North America’s freshwater mussels are both impressively diverse and highly imperiled. Nearly 300 species occur in the United States and Canada, and up to 40 species of the hard-shelled bottom dwellers can be found on a single stretch of a clean, swiftly flowing river.

Freshwater mussels are also a paradoxical group: How can so many species persist side by side while feeding on the same foods—sediments, plankton and other particles in the water column? Elsewhere in nature, a few species would typically gain an advantage over the others and would eventually outcompete them.

A new genetic analysis of North American freshwater mussels by University of Michigan biologists concludes that a key to understanding the coexistence of many freshwater mussel species lies with the distinctive ecology of their microscopic, short-lived larvae, rather than the long-lived adults.

Female freshwater mussels in the tribe Lampsilini have evolved a mind-bending array of lures that use mimicry to attract nearby fish and to “infect” them with mussel larvae, which then metamorphose on the fish’s gills or fins before dropping to the river bottom, weeks later, as juvenile mussels. In biology, a tribe is a taxonomic rank above genus but below family and subfamily.

The larvae of each lampsiline mussel species must infect a specific type of freshwater fish—bass, sunfish, darters, sculpins or drum, for example—to complete their development. In the language of ecology, the mussel larvae are obligate parasites, and the fish that shelter and disperse them are their hosts.

It is this competition for host fishes that best explains the coexistence of multiple lampsiline species, according to the authors of a paper published online Nov. 16 in the journal PeerJ.

The high diversity of available fishes and the ability of mussel species to target specific fish hosts has led, over time, to specialized host-infection behaviors and the evolution of new species through a process called adaptive radiation, according to the U-M researchers.

“This study provides an exciting new paradigm for how much of North America’s rich, but highly endangered, freshwater mussel fauna has evolved, and it highlights the cryptic role their remarkably complicated and interesting larval life histories has played in that process,” said study senior author Diarmaid Ó Foighil, a professor in the U-M Department of Ecology and Evolutionary Biology and a curator at the Museum of Zoology. The study’s lead author is U-M doctoral candidate Trevor Hewitt, who did the work for his EEB dissertation. The other author of the PeerJ study is Amanda Haponski of the U-M Department of Ecology and Evolutionary Biology.

Read full Michigan News press release