wright! (OSUM)). Little Ocmulgee River, ½ mi (0.8 km) W of Ga. Hwy 19, near Lumber City, Wheeler Co.-Telfair Co. boundary (1979, A. H. Clarke and J. J. Clarke! (USNM)).

Ohoopee River Drainage: Ohoopee River, 3 mi (4.8 km) N of Lehman, Emanuel Co. (Johnson, 1970); and at Ga. Hwy. 147 bridge, 5.7 mi (9.1 km) SSW of Reidsville, Tattnall Co. (1976, E. P. Keferl and B. Marzluf! (OSUM)).

Altamaha River (main channel): Altamaha River at U.S. Hwy. 301, 5 mi (8.0 km) NNE of Jesup, Long Co.-Wayne Co. boundary (1965, F. J. Moore!; 1965, E. P. Keferl!; and 1970, W. J. Clench and D. H. Stansbery! (all OSUM)); at Liberty Co. (USNM); at Fort Barrington, McIntosh Co. (ANSP) and “near Darien,” [McIntosh Co.] (Lea, 1863).

The Altamaha arcmussel (Alasmidonta arcula) is a species of freshwater mussel, an aquatic bivalve in the family Unionidae.

It is an endangered species.