AIM

Margaritifera falcata (Gould)


Western pearlshell

Recognition

The shell is elongate, compressed, dark colored, and slightly concave on the ventral edge. It has only weakly developed teeth and purple nacre. The normal size is 50 to 85 mm. It is similar to Ligumia recta, which is larger, thicker and has better developed teeth, pink nacre and occurs only in warmer rivers much further downstream.

Summary

Our smallest species and the only species known from west of the divide. The normal fish hosts in our area are probably Oncorhynchus species, but Salmo and Salvelinus and even Rhinicthys and Catostomus are reported to be suitable. This species occurs in sand, gravel and even among cobble and bolders.

The name Margaritifera margaritifera is normally used for specimens east of the continental divide, but I have concluded that all Montana populations are M. falcata. They likely crossed the divide with the west-slope cutthroat trout, which is the native salmonid of the upper Missouri River drainage. This is a headwater species occuring very near the continental divide on both sides. It does not seem to occur anywhere in the cental part of the continent. All of our Margaritifera have purple nacre, while the true M. margaritifera of the Atlantic slope has white nacre. I can find no differences in the shells of our east and west slope populations. While I am quite convinced, further study would be good.

Distribution

In our trout streams and trout rivers west of the divide and in the Missouri headwaters. Outside of Montana this species is reported from the Pacific drainage from southern Alaska to central California.

17 DEC 1995, updated on 12 JAN 1996 D.L. Gustafson
AIM home page
dlg@rivers.oscs.montana.edu