Despite my aforementioned anti-bivalve bias, the time has once again come for a bivalve to hold the coveted position of Taxon of the Week. What, I hear you ask, is the lucky, lucky bivalve so favoured? None other, I reply, than the North American freshwater genus Fusconaia.
In contrast to the great diversity of freshwater gastropods, surprisingly few groups of bivalves have really made a go of things in the freshwater environment. The superfamily Unionoidea is therefore very distinctive in not only including the largest number of freshwater bivalves, but in fact being only found in freshwater. Fusconaia is a genus of Unionoidea, usually included in the family Unionidae and subfamily Ambleminae, found in many river basins of North America. Members of Fusconaia are rather unfairly lumbered with the vernacular name of "pigtoe", evidently a reference to the shape of the shell. For some reason, North American unionids have garnered more than their fair share of memorable names - Motley (1933) gives us "heelsplitter", "elephant's ear", and for one particularly unfortunate Fusconaia species, F. ebena, "niggerhead". Thankfully, that last name seems to have been universally replaced in the present day by "ebonyshell". Those more charitably disposed towards the family might refer to them as "freshwater pearly mussels" or "naiads". "Naiad" derives from a classical Greek term for a class of water spirit, so Fusconaia would mean 'brown nymph'. 'Brown nymph' is arguably a slight improvement over 'pigtoe'.
Other than their memorable names, unionids are most famed for their distinctive life cycles. After fertilisation, female naiads (unionids have separate males and females) brood their eggs in the gills until they hatch out into bivalved larvae with big teeth on the shells called glochidia. Upon release, glochidia attach themselves to the gills of a passing fish and live as parasites, forming a cyst until they hatch out and develop into a mature mussel. At least some unionids actually develop fleshy outgrowths of the gills that mimic a small fish in order to attract a suitable host for their glochidia - I haven't established whether Fusconaia is one of the unionids that does so. Speaking speculatively, perhaps the biggest advantage unionids gain from this life cycle is overcoming the hurdle of dispersal in fresh water environments. Most marine bivalves have larvae that disperse as plankton, but this is obviously not a suitable option in fresh water, where currents are either more or less non-existent (lakes) or would inevitably carry the larvae into an unsuitable environment (rivers). [Is this why there are so few freshwater bivalves?] Attaching to a fish potentially allows the unionid larva to disperse upstream. Once they do settle down, most unionids grow very slowly - Bruenderman & Neves (1993) found that Fusconaia cuneolus, the fine-rayed pigtoe of Virginia, grew an average of five millimetres per year for the first ten years of its life, then slowed down to a more sedate two millimetres per year. Because of their slow growth, vulnerability to environmental pollutants and changes (they are sessile filter feeders, after all) and reliance on the presence of suitable fish to act as glochidial hosts, many unionid species are critically endangered. About one-eighth of Recent North American amblemine taxa are believed to have become extinct, and another quarter are endangered or threatened (Campbell et al., 2005).
Establishing exactly which species are threatened is another matter. Taxonomically, unionids can only be described as evil. Graf & Cummings (2007) listed fifteen species in the genus Fusconaia, but many widespread unionid species are decidedly variable. Prior to the beginning of the 1900s and the revisionary work in North America by Simpson and Oortman, a large number of species and subspecies were erected based on what are now regarded as ecological variants of other species. According to Graf & Cummings (2007), no less than 4955 species-group names are available to unionoids for 840 currently-recognised valid taxa, giving an average of nearly five synonyms for every valid species*. Graf (1997, in an unpublished thesis) listed no less than forty species-group names of relevance to the taxonomy of Fusconaia flava, the widespread Wabash pigtoe. Even after narrowing the field down to valid species only, the confusion is not yet over. Campbell et al. (2005) suggested based on molecular analysis that many, if not most, of the currently recongised North American unionid genera are polyphyletic. Fusconaia, originally established by Simpson in 1900 primarily on the basis that its species incubated their eggs in all four gills, included representatives of no less than three tribes of Unionidae.
*More than 1000 of these were introduced for supposed European (mostly French) taxa by the so-called Nouvelle École of Jules-René Bourguignat and his successors in France in the late 1800s (Graf, 2007). So extravagant were the efforts of the Nouvelle École that C. T. Simpson, when reviewing the world's unionid fauna, simply refused to consider them, complaining in 1900 that, "Life is too short and valuable to be wasted in any attempt at deciphering such nonsense".
Bruenderman, S. A., & R. J. Neves. 1993. Life history of the endangered fine-rayed pigtoe Fusconaia cuneolus (Bivalvia: Unionidae) in the Clinch River, Virginia. American Malacological Bulletin 10 (1): 83-91.
Campbell, D. C., J. M. Serb, J. E. Buhay, K. J. Roe, R. L. Minton & C. Lydeard. 2005. Phylogeny of North American amblemines (Bivalvia, Unionoida): prodigious polyphyly proves pervasive across genera. Invertebrate Biology 124 (2): 131-164.
Graf, D. L. 1997. Morphology, Zoogeography, and Taxonomy of Fusconaia flava (Rafinesque) (Mollusca: Bivalvia: Unionidae) in the Upper Mississippi, Great Lakes, and Nelson River Basins. MSc thesis, Northeastern University, Boston.
Graf, D. L. 2007. Palearctic freshwater mussel (Mollusca: Bivalvia: Unionoida) diversity and the Comparatory Method as a species concept. Proceedings of the Academy of Natural Sciences of Philadelphia 156: 71-88.
Graf, D. L., & K. S. Cummings. 2007. Review of the systematics and global diversity of freshwater mussel species (Bivalvia: Unionoida). Journal of Molluscan Studies 73 (4): 291-314.
Motley, H. L. 1933. Histology of the fresh-water mussel heart with reference to its physiological reactions. Journal of Morphology 54 (2): 415-427.