Michigan is home to 43 species of native freshwater mussels, 30 of which are considered to be at risk of extinction. Among the many factors that threaten the hard-shelled bottom dwellers are competition from invasive zebra and quagga mussels, water pollution, and—especially—dams.

The Huron River in Southeast Michigan, for example, has 19 dams on its main stem and at least 96 across its entire drainage. Damming completely transforms a river’s ecology, replacing native mussel-rich shoal, riffle, and pool habitats with less suitable lake-like reservoirs.

Two University of Michigan biologists led a recent freshwater mussel study based on fieldwork along the Huron River and the River Raisin, also in Southeast Michigan. The study, published May 24 in the journal PeerJ, investigates freshwater mussels’ remarkable reproductive cycle, which includes the use of fleshy “mantle lures” by pregnant females to attract nearby fish and “infect” them with mussel larvae.

The study’s lead author, Trevor Hewitt, conducted the fieldwork for his doctoral dissertation in the U-M Department of Ecology and Evolutionary Biology. The senior author, Diarmaid Ó Foighil, is a professor in the department and was Hewitt’s adviser.

What background information would someone who is completely unfamiliar with your field need to know to understand the findings of your study?

Hewitt: Freshwater mussels undergo an obligate parasitic larval development in which pregnant females must infect a suitable fish host with her young. The mussel larvae typically attach to the host fish’s gills, and after two to four weeks they metamorphose into juveniles and drop to the riverbed. Many mussel species are host specialists, infecting just one or a few species of fish, and utilizing distinct host infection strategies.

One of the most striking strategies involves use of a mantle lure. This is a pigmented tissue flap displayed by pregnant females to mimic a host prey item (a small fish, an invertebrate, etc.), eliciting an attack by the host fish and resulting in its infection.

Mantle lure displays are a remarkable and understudied example of mimicry in nature that occurs in many of our streams and rivers each spring and summer.

What exact research question did you set out to answer, and what methods did you use?

Hewitt: Our research focused on the mantle-lure diversity present in one mussel species, the wavy-rayed lampmussel, which is found from Michigan to Alabama. This mussel uses smallmouth bass as its primary fish host and, most unusually, it has two highly distinctive types of mantle lures.

One, previously termed “darter-like,” resembles a small fish called a darter, complete with eye spots, mottled body coloration and prominent marginal extensions including a tail. The other, previously termed “worm-like,” is uniformly bright orange underlain with black. Both lure forms, or morphs, co-occur throughout the animal’s range.

Our research aims were to: 1) confirm that the mantle lure diversity represents a true polymorphism, meaning a clearly different form within a population of the species; 2) investigate its ecological persistence through time; 3) identify the range of putative model species targeted by this mimicry system in a natural population; and 4) determine whether the two mantle lure morphs differ in their display behavior in addition to their pigmentation and morphology.