Mussel reproduction is a marvel of evolution. At every step of the process, the likelihood of failure far exceeds the likelihood of success. The process begins when a male mussel discharges a white cloud of sperm into the surrounding water. Most of this sperm will be carried away by the current and go to waste, but some may find its way into the gills of a female, where her eggs lie in wait. Once fertilized, the eggs develop into larvae, called glochidia, which females hold in their gills for a month or more. Some species release their larvae in the cool waters of late summer or early fall; others wait for spring.

In order to mature, the larvae must feed on fish blood. That means when the female releases her glochidia, they must find and latch onto the gills of a fish. But not just any fish will do. Each mussel species requires particular host species; if the larvae latch onto the wrong fish, the fish’s immune system recognizes them as a parasitic invasion to be fought and killed. If the larvae succeed, they will hold on for a couple of weeks until they’re mature enough to drop off and drift to their lifelong home on the river bottom.

To attract a suitable host, mussels have evolved lures that mimic fish food. The lure of the snuffbox mussel, for example, looks like a grub nestled inside its open shells. When a fish swims in for a bite, the mussel snaps shut onto the fish’s head before releasing the larvae into the open mouth of its suffocating prey. A log perch is the preferred fish for a snuffbox; its immune system tolerates the mussel larvae infestation, and it can survive this aquatic chokehold. If a smaller fish, like a darter, goes in for the bait, it may lose its head.

Spectaclecase mussels take a gentler approach. They unreel a six-foot line of mucus with glochidia on the end. When a fish goes in for a bite of the lure, it ends up with a mouthful of larvae instead. That’s at least what biologists suspect. Even though the mechanism is known, Sims says, “no one’s been able to figure out the host fish.” That’s one of the reasons propagating mussels is so finicky. Experiments aren’t technically difficult, but they are time-consuming. They require infesting different types of fish with glochidia and hoping something sticks. If the larvae fail to develop, researchers have to wait another year, until the female mussels have glochidia again, to try with a new host species. The programs simply don’t have sufficient resources, either in time or researchers, to infest every native host fish up front, so they must take a systematic approach.