Written by Denise Shepherd
When I was first assigned to cover a freshwater mussel survey, I wondered why I should care about a ‘shell in the mud?’ Then I met and interviewed Matthew Rowe, a freshwater invertebrate biologist, along with Peter Dimmick and Austin Haney, two aquatic technicians that work alongside him. Their passion for conserving Georgia’s freshwater mussels blew me out of the boat and by the end of the survey, I was hooked on conglutinate.
What I Learned
Altamaha slabshell mussel. Photo credit: Georgia DNR.
It’s true that most species of freshwater mussels live sedentary lives, filter-feeding on microscopic algae and bacteria. Then I learned about their impressive evolutionary biology which includes teeth, fish-like lures, and master angling skills.
Marine ancestors and relatives of freshwater mussels release planktonic larvae into the water column to be carried by currents. While this method works in the ocean, it would be much harder in a river or lake. Impressively, freshwater mussels have solved this by developing a parasitic lifestyle on fish. This evolutionary adaptation is not detrimental to the host fish.
Lures
Some freshwater mussels use a lure attached to their mantle to attract fish. The mantle is tissue that also grows a mussel’s shell. Lures vary among species, but one group of mussels uses a lure that looks exactly like a darter – complete with an eye spot, lateral line, and even a tail.
Shiny-rayed pocketbook. Photo credit: Georgia DNR.
One species in the Northeast has teeth on its shell. When a potential host fish falls for the lure trap, the mussel clamps down on the fish and holds it in place while pumping out larvae known as glochidia, which resemble thousands of little Pac-men.
In Georgia, there is one freshwater mussel species that challenges even our most talented anglers with its lure. It is known as the shiny-rayed pocketbook, and it resides in the Flint River Basin. The shiny-rayed pocketbook operates a superconglutinate, which is essentially a long strand of mucous with several glochidial packages attached. The superconglutinate floats or is fluttered around in the water to attract host fish.
How do mussels use lures?
The mussels nestle themselves in the mud, posterior end out. This places the lure on top. Then, they wiggle the lure which looks – to an unsuspecting fish – like an easy meal. When a fish goes for the lure, the mussel releases its glochidia. The glochidium clamp onto the fish’s gills, fins, or skin and will not release until they become juvenile mussels. This takes a few weeks or months, depending on the species.
Other species release groups of glochidia called conglutinates which resemble insect larvae or a cluster of fish eggs. When the mussel releases the conglutinate, it will move around on the bottom of the water column until a hungry fish swims by to gobble it up.
If these reproductive adaptations aren’t enough to convince you that mussels are cool, freshwater mussels play a major role in indicating water quality and stream health in Georgia. If they are settled in an area, it tells biologists that the habitat has adequate oxygen, low contaminant levels, and has remained relatively intact from construction activities that disturb the river or lake bottom. That’s good news for mussels, fish, and anglers alike!
Want to learn more about how Georgia DNR surveys for mussel species? Check out the Study Spotlight: Mussel Mania in the Lower Flint River blog.