Alasmidonta atropurpurea (Rafinesque 1831)

Alasmidonta (Decurambis) atropurpurea (Rafinesque, 1831)

Alasmodon atropurpureum Rafinesque, 1831:9. [Type-locality: “river Cumberland, very rare.” Type-material not in Academy of Natural Sciences (Johnson and Baker, 1973) and apparently lost. A neotype, herein selected, is in the Smithsonian Institution (150522). See “Remarks.”]

THE SHELL

DESCRIPTION.—Shell subovate; up to about 90 mm long, 40 mm high, and 25 mm wide; rather thin-shelled throughout but not fragile; and up to about 2 mm thick anteriorly. Anterior margin sharply rounded; ventral margin flatly curved, nearly straight, or concave centrally; posterior margin bluntly pointed and biangulate below, rounded above, and continuing into the dorsal margin that is broadly convex posteriorly and slightly concave anteriorly. Maximum inflation at the posterior ridge a little behind the center of the shell. Beaks of medium width, rounded, located about 1/4 the distance from anterior to posterior, and projecting somewhat above the hinge line. Posterior ridge well marked, broad, rounded or flattened centrally, and becoming double near the margin in some adult specimens. Posterior slope of moderate width, slightly concave proximally, and flattened distally. Growth increments marked by concentric ridges and grooves. Additional post-juvenile sculpturing consisting of faint, moderate, or well-developed, diagonal corrugations (about 6 to 10 in 10 mm) on the posterior slope, approximately perpendicular to the shell margin, and either restricted to the proximal area or more generally distributed over the posterior slope. Periostracum smooth and glossy between growth rests except roughened posteriorly, with clearly visible, extensive, greenish rays spread over a yellowish brown background in juveniles. The periostracum is thicker and dark blackish brown in adults but rays are still apparent in most specimens, particularly by transmitted light. Ligament strong, broad, thick, and of medium length.

Hinge teeth incomplete and variable. Pseudocardinal teeth rather small but conspicuous, pyramidal, and compressed; one in the right valve, anterior-ventrally directed, and two partially confluent teeth in the left valve. Interdental projection variable but in most specimens clearly developed in the left valve, and articulating with a depression in the right valve. In some specimens a small interdental projection also occurs in the right valve behind this depression. Lateral teeth absent or rudimentary. Beak cavity quite shallow. Anterior muscle scars variably shaped and variably impressed, pallial line continuous and well-marked, and posterior muscle scars shallow but clearly apparent. Scars within beak cavity consisting of one or two short, deep grooves on the back of the hinge plate in the left valve and, in many specimens, of a prominent, deep groove on the lower side of the hinge plate in the right valve placed diagonally and clearly indenting the plate margin when viewed perpendicular to the plane of the valve margin. Nacre shiny; bluish or bluish white; salmon, pinkish, or brownish centrally and in the beak cavity; and in some specimens with irregular brownish blotches.

Beak sculpture obliterated in available material.

TOPOGRAPHIC ANATOMY

(not figured)

SPECIMEN DESCRIBED.—USNM 801870, from Marsh Creek, near Brushy Creek, McCreary County, Kentucky, collected 12 July 1980 by S. M. Call and R. D. Stefano; pegged, fixed in 10% formaldehyde and preserved in 70% ethyl alcohol; shell length 77.8 mm, sex male (inferred).

DESCRIPTION.—Mantle whitish, opaque, and slightly thickened between edge and pallial line; grayish, semi-transparent, and very thin within pallial line with the yellow color of the foot and the brownish gray of the demibranchs clearly showing through. The slightly (0.5 mm) thickened edge is about 5.5 mm wide anteriorly and ventrally and 9.5 mm posteriorly; it is suffused with pigment on its inner side, the color being purple-brown and dense near the posterior openings and orange and less dense elsewhere. A broken band of blackish brown pigment extends around the posterior edge of the mantle externally from the mid-ventral area to the hinge; it is about 1.8 mm wide near the incurrent opening (where it appears as a series of squarish spots that are dark toward the mantle edge and paler away from it) and becomes narrower dorsally and even narrower ventrally.

Incurrent opening 11.3 mm long, purplish brown internally, and surrounded within by a (predominantly) single row of short subcylindrical papillae that are about 0.7 mm long and 0.3 mm wide at their bases. Separation between incurrent and anal openings achieved entirely by the diaphragm; there is no mantle connection. Anal opening 10.5 mm long, with crenulated edges, and darkly pigmented within. Connected portions of mantle edges 5.5 mm long between anal and supra-anal openings. Supra-anal opening 9.4 mm long, with a narrow orange-brown band of pigment between the internal flange and the mantle edge; narrow, and without papillae or crenulations.

Demibranchs brownish gray in the preserved specimen. Outer demibranch 50.2 mm long, 18.7 mm high centrally, with low, dorso-ventral wrinkles overall and a convex, curved central margin. The demibranchs narrow more-or-less evenly from near the center to the anterior and posterior extremities, which are acute. There are about 1.2–1.5 water tubes per mm (with approximately every third septum thicker than those between); about 16 double dorso-ventral surface filaments per mm, and about 5 cross-filaments per mm. Inner demibranch with an obtusely angled anterior extremity, a roundly convex anterior-ventral margin that extends about 8.5 mm beyond the outer demibranch, and a posterior ventral margin that tapers from the point of maximum convexity (located 1/3 of the distance from anterior to posterior) to the acute posterior extremity. The anterior-ventral margin extends about 8.5 mm beyond the outer demibranch but the posterior-ventral margin is flush with that of the outer demibranch. There are also about 1.2–1.5 water tubes per mm, 16 double surface filaments, and 5 cross-filaments per mm in the inner demibranch. The inner lamina of the inner demibranch is fully attached by a membrane to the visceral mass.

