Page last updated
2 January 2008

Mussel of the Month

The January 2008 Mussel of the Month is Gonidea angulata. Gonidea is a monotypic genus restricted to Pacific drainages in western North America.

Gonidea angulata is an interesting mussel. It was originally described as an Anodonta. Based upon the shell, that really isn't too unreasonable since the hinge is complete edentulous. But the soft-parts of this mussel are completely unlike any anodontine: it broods in all four demibranchs and the larvae are the smaller semi-circular type that lack hooks. Except for lacking hinge teeth and having such a thin shell, Gonidea shares more traits with Amblema than Anodonta. However, Gonidea takes this puzzle even further: the septa that divide the demibranchs into water-tubes are perforated rather than continuous, and that is more like a mussel from southeast Asia or Africa than other species from North America!

Arnold Ortmann, the Grand-Daddy of Modern Mussel Research, provided a detailed description of the anatomy of Gonidea angulata in 1916. The weirdness of that species broke the classification that Ortmann has been using to arrange the genera of the Unionidae. As he acquired more and more specimens of taxa, Ortmann continually modified his arrangement to account for new data. Eventually, this transformed his fairly simple system devised for the North American fauna into one that was complicated but able to accommodate the Old World specimens his colleagues were sending him. It was Gonidea angulata that finally broke the classification. Ortmann decided that he couldn't adequately delimit the subfamilies of the Unionidae, and in future anatomical descriptions didn't even try.

With the introduction of molecular data and cladistic methods, we have been able to get a more accurate handle upon the phylogenetic position of Gonidea. We will know more about Gonidea and the evolution of the Nearctic Unionidae when we have sampled more taxa from the Indotropics.

For more information about Gonidea and its phylogenetic position, check out:

• Ortmann, A.E. 1916. The anatomical structure of Gonidea angulata (Lea). Nautilus 30(5): 50-53.
• Graf, D.L. 2002. Molecular phylogenetic analysis of two problematic freshwater mussel genera (Unio and Gonidea) and a re-evaluation of the classification of Nearctic Unionidae (Bivalvia: Palaeoheterodonta: Unionoida). Journal of Molluscan Studies 68: 65-71.