Alan D. Christian, Ph.D. & Aquatic Ecology Laboratory (AEL)

Key to Freshwater Mussels in the St. Francis River Basin
Arkansas and Missouri

John L. Harris, and Cristin D. Milam, 2002, Field Identification Notebook Mussels of the St. Francis River Basin, Arkansas and Missouri. Version 1.0.

 

  1a. External portion of shell having knobs, pustules or parallel ridges

  1b. External portion of the shell mostly smooth; (posterior wing may have ridges)

 

  2a. Shell with single row of knobs on the posterior ridge or on the middle portion of the shell

  2b. Shell not as above

 

  3a. Shell elongate, more than twice as long as deep

  3b. Shell not elongate, knobs prominent and high

  3c. Posterior ridge with large knobs, ridge extends to margin of the shell; external shell coloration usually with down-pointing pigmented markings (sometimes absent in larger specimens)

 

  4a. Shell with two rows of knobs or pustules in the umbo region, more distinct in smaller specimens; shell sculptured with a rough texture; lateral teeth poorly developed

  4b. Shell with two prominent rows of knobs or pustules; pseudocardinal teeth and lateral teeth well developed

  4c. Not as above

 

  5a. A definite sulcus between the two rows of large pustules; sulcus without small pustules in the umbo region

  5b. Pustules in definite rows, no sulcus between the rows; pustules sparse, one to three per row, sometimes pustules may be present only on one row

 

  6a. Many pustules on shell; margin of shell nearly round; nacre purple

  6b. Rounded to slightly triangular shell with sulcus betweeen posterior ridge and anterior half of shell; sulcus with wrinkeled or striated appearance; shell colored with tiny flecks that become dense enough to form fines rays; posterior slope fluted

  6c. Shell with parallel ridges or with pustules more or less randomly scattered

 

  7a. Shell elongate, posterior ridge prominent and extends margin of the shell; pustules usually present, sometimes extremely prominent

  7b. Pustules randomly distributed on shell, abundant to almost absent; usually green pigmented area on umbo, especially in younger individuals; shell nearly round

  7c. Not as above

 

  8a. Shell with 3 or more parallel ridges beginning on the umbo and directed towards the posterior ventral margin, pustules and/or crenulations absent from anterior; nacre usually white, sometimes with a purplish iridescence on the posterior margin

  8b. Shell quadrate; usually with pustules and/or ridges

 

  9a. Shell quadrate and with fluting, pustules, ridges or corrugation on posterior slope and other portions of the shell

  9b. Shell quadrate to rounded with fluting and ridges more or less restricted to posterior slope and posterior ridge

 

10a. Umbo covered with small zigzag ridges; posterior slope fluted; exterior coloration black, not shiny; nacre usually white, often with copper or gold colored spots

10b. Posterior ridge prominent, posterior ventral margin appears pointed (especially in smaller individuals); pustules and ridges usually present but not prominent; nacre purple to copper colored

 

11a. Posterior slope fluted; shell compressed and valves relatively thin; beaks low, narrow, not raised above hingeline; external coloration chestnut brown to dark brown, rays often present in smaller specimens; pseudocardinal teeth present, lateral teeth reduced or absent

11b. Shell oval with large, prominent posterior dorsal wing, often with small flutations or corrugations; pseudocardinal teeth moderately to well developed, lateral teeth absent; nacre white

 

12a. Shell decidedly more elongate than rounded; posterior ridge not sharply angled; shape rhomboidal to elliptical

12b. Shell triangular, round or oval in shape

 

13a. Shell without distinct color rays, base color banana yellow; posterior ridge rounded and posterior slope smooth

13b. Shell background coloration yellow, yellow-green, green to brown; color rays present

13c. Shell not as above

 

14a. Shell background coloration yellow, faint to prominent green rays usually present; posterior wing prominently to poorly developed; shell relatively thin; lateral and pseudocardinal teeth thin

14b. Shell with distinct green or black color rays

 

15a. Rays prominent, thin to broad in width but broken – not continuous; females with indentation to shell anterior to posterior ridge

15b. Rays prominent to obscure, thin to broad in width but continuous – not broken or interrupted; rays not noticeably concentrated in posterior one third of shell

