Simpsonaias ambigua

Clarke (1985) gives the following description. "Shell lenticular-ovate, moderately inflated, unsculptured, with only rudimentary hinge teeth; up to about 48 mm long, 22 mm high, and 16 mm wide. Much thicker (2.9 mm) anteriorly than posteriorly (0.5 mm). Anterior margin more or less evenly rounded; ventral margin flatly rounded, flat, or slightly concave just posterior of center; posterior margin also more or less evenly rounded and similar to anterior margin or a little broader; and dorsal parallel with ventral margin except indented in front of the umbones. Maximum inflation (and maximum height) both behind the middle of the shell. Beaks somewhat pointed, directed inwardly and toward the anterior, not inflated, located about 24% to 26% the distance from anterior to posterior, and projecting only a little above the hinge line. Area of posterior ridge convex and inflated but no distinct posterior ridge is discernable in most species. Area of posterior slope slightly concave near margin. Growth increments indicated by dark concentric periostracal bands covering barely discernable concentric grooves. Additional post-juvenile sculpturing consisting only of concentric threads (especially anteriorly), a few obscure radial lines posteriorly, and low lines and grooves of growth. Periostracum predominately brown or yellowish brown but blackish posteriorly in some specimens. Rarely one sees faint traces of narrow rays over the center of the shell, but most specimens, including juveniles, are entirely unrayed. Ligament rather long or of medium length, of moderate thickness, brown, and fragile when dry. Hinge teeth unusual, small, and incomplete. The right valve has a single, small, low, rounded, slightly elongated pseudocardinal tooth that arises from the shell wall (not from a thickened hinge plate as in other specimens) just in front of the umbone. The left valve of some specimens has an even smaller tooth that arises below and posterior to the umbone, i.e., in the same position as the interdental projection in Lasmigona species. This tooth, where present, is low, short, somewhat flange-like, rounded, and irregular. There are no articulating lateral hinge teeth, although the edge of the shell is a little thickened below the ligament and, in some specimens, a poorly defined lateral ridge may be present. Beak cavities somewhat excavated but not deep, and with a variable number of small, irregular muscle scars within. Major anterior muscle scars small and shallow but well marked; pallial band well marked, located quite far from the margin (and more clearly defined) anteriorly but closer to the margin posteriorly, and in some specimens with several tiny, parallel, collabral ridges within the anterior portion; and posterior muscle scars very lightly etched and located distinctly forward of the most posterior portion of the pallial band loop. Nacre bluish white, iridescent posteriorly, with yellowish, salmon, or purplish suffusions in the center and near the beak cavities, and thin in a narrow band around the edge of the shell with the periostracal color showing through. Beak sculpture composed of about six parallel, predominately inverted, V-shaped ridges, apparently corresponding to the middle portions of the double-looped ridges in Lasmigona . The earliest two ridges ore obscure in available material: they appear to be single-looped and especially expanded anteriorly. Later ridges are inverted V- shaped, with anterior arms short and directed ventrally in their proximal portions but curving anteriorly distally, and with posterior arms also short and more or less straight and parallel with the ligament or slightly curved upward. In some specimens the bars are in the shape of shallow V's and the later bars are simple undulating ridges but in most specimens the bars are deeply indented centrally."

Although occasionally found elsewhere, there is little doubt the preferred habitat is under large, flat stones in areas of swift current in medium to large rivers and lakes (Call 1900, Howard 1915, Buchanan 1980, Clarke 1985, Oesch 1984, Oesch 1995, Parmalee and Bogan 1998). Its presence is linked to the mudpuppy (Necturus maculosus) and it is usually rare though can be abundant in patches. This species generally occupies rivers, but can also be found in creeks, streams, and lakes on a variety of substrates (mud, silt, sand, gravel, cobble or boulder) in areas of swift current (Cudmore et al. 2004).

Refer to the General Freshwater Mussel ESA for general ecology of mussels.

Howard (1915, 1951) demonstrated that the host is the mudpuppy (Necturus maculosus). He suspected "that Necturus eats the adult mussel and in seeking food visits one rock after another. In satisfying its appetite it becomes infected with the mussel glochidia, nourishing them, and when they have matured serves as a transporting and distributing agent for the young mussels". Glochidia were found deeply imbedded in the external gills of the mudpuppy, which was confirmed as a host by Bequaert et al. (1998). There is some evidence that the glochidia are released in the fall (Clarke 1985).

Simpson (1914) described gravid specimens; "Brood pouch filling the entire outer gills and forming enormously thickened pads, the upper part finely vertically striate, the lower part of different texture, lighter colored, wrinkled and granular on the surface: embryos very large; outer and inner gills nearly alike in size, the latter free from the abdominal sac, all united to the mantle to their posterior ends..."