17 June 2026

Now that western Pennsylvania’s rivers are warming up for the summer, female plain pocketbook mussels (Lampsilis cardium) are ready to complete last year’s reproductive cycle by luring a fish. As soon as it strikes she’ll release her larvae into the fish’s gills where they’ll feed and mature and fall off as tiny mussels. Amazingly both sides of this parasitic relationship are unharmed. Here’s how it works.

The mantle in mussels is a fleshy sheet-like tissue that lines the inside of the shell and uses minerals from the water to build and repair the shell. Because she needs a fish lure, the plain pocketbook mussel (member of the Unionidae family) has fancy flaps on the edge of her mantle that stick out in the water.

Wikipedia explains:

All Unionidae are known to use the gills, fins, or skin of a host fish for nutrients during the larval glochidia stage. Lampsilis cardium accomplishes this by having the inner sides of its mantle flaps marked with longitudinal stripes, resembling a small fish [eastern shiner] of the genus Notropis . When these are attacked and ruptured by a striking predatory fish, especially [Redeye bass] Micropterus coosae, the mussel larvae are released into the gills of the host fish, where they feed and develop.

— Wikipedia: Lampsilis cardium

Her mantle flaps have eye spots and stripes that make them look fish-y.

She moves them like a fish!

And here’s her favorite host, redeye bass.

The mussel larvae, called glochidia, typically clamp on to the gills of the fish, but may also attach to tails or fins, where they will complete their next transformation. For several weeks the mussel larvae harmlessly ride along on the fish as the fish swims around doing fish things. Eventually their metamorphosis is complete and they drop to the river bottom as a mussel.

— USFW: Freshwater Mussels and Fish: A Timeless Love Affair

This is approximately what they look like when they drop off (rainbow mussels, related to plain pocketbooks).

After she’s released her glochidia she’s ready to start over with fertilization (warmer water triggers male sperm release), brooding (in her gills for as long as 11 months) and release of the glochidia next year.

Plain pocketbook mussels live in our area. Here’s a female in the Kiskiminetas River in a video posted by the Pennsylvania Fish and Boat Commission on Facebook.

Read more about Freshwater mussels and fish: A timeless love affair at US Fish and Wildlife.