15c. Rays prominent, thin to broad in width but continuous – not broken or interrupted; rays more concentrated in posterior one third of shell, also often with green or black background coloration in this area

 

16a. Rays prominent, very thin (like a fine-point pen line), continuous and closely spaced; valves moderately thick; shell laterally compressed

16b. Rays prominent to usually obscure; background coloration tan to brown; shell stout, compressed at dorsal margin; nacre purple to white; beak cavity absent

16c. Rays prominent; shell moderately inflated in umbo region

 

17a. Rays prominent, thin to broad in width and continuous – not broken, more or less widely spaced; valves moderately thick to thin; shell laterally inflated; umbo located anteriorly; pseudocardinal and lateral teeth well developed

17b. Rays prominent, usually broad in width and continuous – not broken, valves moderately thick to thin; shell laterally inflated; umbo located greater than one third of way toward center of shell; pseudocardinal and lateral teeth poorly developed

17c. Rays prominent to somewhat obscure; shell quite elongate and valves moderately thick; umbo located anteriorly, very broad but not raised much above the hinge line, moderately inflated; nacre white except in beak cavity where it is purple or salmon

 

18a. Shell distinctly pointed on posterior end, males acutely pointed, females bluntly pointed; posterior ridge with an upturned appearance at posterior end; shell thin but solid; nacre iridescent; pseudocardinal teeth thin

18b. Shell bluntly rounded on posterior end; nacre iridescent; pseudocardinal teeth relatively stout

 

19a. Shell decidedly elongate, usually more than twice as long as deep

19b. Shell elongate, rhomboidal or elliptical in shape

 

20a. Shell coloration dark green, brown or black; pseudocardinal and lateral teeth well developed

20b. Shell coloration tan, yellow-green, green or light brown; pseudocardinal and lateral teeth poorly developed or absent

 

21a. Shell with broad, green rays in small specimens, larger specimens uniformly black; dorsal margin rounded, somewhat inflated in large specimens; posterior end rounded in females, bluntly pointed in males; nacre usually white except in beak cavity where it is purple or salmon

21b. Shell stout, external coloration uniformly dark; compressed at the dorsal margin; nacre dark purple to white; beak cavity absent

 

22a. Shell fragile; ventral margin broadly rounded; shell coloration yellow, olive, or brown; nacre rose colored in the beak cavity, iridescent bluish color over the remainder; pseudocardinal teeth reduced to thickened ridge, lateral teeth low and indistinct; when viewed from above (dorsal aspect), the shell has a somewhat twisted appearance, almost never is the shell straight

22b. Shell fragile; hingeline straight; umbos not extending above hingeline; coloration yellow, yellow-green to green; pseudocardinal and lateral teeth absent

 

23a. Pseudocardinal and lateral teeth absent; shell greatly inflated in the umbo region and central part of shell; external coloration tan, olive, black; shell thin

23b. Pseudocardinal and lateral teeth present and well developed

 

24a. Nacre purple

24b. Nacre white, salmon or iridescent

 

25a. Shell inflated at the posterior end, broadly truncate in females; external coloration dark, black or green, sometimes with broad color rays in small specimens; pseudocardinal and lateral teeth well developed; nacre

25b. Shell small, relatively solid and inflated; pseudocardinal and lateral teeth well developed; nacre purple and usually lighter toward the ventral margin

 

26a. Color rays absent, shell texture shiny, satiny or cloth-like

26b. Broad green color rays present, often obscure

 

27a. Shell small, relatively solid and inflated; pseudocardinal teeth well developed, lateral teeth long, thin, and straight; beak cavity moderately deep; nacre white and iridescent

27b. Shell small to medium size, relatively solid and moderately inflated; beak cavity shallow; nacre white to salmon

27c. Shell compressed; base color tan, brown to occasionally black; posterior ridge gently rounded but posterior slope with two parallel grooves with slightly raised ridge in between

 

28a. Posterior end terminates in a rather distinct, sharp point

28b. Posterior end is bluntly pointed

 

29a. Small shell with pronounced, regularly spaced, concentric ridges over the entire surface; external coloration greenish yellow to black and shiny; solid and strong at all sizes

29b. Shell with posterior ridge sharply angled (approaching 90 degrees); posterior slope prominent

