Alasmidonta robusta A. H. Clarke 1981

Alasmidonta (Decurambis) robusta

THE SHELL

DESCRIPTION.—Shell subovate, up to about 66 mm long, 43 mm high, and 33 mm wide, rather thin-shelled throughout but not fragile, up to about 2 mm thick anteriorly. Anterior margin well rounded; ventral margin broadly curved (more convex anteriorly than posteriorly); posterior margin terminating in a rounded point below the center and flatly convex and irregular above; and dorsal margin of medium length and curved. Maximum inflation in front of the posterior ridge near the center of the shell. Beaks of medium width, rounded, located about 32% to 39% of the distance from anterior to posterior, and projecting to a variable extent above the hinge line. Posterior ridge well-marked, sharply rounded, becoming more gently rounded near the margin and terminating posterior-ventrally. Posterior slope of moderate width, sharply defined by the posterior ridge, concave proximally and flattened distally. Growth increments strongly marked by concentric ridges and grooves but disc otherwise unsculptured. Additional post-juvenile sculpturing consisting of well-developed, diagonal, moderately coarse (about 8 to 15 in 10 mm) corrugations on the posterior slope, approximately perpendicular to the shell margin, and generally distributed over the whole posterior slope. Periostracum smooth and glossy except roughened posteriorly, with clearly visible, extensive, wavy, greenish rays spread over a yellowish brown background. The periostracum is darker in adults but rays are still clearly apparent. Ligament strong, broad, thick, and of medium length.

Hinge teeth incomplete and variable. Pseudocardinal teeth of moderate size, conspicuous, pyramidal, and compressed; one in the right valve, anterior-ventrally directed, and one in the left valve that is confluent with the interdental projection. Interdental projection prominent, long, centrally acute, located in the left valve, more elevated (in most specimens) than the adjacent pseudocardinal tooth, and articulating with a depression in the right valve. Lateral teeth absent and replaced by a single nonarticulating thickening of the hinge plate in each valve. Beak cavities moderately wide and deep. Anterior muscle scars variably shaped, impressed, and bounded by a low, rounded ridge which is decurrent below the pseudocardinal tooth in each valve; pallial line continuous and well-marked; posterior muscle scars shallow but clearly apparent and iridescent, and scars within beak cavity consisting of one or two short, deep grooves on the back of the hinge plate in each valve. Nacre dull or somewhat shiny; bluish or bluish white; and salmon, pinkish, or brownish centrally and in the beak cavity.

Beak sculpture mostly obscure in available material but apparently consisting of a few moderately heavy, concentric, single-looped bars that are angular on the posterior ridge.

TYPES.—The holotype of A. robusta is from Long Creek (near Charlotte), Mecklenberg County, North Carolina, originally from the large nineteenth century collection of Charles M. Wheatley. It was later deposited at the University of Pennsylvania and finally in the Philadelphia Academy of Natural Sciences (catalog number 126755). The specimen is 66.0 mm long, 43.0 mm high, 33.2 mm wide (both valves appressed), and the umbones are 25.7 mm from the anterior extremity (measured parallel to the long axis of the shell). There are also four paratypes in the lot (see below). A comparison of A. robusta and other species is included under Remarks.

VARIATION.—Measurements of the type lot are given in Table 20.

GEOGRAPHICAL RECORDS

The type lot contains the only known specimens of this species. It is probably now extinct.

Long Creek is a tributary of the Catawba River, which is part of the Wateree-Santee River System of North and South Carolina.

REMARKS

No information is available about the anatomy, glochidia, or life history of A. robusta. About its ecology we know only that it is a localized creek species and that it is now either very rare or extinct. Long Creek traverses an area very close to, and north of, Charlotte, North Carolina. So many of its tributaries have been impounded by dams that it now carries little water. On 29 May 1979, after several days of rain in the area, the main creek channel in a mile-long stretch about 5 miles west northwest of Charlotte carried only 4 to 6 inches of water throughout most of its area. Corbicula was common in some depressions and deep channels, and a few empty shells of Elliptio and Lampsilis show that some unionids may be present. It seems probable that unionid populations would not persist in Long Creek during times of unusual drought and that present populations there, if they exist, are not descended from the same populations that existed there 100 years ago.

