The process of encystment on the fish host occurs as the tissue of the fish grows over the glochidia. Metamorphosis takes place within days or weeks, depending on species and temperature. Glochidia transform into microscopic juveniles and drop off. If by chance they settle into suitable habitat, a new mussel bed is created. Photo by M. Barnhart Glochidia of Lampsilis encapsulated on the gills of largemouth bass.
Glochidia are parasites but they usually do no harm to the host. The glochidium clamps on, and the epithelial cells of the gill then migrate to form a capsule around it.
This structure is often erroneously called a cyst, but technically cysts are structures made by a parasite, while capsules are made by a host. Obviously, if they aren't grouped fairly closely, reproduction is hard to achieve.
After fertilization, the female then holds up to several thousand eggs at a time in her gills. There they can obtain oxygen and have a place to brood until they develop into glochidia—the larval stage of mussels. It is the larval stage that is more exciting! In the late spring or early summer, the glochidia are expelled into the water where they have to fend for themselves. They need to attach themselves to the gills of a host fish within a couple days. Most freshwater mussels team up with only one type of fish.
Our northwest species favor salmon and without salmon as hosts, mussels cannot successfully reproduce. Once the larval mussels attach to the fish, the fish body reacts to cover them with cells—an unconscious action that forms a cyst, where the glochidia remain for two to five weeks depending on the temperature.
Hitchhiking on a fish is a baby mussel's only opportunity to travel and experience more of the world, and traveling this way results in a free ride to a new home. After the mussels change from the larval form and begin to resemble adults, they break out of the cyst and fall to the bottom of the stream where they bury themselves in the bottom and begin to live an independent life.
Only one in a million survive to the adult stage, but to offset these low odds, mussels lead a very long reproductive life and produce millions of eggs per year! Although one would expect to see mussels in most of our streams and rivers, they often aren't there. When the mussel is mature enough to face river life, it releases from the gills, the fish none the worse for the experience.
The mussel attaches to the bottom, having evaded many of its predators. Pollutants and altered river flows threaten many freshwater mussel species. Conservationists call them a canary in the coal mine. Historically, a few people ate them. A few more made buttons with their shells. Freshwater mussels are the kind of creatures that welcome scorn from those opposed to endangered species funding: Conservationists put the shinyrayed pocketbook ahead of people! The road to mussel conservation funding and policy is thus littered with bitter debates and ridiculous sound bites.
Some of us may argue that mussels — as irreplaceable and fascinating creatures of evolutionary history — deserve our appreciation, too. But that argument fails to persuade most. Since the mussel needs fish, restoring fish could help restore mussels. On many rivers, a lot of the restoration efforts have focused on the benefits to native fish species.
But restore habitat for fish, and you often restore it for mussels, too. And not only that, but mussels need specific native fish species to survive. Science has only recently revealed the ways mussels attract fish, and use their gills to carry their young. That knowledge not only deepens our understanding of the natural world; it also likely leads to more effective — and uncontroversial — conservation.
Please note that all comments are moderated and may take some time to appear. Hey Matthew, That must be one heck of a bass to bite on an 18 inch lure. And how does the mussel stow such a massive appendage in its shell. Or was that supposed to be 18mm?
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