Alasmidonta varicosa

The shell is kidney-shaped. The anterior margin is abruptly curved. The ventral margin is straight to slightly concave, descending somewhat. The posterior margin is biangulate below and broadly curved to straight above. The dorsal margin is slightly convex. The beaks are located well forward, approximately one fourth of length from anterior margin. There are numerous short, low, corrugations or ridges on the posterior slope. The ridges tend to be oriented radially. The shell is moderately elongate and is firm but not thick. The periostracum tends to be smooth centrally and rough elsewhere. In adults, the periostracum is brownish with dark fine rays sometimes present; in juveniles it is yellowish with green rays (Clarke, 1981). The ligament is located posterior of beaks, and is well developed. The nacre is rose or yellowish olive centrally and bluish white or blue at margins. Dentition is rudimentary. There is typically one small pseudocardinal in each valve that is poorly developed and of compressed pyramidal shape. If present, the lateral teeth are vestigial. See Clarke (1981) or Johnson (1970) for a complete description. Identification keys are provided in Bogan (2002) for North Carolina, Bogan and Alderman (2004) for South Carolina, Clarke (1981) for Canada, Fichtel and Smith (1995) for Vermont, Nedeau et al. (2000) for Maine, Nedeau and Victoria (2000) for Connecticut, Smith (2000) for New England, and Strayer and Jirka (1997) for New York.

Diagnostic characters include a flat or slightly indented ventral margin, swollen to moderately inflated valves, yellowish-green (young specimens) to brownish-black periostracum with prominent green color rays (eroded away in older specimens), series of corrugations along the dorso-posterior slope perpendicular to growth lines, pseudocardinal teeth poorly developed with only one small knob-like tooth in each valve and no lateral teeth (Strayer and Jirka, 1997; Nedeau et al., 2000; Smith, 2000; Nedeau, 2008).

This species can sometimes be confused with Alasmidonta undulata, Strophitus undulatus, and Alasmidonta heterodon (Nedeau, 2008). Alasmidonta marginata is similar but is more inflated, grows larger, and has a distinctly truncate posterior slope. No other shell within its range has distinctive radial ridging on posterior slope except Alasmidonta robusta, a localized and believed extinct species of the Santee River system in North and South Carolina. Alasmidonta robusta has distinctive wide and wavy green rays on its shell (Strayer and Jirka, 1997).

This species is found in creeks and rivers of varying sizes with moderate flow and stable substrates along intact riparian corridors (USFWS 2018).

This species is a long-term brooder with gravid females found from August to May (Clarke 1981; Ortmann 1919). Fertilization occurs in summer with glochidia released the following spring (Nedeau 2008).