The labial palps have undulating dorsal margins, rather sharply rounded posterior margins, and flatly rounded ventral margins. They broadly overlap the inner demibranchs. The outer surfaces are smooth and the inner opposing surfaces of each member are radially furrowed (about 8 furrows per mm at the ventral margin). The outer palpus of each pair is fused to the mantle anteriorly and, for about 3/4 of its length, to the inner palpus.

VARIATION.—Table 16 shows the extent of variation in several characters. The specimen described above in detail (#1 in table) was somewhat unusual in ralative length of A-SA and SA. A sexually related dichotomy clearly exists with respect to number of water tubes per mm in the outer demibranchs. In specimen 6, the septa were not parallel but markedly diagonal to the surficial filaments, and in specimen 4 this condition also obtained (particularly in the posterior part of the demibranch) but to a reduced extent.

In addition to the data presented in the table, it is significant that the anal opening was strongly crenulated in specimen 3 (with tiny papillae 0.2 mm long); moderately crenulated in 1, 2, 5, and 6; and weakly crenulated in 4. The inner demibranch in specimen 3 extended beyond the outer demibranch (by 2.5 mm) in the posterior region as well as in the anterior region. In all specimens the inner demibranch was fully attached, by a membrane, to the visceral mass.

LIFE HISTORY

Alasmidonta atropurpurea is a rare species and, until 1980, almost nothing was known about its ecology. After its discovery in Marsh Creek, McCreary Co., Kentucky in 1979 (see below), however, an extensive description of the physiography, botany, zoology, and water chemistry of Marsh Creek was published (Harker, et al, 1980). Living specimens of A. atropurpurea were found there in slow-flowing water among cobbles where they were buried about 1/2 way down in the substrate. Details about the specific ecology of the species will be published by Mr. S. M. Call, Kentucky Nature Preserves Commission, Frankfort, Kentucky.

GEOGRAPHICAL RECORDS

CUMBERLAND RIVER SYSTEM.—North Fork Cumberland River (B. H. Wright! (USNM)). South Fork Cumberland River, Armathwaite, Fentress County, Tenn. (J. Lewis! (MCZ)) and Fentress County, Tenn. (B. Walker! (UMMZ)). Lynn Camp Creek, Corbin, Whitley County, Ky. (M. D. Barber! (MCZ)). Marsh Creek (alive) and Rock Creek (an empty shell), both McCreary Co., Ky. (1980, S. M. Call!, not included in Figure 23). Collins River, near Mount Olive, Grundy County, Tenn. (1978, A. E. Bogan!).

REMARKS

Rafinesque's description and his remarks appear to clearly indicate this species. Previous authors have not recognized it, probably because it is rare and because no type material exists. Selection of a neotype is therefore necessary to establish its identity. A neotype from near the original type locality (Cumberland River) and in the same river system, and closely matching Rafinesque's description and measurements, is hereby selected. It is in the Smithsonian Institution (catalog number 150522) and is from South Fork, Cumberland River, Fentress Co., Tennessee collected by B. H. Wright. Its measurements are: length, 74.3 mm; height (including ligament), 43.8 mm; width of both valves appressed, 28.3 mm; distance from umbone to anterior end measured parallel to long axis (B-A), 16.4 mm.

The lot (USNM 783317) from which the neotype was selected contains three other specimens whose measurements are: (a) length 80.0, height (without ligament) 38.6, width 26.5 mm; (b) length 76.7, height (without ligament) 39.8, width 22.5 mm; (c) length 55.3, height (with ligament) 32.3, width 17.8 mm. While specimen (a) is closer to Rafinesque's original measurements (length 3 inches, height 1½ inches, width 1 inch) than the neotype, only the neotype lacks prominent corrugations on the posterior slope. (Rafinesque described atropurpurea as smooth.) The neotype is faintly sculptured with corrugations but that is obviously a variable feature in this species.

The neotype and the other specimens in that lot were collected prior to January 1897, the date they were originally catalogued in this museum. No further specimens had been found until 1978, when a pair of disarticulated valves from an apparently freshly-dead specimen were collected from Collins River near Mount Olive, Tennessee by Mr. and Mrs. Arthur Bogan of the University of Tennessee at Knoxville. Soon thereafter (September, 1979), the species was discovered in Rock Creek and Marsh Creek, McCreary Co., Kentucky, by Mr. S. M. Call of the Kentucky Nature Preserves Commission. Later, Mr. Call kindly provided specimens for dissection, but they were received in July 1980, too late to be illustrated here.

Alasmidonta atropurpurea differs from A. marginata in that it is much smaller, lacks the high, angular posterior ridge and the truncated and flattened posterior slope, and the corrugations on the posterior slope are much less pronounced. It is similar in some respects to A. varicosa, but the posterior ridge in that species is also more elevated than in A. atropurpurea and the posterior margin is also more truncated.

Alasmidonta atropurpurea, common name Cumberland elktoe, is a species of freshwater mussel, an aquatic bivalve mollusk in the family Unionidae, the river mussels.

Alasmidonta atropurpurea er en art av dammuslingfamilien innen klassen muslinger som lever i ferskvann.

Alasmidonta atropurpurea − gatunek małża z rodziny skójkowatych (Unionidae). Jest zagrożony wyginięciem.