The holotype of A. robusta bears a striking resemblance to A. mccordi. They are similar in shape and size, and are both covered with wide and narrow, wavy, green rays on a yellowish-brown background. They differ markedly in tooth structure, however. A. mccordi has strong and unique pseudocardinal teeth, clear remnants of lateral teeth, and no interdental projection. A. robusta has very different pseudocardinal teeth, no remnants of lateral teeth, and a conspicuous interdental projection in the left valve.

Alasmidonta robusta is also similar to some specimens of A. varicosa, particularly to those in a lot of unusually large and high specimens collected by C. H. Conner many years ago in Ridley Creek near Philadelphia, Pennsylvania. They differ, however, in that A. robusta has a large interdental projection in the left valve; a single, rounded posterior ridge; and mostly wide, wavy, prominent greenish rays even in adult specimens. In A. varicosa from Ridley Creek the interdental projection is either absent or vestigial, the posterior ridge has a tendency to be double and rather angular, and the periostracal rays, where visible in adult specimens, are narrow and not wavy.

A. varicosa also occurs in the same drainage system as A. robusta and the two species are easily separable. Twelve specimens of A. varicosa were collected from the Catawba River at Bridgeport, Burke Co., N.C. by A. E. Ortmann in 1914 (Carnegie Museum, catalog No. 61.7132). They are similar to A. robusta except that they lack the prominent interdental projection seen in that species and their shells are thinner, much more quadrate, and their ventral margins are much less convex than in A. robusta.

Although A. robusta is obviously related to A. varicosa and to A. marginata, the interdental projection is so striking that one is reminded of the genus Lasmigona, in which that feature is characteristic. In 1863 Issac Lea described several unionid species from the Catawba River drainage near Charlotte, North Carolina. One of these was Unio charlottensis (now Lasmigona subviridis (Conrad, 1834)). Specimens of Lasmigona “charlottensis,” from the vicinity of Charlotte, are up to 116 mm long ((Johnson (1970), also Smithsonian Institution specimens (e.g. USNM 85402)), however, and this is nearly twice as long as the usual maximum length of L. subviridis from all other localities. At any rate, Alasmidonta robusta differs from L. subviridis, (including specimens of L. “charlottensis”) in that L. subviridis has well-developed lateral teeth while A. robusta has none, in L. subviridis the interdental projection is poorly developed while in A. robusta it is conspicuous, and the shell in L. subviridis is compressed whereas in A. robusta it is moderately inflated.

It has been a problem to decide if Alasmidonta robusta is specifically distinct from A. varicosa. Analysis of a sample of Long Creek water, taken on 29 May 1979 following several days of rain, revealed a total hardness of 40 ppm. Experience has shown that stream water analyzed immediately following substantial rain may be expected to have a much lower CaC03 concentration than one encounters at the same locality at other times. Therefore, before headwater impoundments were carried out, the average total hardness of Long Creek water may have been much higher than 40 ppm. The fact that Lasmigona subviridis from the same area is so large also implies that unusually favorable water quality may occur in the region. (Empty shells of Elliptio complanata and Lampsilisradiata collected in Long Creek on 29 May 1979, however, were entirely normal.)

Nevertheless, A. robusta differs substantially from A. varicosa, even when presumably excellent conditions for growth of that species occur, as demonstrated by lots from Ridley Creek and elsewhere. There appears to be no feature of ecology alone that could cause, for example, the interdental projection in A. varicosa to become as disproportionately enlarged as we see in A. robusta. Significant genetic differences between the taxa are therefore presumed to exist and A. robusta is considered a distinct species.

