Mussel of the

Page last updated
4 October 2021

Mussel of the Month

The December 2012 Mussel of the Month is Trapezoideus exolescens. Trapezoideus is a genus of only four species from southern and southeastern Asia.

[Since December 2012 when this species was selected as Mussel of the Month, T. exolescens has been moved to the genus Trapezidens. This specimen is now known as Lens misellus. — DLG]

Trapezoideus exolescens
USNM 85893. Siam (type of Unio siamensis Lea).

The last Mussel of the Month of 2012, Trapezoideus exolescens, is dedicated to Mr. John M. Pfeiffer III in honor of the defense of his master's thesis at the University of Alabama. John has spent the last couple years exploring the phylogenetic relationships of southeast Asian freshwater mussels, and Trapezoideus has figured prominently in his research.

Solenaia is another genus of SE Asian freshwater mussels having asymmetrical glochidia (Image from Deein et al., 2008)Trapezoideus is one of the five genera of tropical Asia freshwater mussels reported to have asymmetrical glochidia (Ortmann, 1916; Panha & Eongprakornkeaw, 1995; Deein et al., 2008).The asymmetry of the larva is the result a pronounced flange (appendix, appendage, process, hook, &c. &c.) on only one of the valves. As John explained last summer in an award-winning presentation to the American Malacological Society, these genera are polyphyletic. That is, this odd larval morphology appears to have evolved more than once in the same geographical area. However, there are more interesting twists to this story. Watch this site for updates, and we will let you know when the paper on the subject is published.

Congratulations, John!!



Phylum Mollusca
Class Bivalvia
Subclass Palaeoheterodonta
Order Unionoida

Family UNIONIDAE Rafinesque, 1820
Subfamily RECTIDENTINAE Modell, 1942

Genus Trapezoideus Simpson, 1900

Species Trapezoideus exolescens (Gould, 1843)

To find out more about Trapezoideus and asymmetrical glochidia, check out:
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