The Ohio River is full of tiny monsters that “resemble a vicious Pac-Man”

Sitting stationary just beneath the surface of the silt and gravel of the Ohio River in the heart of the USA is a kind of mussel. It doesn’t appear to do much. Day in, day out, year after year, it sucks in water, then blows it out again.

There is, on the face of it, not much to get excited about. But beneath this humble routine lurks a secret. This is the pocketbook mussel (Lampsilis ovata) – no bigger than your palm, but with a devious, dazzling trick up its shell.

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The mussel is named for its close resemblance to a pocketbook, a term used in the USA to describe a small purse or bag.

Like other bivalves, the species is a filter feeder. Buried in sediment, it opens its incurrent siphon and draws in water, straining it for microscopic meals.

The inflowing water is passed through a fine-mesh filter of gill plates, where mucous traps minute organic particles. These are then balled up and, via the labial palps, passed into the mouth. The now-cleaner water and any undigestible waste are blown out via another siphon. 

For the pocketbook mussel, eating and breathing are one simple, seamless act. And this is its life. Some endure for 20 to 30 years, anchored in place and quietly doing their thing, like tiny living filter units.

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The species’ sex life is, as you might predict, quite dull, too. Pocketbook mussels, like many freshwater bivalves, are Bradytictic breeders, which means they play the long game.

Sexually mature males and females (more than four years old) develop sperm and eggs in the summer. Taking environmental cues such as water temperature and day length, a male ejaculates his sperm into the water – a couple of puffs of milky fluid, and that’s him done.

Downstream, a female quietly inhales the sperm, fertilising the eggs in her gill chamber. Not much to gossip about there.

But from here, things start to get weird. The fertilised eggs hatch into larvae and stay hidden in a pouch (the marsupia) in the mother’s gills until spring.

Each larva  is a vicious-looking Pac-Man, but with a wiggling tail to drive it forward. The task now is for the female to eject her offspring into the water, where their sole job is to find a fish, such as a largemouth bass, get sucked into its mouth and attach to the blood-rich, well-vasculated gills. 

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When inside the fish’s mouth, they chomp down with their Pac-Man jaws (which will eventually become their valves) and burrow into the tissue until they are encapsulated by it. The parasitic larval molluscs then go wherever their host does until they have grown big enough to drop off and settle in the sediment. 

But even with thousands of larvae launching each year, getting into a fish isn’t easy. This is where the pocketbook mussel’s showmanship shines, with the female putting on a sort of puppet show.

The mantle of a mussel is the bit you see when it opens its valves. In most species, it resembles a lacy frill. In Lampsilis, however, the mantle flaps look like a minnow, complete with a tapering body, a flank stripe and, most impressively, a pair of ‘cartoon’ eyes at one end.

The dupe is then wiggled to catch the eye of a predatory fish. As the fish approaches and moves in to snap up what it thinks is dinner, the female mussel blows a cloud of larvae right into its face.

This aggressive mimicry is so effective that you can find bass with 30 or more larvae encapsulated within their gills. But the stowaways don’t appear to harm the fish. They normally hang on for just a few weeks, until they’ve absorbed enough nutrients from their host’s blood. 

During this short period of temporary parasitism, the fish acts as a taxi, giving the mussel larvae, quite literally, the trip of a lifetime, before they finally drop off to resume the sluggish and rather uneventful
life of an adult river mussel.