Arcidens Simpson, 1900:661. [Type-species: Alasmidonta confragosa Say, by original designation.]

The glochidium of Arcidens confragosus (Figure 31) is pyriform, strongly asymmetrical, with malleated and pitted surfaces and with lingulate stylets that bear about 6 longitudinal rows of major microstylets on both the proximal and the distal halves. The glochidium of A. wheeleri is unknown.

Comparative features of the adults are: shells medium-sized to large (about 70–140 mm long), relatively high (H/L >0.70) without sexual dimorphism, with heavy diagonal ridges on the posterior slope and anterior to the posterior ridge, with non-dehiscent periostracum, and with hinge teeth that are well-developed and complete. The anterior-ventral edge of the mantle is not lobate and the mantle edges between the anal and supra-anal openings are fused together.

Arkansia Ortmann and Walker, 1912:98. [Type-species: Arkansia wheeleri Ortmann and Walker, by original designation.]

The subgenus Arkansia is differentiated from Arcidens (sensu stricto) by the presence in the former of a lunule; of an anterior pseudocardinal tooth in the left valve, and a pseudocardinal tooth in the right valve, which are both curved and parallel to the lunule; heavy sculpturing only on the posterior half of the shell; and barely perceptible beak sculpturing. In Arkansia the external membrane of the outer demibranch is unique: it is openly porous, like a loosely-woven net. The glochidia are unknown.

Arcidens (Arkansia) wheeleri (Ortmann and Walker, 1912)

Arkansia wheeleri Ortmann and Walker, 1912:98, pl. 8. [Type-locality: “Old River [an oxbow of Ouachita River], Arkadelphia, Arkansas.” Holotype in the mollusk collection, Museum of Zoology, University of Michigan, Ann Arbor, Michigan, catalog number 105514, labelled “Ouachita Road, 3 miles [4.8 km] above Arkadelphia, Clark Co., Arkansas.”]

THE SHELL

DESCRIPTION.—Shell quadrate-ovate or subcircular, subinflated, up to about 110 mm long, 73mm high, and 48 mm wide, moderately heavy, somewhat thickened anteriorly (up to about 6 mm thick) and half as thick posteriorly. Anterior margin broadly rounded and continuing smoothly into the ventral margin, which is also evenly rounded throughout or rounded anteriorly and centrally but flattened posteriorly; posterior margin wide, truncated, oblique, and angular above and below; and dorsal margin long, straight behind the umbones and sharply concave for a short distance below the beaks. When viewed with both valves appressed, this anterior-dorsal depression (the lunule) is round, hemispherical, and unusually well-defined. Maximum inflation near the midline, slightly above the center. Beaks inflated, rounded, inclined forward, and located about 26% to 31% the distance from anterior to posterior, and projecting somewhat above the hinge line. Posterior ridge single or double but low and inconspicuous. Posterior slope rather broad and expanded centrally; the shell has a tendency toward being slightly alate. Growth increments marked by concentric, flattened, annual rings that tend to interrupt the surface sculpturing and numerous fine ridges and grooves. Additional post-juvenile sculpturing consisting of about 3 to 5 broad, rounded, diagonal ridges (similar to those in Amblema plicata (Say), but lower) that cross the center and the posterior of the shell; irregular, crowded, flattened tubercles or flattened tubercular ridges that cover the posterior half; and broad, rounded corrugations on the posterior ridge and the posterior slope that cross the lines of growth at a 90° angle, and are interrupted by a flattened, radial groove in the center of the posterior slope. In some specimens the corrugations are reduced and appear as non-aligned swellings. Periostracum chestnut brown, with darker concentric bands, and somewhat lustrous. Ligament dark brown, rather long, and fragile when dry.

