After the divers suit up, they each grab a mesh bag. I tell them that they look like they’re going scalloping and Dr. Nathan Johnson says that, actually, he had done just that over the weekend. Recreational and professional lives align. Scallops in Crystal River over the weekend, freshwater mussels in the Apalachicola River today.
Nathan is a mussel biologist with the U.S. Geological Survey. He’s joining a team from the U.S. Fish and Wildlife Service, and student divers from the Florida State University Panama City Campus scientific diving program. That’s three boats’ worth of diving teams tasked with surveying dozens of locations up and down the Apalachicola. Today, they will focus on an area around Wewahitchka, the middle stretch of the river.
The Apalachicola basin is home to almost 30 species of freshwater mussels (see list below). Six are federally listed as endangered or threatened, while a handful have gone extinct or have been extirpated from the river in recent decades. Mussels face many of the same challenges as oysters and Ogeechee tupelo trees, the species we most often talk about in relation to drought, dredging, and dams on this river.
Unlike those species, freshwater mussels spend their lives away from human eyes, and off of human plates. With the help of these divers, we’ll meet a few species, and learn of their lives at the bottom of the Apalachicola River. And also, surprisingly (to me, anyway), their lives moving around this river system.
Like other stories we tell about this Apalachicola River basin, this is a story about those aforementioned challenges. But it is also a story with some good news. One of these mussels might be leaving the Endangered Species list.
“The livers of the river”
“Mussels are just a good indicator of the biodiversity of your system,” says Andy Hartzog, a fish biologist with the U.S. Fish and Wildlife Service. “And so if you have some mussels in your river, you probably got a healthy, healthy system.”
Mussels are known as the “livers of the river.” Like all mollusks, they are filter feeders. This is a river full of sediment: sand mixed with decaying plant matter. Mussels eat that plant matter and plankton swimming within it. In doing so, they remove nutrients that could feed algae blooms. Mussels also filter toxins from the water column.
Meet the fat threeridge mussel
When the divers dump the contents of their mesh bags, one mussel stands out from the rest. The others have that standard, clam-like bivalve look. There are variations of shapes and shading, but even a neophyte can see that there are different species here, even if the differences are subtle.
The fat threeridge is bulkier, and less rounded. The namesake ridges look like they were chiseled from rock. As Nathan and Dr. Adam Kaeser sort their catch, the fat threeridge pile becomes the largest, as it will be throughout the day. Not bad for a species that was thought to be in decline twenty five years ago. This mussel is, as of now, federally listed as endangered.
I love any excuse to break out a physical copy of an old nature book. In this case, it’s the Field Guide to the Rare Animals of Florida, printed by the Florida Natural Areas Inventory in 2001. The book, and its companion book on rare plants, has a wealth of information that hasn’t changed. Something has changed for this animal, though, and here we have a snapshot of a mussel at risk.
The entry on fat threeridge mussels describes it as declining, facing threats from dredging and invasive Asian clams (Corbicula fluminea). A visit to an Apalachicola sand bar lets us know that Asian clams are still present here, and today, divers have to pick around them to harvest mussels.
The dredging of the Apalachicola is another story.
The Dredging of the Apalachicola River
Let’s travel back to 2001, when the field guide was written. This was around the tail end of the barge era of the Apalachicola River. Large vessels carried goods down the Chattahoochee River and into the Apalachicola, from as far away as Atlanta. For decades, the Army Corps of Engineers managed the river channel to accommodate barges. One feature they added was wooden dikes designed to focus the flow of the river to the center, ostensibly to create a faster current to speed boats along.
The Corps also dug sand out from the center of the channel to make it deeper for large boats. If sand bars on the Apalachicola seem wider than those on the Suwannee or other rivers, it’s because the Corps augmented them with dredge sand. They also built Sand Mountain six miles south of Wewahitchka. These features have recreational value: places to camp, climb, and play. But that sand at the bottom of the river is a habitat.
The dredging process also pushed sand to the banks of the river, where the main channel is fed by creeks and sloughs. These smaller waterways connect the river to the floodplain, adding hundreds of miles of habitat for fish, mussels, and other freshwater fauna. Apalachicola Riverkeeper has been spearheading projects to remove dredge sand and restore these connections.
There hasn’t been regular barge traffic, or dredging, for over twenty years. The Army Corps of Engineers has recently proposed new dredging that they claim would be less environmentally destructive. The proposal has met with resistance, and that part of the story is still unfolding.
For now, though, the bottom hasn’t been disrupted for some time. This could be a reason for the healthier fat threeridge numbers.
Mussel reproductive ecology
Dredging disturbs mussels where they lay, at the bottom of the river. Dams disrupt their movement.
As Adam and Nathan fill their bags in the waters beneath us, Andy Hartzog shares the surprising way that mussels move up and down a river. Andy is a fish biologist with USFWS and, appropriately, this is where fish enter the story.
“Each species of mussel requires a specific fish host to complete its life cycle,” Andy says. “Once their larvae are fertilized, the female will [use] varying strategies to release her larvae. And then, those larvae have to encounter a fish. Once they do encounter a fish, they attach to the fish’s gill and it’ll become insistent. It’ll stay on there for 2 to 4 weeks, and it will complete its life cycle as a parasite.”
It’s akin to butterfly caterpillars and their larval host plants, but maybe not as specific a relationship as monarchs and milkweed. A study conducted by USGS and the University of Georgia found that fat threeridge larvae attached to and metamorphosed on over twenty different fish species. Most were darters and minnows, but I picked striped bass for one of our student illustrators to draw for the video.
Some mussels bring the fish to them
That fat threeridge broadcasts larvae into the water column, as do many mussel species. Others use a different reproductive strategy. Nathan selects a Florida sandshell (Lampsilis floridensis) from the pile to show me.
“The gills inside the females hold all the larvae, and they push the gills out and put pressure on them,” Nathan says. “And then they have this little mantle flap that they’ll twitch and move, and it mimics some sort of prey item.
“This species primarily uses gar as host. So the gar comes down and tries to eat the lure, ruptures the gill, and then all the larvae go on to the gills of the fish. They live on the fish for roughly 20, 30 days. And that’s their main method of dispersal.”
Whether the mussels lure the fish to them or flood the water with larvae, the goal is to attach larvae to a host. In the weeks after attachment, larvae metamorphose into young mussels and drop off. Different mussels thrive in different areas along the river bottom. To learn a little more about mussel ecology, I watched a presentation by Dr. Michael Gangloff, who has studied mussels in this river for over 20 years. Dr. Gangloff has found that fat threeridge are abundant next to sandbars, in areas where sand is captured by the bar and willows grow.
The Jim Woodruff musse