29c. Shell with posterior ridge gently angled; posterior slope not prominent

 

30a. Shell triangular to somewhat oval; external coloration green, yellow-brown, to brown, usually with distinct fine color rays often grouped together to form broad bands of color

30b. Shell broadly triangular and elongated from anterior to posterior; posterior ridge sharply angled; posterior slope relatively wide

 

31a. Posterior ridge sharply angled; posterior slope wide, expanded and ribbed; umbos swollen and slightly elevated above hingeline, located slightly toward anterior of shell; external coloration yellow or yellow-green with dark green rays, blotches or chevron-shaped markings; two pseudocardinal teeth in each valve; lateral teeth short but elevated; beak cavity deep

31b. Posterior ridge sharply angled; posterior slope wide; expanded and ribbed; umbos swollen and slightly elevated above hingeline, located near the center of the shell; external coloration yellow to yellow green with dark green spots, posterior slope often lighter than rest of shell; one pseudecardinal tooth in right valve, ocassionally two in left valve; lateral teeth absent or greatly reduced

31c. Shell elongate, described as boat shaped; dorsal end pointed; external coloration yellow, green, tan to black, usually with dark wavy rays; no lateral or pseudocardinal teeth present; byssal threads present for attachment

 

32a. Shell laterally compressed (thin) with posterior dorsal wing present, small to very prominent

32b. Not as above

 

33a. Shell nearly circular; shell thin even in large specimens; small posterior wing present; external coloration yellowish tan to olive, shiny; lateral and pseudocardinal teeth absent

33b. Shell oval to oblong; prominent posterior dorsal wing present

 

34a. Shell oval with large, prominent posterior dorsal wing, often with small flutations or corrugations; pseudocardinal teeth moderately to well developed, lateral teeth absent; nacre white

34b. Shell oblong to ovate with prominent posterior dorsal wing; nacre pink to purple

 

35a. Shell oblong to slightly ovate; small anterior dorsal wing present; external coloration greenish, reddish brown or light brown, and shiny; nacre color light purple to pink; pseudocardinal teeth thin, one in left valve and two in right valve; lateral teeth short and curved

35b. Shell oblong to somewhat rectangular; anterior dorsal wing absent; external, coloration dark green to black; nacre color light to dark purple; two thin pseudocardinal teeth in each valve

 

36a. Shell oval to elliptical, often greatly inflated; external coloration yellow in small individuals to brownish yellow in larger specimens, almost always with prominent color rays; nacre white; beaks broad and raised above the hinge line; pseudocardinal and lateral teeth well developed

36b. Shell oval and greatly inflated; dorsal margin s-shaped; beaks broad, high and turned decidedly inward; external coloration tan, gray or olive, and color rays absent, shell shiny; nacre bluish white tinged with salmon

36c. Not as above

 

37a. Shell acutely to broadly triangular, thick; umbos inflated; coloration reddish brown to black

37b. Not as above

 

38a. Shell acutely triangular (distinctly taller than wide); coloration dark brown to black; beaks elevated and hooked; sulcus absent in front of posterior ridge

38b. Shell broadly triangular; coloration reddish brown to brown; beaks broad but not extremely elevated; posterior ridge prominent and often sharply angled; broad flat sulcus in front of posterior ridge

 

39a. Shell broadly triangular, laterally compressed; shell thick and stout; posterior ridge prominent and acutely angled; beaks broad and flattened on the side but pointed at the apex; external coloration yellow, yellow brown or greenish brown; color rays narrow to broad and covering entire shell, rays discontinuous with spots, bars, and chevron shapes

39b. Shell broadly triangular, moderately inflated; shell thin to moderately thick; posterior ridge not prominent, posterior slope gentle; external coloration yellow to yellowish green, color rays form vertical bands, w-shaped markings aligned horizontally on shell

39c. Not as above

 

40a. Shell circular to oval or quadrate, often somewhat elongated posteriorly; beaks thick and inflated; external coloration tan, brown to black, color rays sometimes present

40b. Not as above

 

41a. Shell thick and stout, anterior end considerably thicker than posterior half; beaks elevated and hooked, strongly directed forward; beak cavity well developed; nacre white; pseudocardinal teeth parallel to long axis of lateral teeth