Hinge teeth heavy but not entirely complete. The left valve has two strong pseudocardinal teeth. The anterior pseudocardinal is compressed, serrated, and parallel with the sharply defined lunule; the posterior pseudocardinal is pyramidal, rounded, thick, irregular, serrated, and not parallel with the lunule. The pseudocardinal teeth are separated by a deep cavity and the posterior member is more or less confluent with a broad, high, arched, serrated, interdental projection. The lateral teeth are double in the left valve and of medium length; the lower lateral is, in fact, confluent with, or an extension of, the interdental projection. The right valve has one heavy, erect, pseudocardinal tooth, convex below and supported by a buttress and concave above and roughly parallel with the lunule, and one strong, straight, lateral tooth of moderate length. Beak cavity excavated, deep, and of moderate width. Anterior adductor muscle scar impressed and rather small, pallial line clearly defined and lightly impressed, posterior adductor muscle scar rather large and well-marked but not impressed, and scars within the beak cavity consisting of 1 to 3 ovate pits on the underside or on the back of the hinge plate. Nacre white or salmon-colored and porcellaneous throughout but iridescent posteriorly. A broad greenish band occurs around the edge where the nacre is thin.

Beak sculpture reduced and fine, consisting of 2 or 3 oblique, double-looped bars that are restricted to the extremity of the beaks and extend only about 4 mm from the umbonal apex. It is visible only on young specimens.

VARIATION.—Table 21 shows that among the material available there is some variation in relative shell dimensions. The largest specimen is relatively the most depressed and the smallest specimen the most elevated (i.e. the most circular) but no clear general trend among other specimens occurs. Relative inflation appears to be inversely related to increased length, however. In most specimens the heavy, diagonal ridges across the middle of the shell are obvious and in other specimens, particularly in young ones, they are obscure.

Wheeler (1918), in describing the nacre of this species, says: “In young shells the entire margin is widely bordered with a rich salmon, in most adults it is a warm cream color, while in some specimens it is an opalescent blue.” Normal variation in periostracal color also occurs, the older specimens being the darker.

TOPOGRAPHIC ANATOMY

SPECIMEN DESCRIBED.—From Kiamichi River, 1.2 mi (1.9 km) SE of Clayton at US Rt. 271 bridge, Pushmataha Co., Oklahoma, collected 22 August 1971 (OSUM 32816.2, D.H.Stansbery!); preserved in 70% ethyl alcohol; length of soft body 66 mm, sex female.

DESCRIPTION.—Mantle pale orange, somewhat translucent in the center, and with color of branchiae and foot showing through. Mantle edge without additional pigment except for a narrow band of pale and dark shades of brown adjacent to the incurrent and anal openings and continuing as an even narrower band around the supra-anal opening. Incurrent opening 11.1 mm long and surrounded by about 3 rows of flattened, small, papillae, the inner papillae the longest (1.1 mm). Separation of the incurrent and anal openings is achieved, at the mantle edge, by appression in life of opposing mantle edges over a very narrow (0.5 mm) distance. The anal opening is 12.1 mm long and is surrounded by a single row of tiny, flattened papillae that are appressed to the sides of the opening and project slightly beyond its edge. The mantle connection between the anal and supra-anal openings is 3.5 mm in length and the supra-anal opening is long (10.3 mm opening, with dorsally expanded edges) and slit-like.

Demibranchs of preserved specimens pale brown. Outer demibranch 43 mm long, 19 mm high, with narrow anterior and posterior extremities and openly curved free margin and with irregular, horizontal wrinkles. The external membrane of the outer demibranch is fragile and porous, appearing more like a closely-woven net (see figure 28d) than in any other species seen, with about 11 double sets of radial filaments and about 3½ horizontal cross filaments per mm. The underlying water tubes are thick-walled and number about 3.0 per mm. Inner demibranch 45 mm long, 23 mm high, of the same shape as the outer demibranch but extending beyond it anteriorly (but not reaching the labial palps) and ventrally, and with a few radial wrinkles in the anterior-ventral region. The external membrane of the inner demibranch is also fragile but not so openly porous, and has about 15 double rows of radial filaments per mm, about 2½ horizontal cross filaments, and about 1.0 underlying water tube per mm. The inner laminar of the inner demibranch is not attached to the visceral mass. The diaphragm is split for a short distance at the posterior extremity and perforated there by the open ends of the water tubes.

Labial palps pale brown, wide, flatly curved above, distally rounded below, and pointed above the midline, and with the outer surfaces smooth and the inner surfaces of opposing members radially furrowed (about 5 furrows per mm at margin). The outer palpus is attached to the mantle for ⅔ of its length and the subdorsal interpalpal connection extends for of the length of each member.

VARIATION.—The specimens from the Kiamichi River are the only ones available for anatomical study and they show little variation. They agree in general with Ortmann and Walker's (1912:97–98) description except for some details. In the material examined, the mantle connection between the anal and supra-anal opening is no more than half as long as the anal opening (Table 22); Ortmann and Walker state that A-SA is a little shorter than, or somewhat longer than, the anal opening. The labial palps in one of our three specimens touches the inner demibranchs; according to Ortmann and Walker they do not touch. There is also some variation in strength of mantle pigmentation among Kiamichi River specimens; mantle pigmentation is not mentioned by Ortmann and Walker.

No glochidia from A. wheeleri have been available for study and nothing has been published regarding them.

LIFE HISTORY

Nothing is known about the timing or duration of the gravid period of A. wheeleri, or about its natural hosts.

Wheeler (1918:112–3) has described Old River, the type locality of A. wheeleri, as follows: “Old River…is really an ox-bow lake, a former channel of the Ouachita, and is still connected to it by a small creek which does not appear to dry up in summer. Its mouth is about two miles north of Arkadelphia on the left bank, almost lost in a rather dense and difficultly passable swamp. Here, and for a mile or more upstream, Old River is deep and rather wide, with a very sluggish current. In this habitat are found very large specimens of Anodonta suborbiculata Say, which are of great beauty, and the largest specimens of Arkansia wheeleri Walker and Ortmann [sic]. One of the latter measured 109.25 by 87 by 58 mm. In the summer “Half-Moon Lake,” the upper channel of Old River, is set off by the subsidence of water on the sand bars, and through the narrow creek which connects it with its lower course it is quite impossible to navigate even a small canoe. Young Arkansia are found in the shallow waters both on the sand bars and muddy bottoms, but like other anodontine species they prefer the oozy mud of the river margins where there is little or no current.”

Mr. Mark Gordon, a graduate student at the University of Arkansas, visited Old River in May 1979 and found the locality now much the same as it was when described by Wheeler. No Arkansia were found and high water prevented a more thorough search.

GEOGRAPHICAL RECORDS

OUACHITA RIVER SYSTEM.—Old River, an oxbow of Ouachita River, near Arkadelphia, Clark Co., Ark. (type-locality) (ANSP, MCZ, USNM). Ouachita River below Arkadelphia (Wheeler, 1918).

RED RIVER SYSTEM.—Kiamichi River, 1.2 mi (1.9 km) SE of Clayton, Pushmataha Co., Okla. (1971, D. H. Stansbery! (OSUM)) and 8.5 mi (13.6 km) NE of Hugo at Spencerville Crossing, Choctaw Co., Okla. (1968, B. Valentine! (OSUM)). Little River, Whitecliffs, Stevier Co.-Little River Co. boundary, Ark. (ANSP).

Arcidens (sensu stricto) may be distinguished from the subgenus Arkansia by (in the former) the absence of a lunule, the presence of pyramidal, dorso-ventrally compressed pseudocardinal teeth that are not curved, by heavy sculpturing over nearly the entire surface, and very heavy, pustulous, beak sculpturing. The outer demibranch is not openly porous. The glochidia are pyriform, markedly asymmetrical, and otherwise unusual (see text).