41b. Shell thick and stout, posterior end nearly as thick as anterior half; beaks stout, but not extensively elevated above hingeline; external coloration yellowish brown to brown, often with broad color rays; pseudocardinal and lateral teeth stout, not parallel; nacre white with pink in beak cavity or entirely pink

 

42a. Shell outline quadrate to elliptical; shell stout and thick to very thick, compressed to inflated; beaks broad and only slightly elevated; external coloration tan, brown to black, often with broad color rays; pseudocardinal and lateral teeth thick; nacre white to pink

42b. Shell outline rhomboid to quadrate; shell thin to moderately thick; umbos full and elevated above hingeline; coloration yellow-green with numerous wavy green rays; pseudocardinal teeth triangular and lateral teeth poorly developed to absent; nacre iridescent

42c. Not as above

 

43a. Shell outline broadly triangular; shell moderately inflated, valves thick and stout; beaks are compressed and only slightly elevated; sulcus anterior to posterior ridge broad and flat; pseudocardinal teeth stout, lateral teeth moderately high; beak cavity is very shallow

43b. Shell outline elliptical to somewhat cylindrical; color rays generally absent and background coloration brown to black

 

44a. Shell small, relatively solid and inflated; pseudocardinal and lateral teeth well developed; nacre purple and usually lighter toward the ventral margin

44b. Shell small, relatively solid and inflated; nacre white and iridescent

 

45a. Color rays absent, shell texture satiny or cloth-like

45b. Broad green color rays present, often obscure

 

46a. Shell small, relatively solid and inflated; pseudocardinal teeth well developed, lateral teeth long, thin, and straight; beak cavity moderately deep; nacre white and iridescent

46b. Shell small to medium size, relatively solid and moderately inflated; beak cavity shallow; nacre white to salmon

 

Bibliography

St. Francis River Basin, Arkansas and Missouri Mussel Literature

Ahlstedt, S. A. and J. J. Jenkinson.  1987.  Distribution and abundance of Potamilus capax and other freshwater mussels in the St. Francis River system, Arkansas and Missouri.  U. S. Army Corps of Engineers, Memphis.  Project # PD-86-6052.  67 pp.
 

Ahlstedt, S. A. and J. J. Jenkinson.  1991.  Distribution and abundance of Potamilus capax and other freshwater mussels in the St. Francis River system, Arkansas and Missouri, U.S.A. Walkerana 5(14)225-261.
 

Arkansas Game and Fish Commission.  1991-1999.  Annual mussel harvest reports.  Arkansas Game and Fish Commission, Little Rock.
 

Brann, W. P.  1947.  Fresh-water mussel shells, the basis for an Arkansas industry.  University of Arkansas Bull. 40(20):1-37.
 

Call, R. E.  1895.  A study of the Unionidae of Arkansas, with incidental references to their distribution in the Mississippi Valley.  Trans. Acad. Sci. St. Louis 7:1-65.
 

Clarke, A. H.  1985.  Mussel (naiad) study: St. Francis and White rivers.  Final Report (Contract No. DACW66-84-M-1666-R), U.S. Army Corps of Engineers, Memphis District.  28 pp.
 

Coker, R. E.  1919.  Fresh-water mussels and mussel industries of the United States.  Bull. U. S. Bur. Fish. 36:13-89.
 

Dennis, S. D.  1984.  A recovery plan for the fat pocketbook pearly mussel Potamilus (=Proptera) capax (Green, 1832).  U. S. Fish and Wildlife Service, Jackson, MS.  43 pp.
 

Ecological Consultants, Inc.  1983.  Mussel (naiad) survey – St. Francis, White, and Cache rivers, Arkansas and Missouri.  Final report (Project # DACW66-78-C-0147).  U. SArmy Corps of Engineers, Memphis District.  88 pp. + appendices A-E.
 

Ecological Consultants, Inc.  1984.  Handbook of the mussels of the St. Francis, White and Cache rivers, Arkansas and Missouri.  Supplement to the Final Report (Contract No. DACW66-78-C-0147), U.S. Army Corps of Engineers, Memphis District.  62 pp.
 

Gordon, M. E., L. R. Kraemer, and A. V. Brown.  1979.  Unionaceae of Arkansas:  historical review, checklist, and observations on distributional patterns.  Bull. Am.  Malacol. Union, Inc., 1979:31-37.
 

Gordon, M. E., L. R. Kraemer, and A. V. Brown.  1980.  Unionacea of Arkansas:  historical review, checklist, and observations on distributional patterns.  Bull. Am.  Malacol. Union, Inc.  1979:31-37.
 

Gordon, M. E. and J. L. Harris.  1983.  Distribution and status of fourteen species of freshwater mussels considered rare or endangered in Arkansas.  Arkansas Natural Heritage Commission, Little Rock.  Project #G6301.  23 pp. + appendices I-II.
 

Harris, J. L.  1986.  Relocation of the fat pocketbook pearly mussel, Proptera capax (Green) in the St. Francis River at Madison, St. Francis County, Arkansas.  Arkansas Highway and Transportation Department and Arkansas Game and Fish Commission, Little Rock.  14 pp.
 

Harris, J. L.  1990a.  Survey of the St. Francis River for the endangered fat pocketbook (Potamilus capax) at the proposed crossing for the Oklahoma-Arkansas pipeline project, St. Francis County, AR.  Final report to Woodward-Clyde Consultants, Houston, TX.  11 pp.
 

Harris, J. L.  1990b.  Survey of Tulot Seep Ditch and Ditch 60 for the fat pocketbook (Potamilus capax) in the vicinity of Trumann, Poinsett County,  Arkansas.  Final report to Crist Engineers, Inc., Little Rock, AR.  18 pp.
 

Harris, J. L.  1997.  A population assessment of recolonization by the fat pocketbook mussel of dredged habitat in the St. Francis Floodway, Arkansas.  Final report to the U. S. Fish and Wildlife Service, Jackson, MS.  14 pp. + appendices.
 

Harris, J. L. and M. E. Gordon.  1987.  Distribution and status of rare and endangered mussels (Mollusca: Margaritiferidae, Unionidae) in Arkansas.  Proc. AR Acad. Sci. 41:49-56.
 

Harris, J. L. and M. E. Gordon.  1990.  Arkansas Mussels.  Arkansas Game and Fish Commission, Little Rock, AR.  32 pp.
 

Harris, J. L., P. J. Rust, A. D. Christian, W. R. Posey, II, C. L. Davidson, and G. L. Harp.  1997.  Revised status of rare and endangered Unionacea (Mollusca: Margaritiferidae, Unionidae) in Arkansas.  J. AR Acad. Sci. 51:66-89.
 

Jenkinson, J. J.  1989.  Relocation of Potamilus capax from a 4-mile reach of the St. Francis Floodway in Arkansas.  Tennessee Valley Authority, Water Resources, Aquatic Biology Department, Norris, TN.  Final Report for Memphis District, U. S. Army Corps of Engineers.  Contract No. PD88-CO44.  53  pp.
 

Jenkinson, J. J. and S. A. Ahlstedt.  1987.  A search for additional populations of Potamilus capax in the St. Francis and Cache river watersheds, Arkansas and Missouri.  Final report for Memphis District, U. S. Army Corps of Engineers.  304 pp.
 

Johnson, R. I.  1980.  Zoogeography of North American Unionacea (Mollusca: Bivalvia) north of the maximum pleistocene glaciation.  Bull. Mus. Comp. Zool. 149(2):77-189.
 

Meek, S. E.  1896.  A list of fishes and mollusks collected in Arkansas and Indian Territory in 1894.  Bulletin U. S. Fish Commission.  15:341-349.
 

Oesch, R. D.  1984.  Missouri Naiades, A Guide to the Mussels of Missouri.  Missouri Department of Conservation, Jefferson City.  270 pp.
 

Posey, W. R. II.  1997.  Location, species composition, and community estimates for mussel beds in the St. Francis and Ouachita rivers in Arkansas.  M. S. thesis, Department of Biological Sciences,  Arkansas State University,  State University, AR.  178 pp.
 

Stansbery, D. H. and C. B. Stein.  1982.  The unionid mollusks of the lower St. Francis River in Arkansas.  Final Report.  The Ohio State University Research Foundation, Columbus, OH.  Prepared for the U. S. Fish and Wildlife Service, Atlanta, GA.  24 pp. + 12 appendices.