Mollusking in the Fakahatchee – Part 1
By Patrick Higgins
As I was recently stalking through one of Fakahatchee’s sloughs, I found myself fantasizing that I was a great big wading bird. It seemed perfectly natural to move ponderously so as not to create a ripple in the golden water. I slid each foot forward in slow motion, conscious of everything around me: the Sun streaming through a leafless winter canopy, a single red maple leaf floating by, a movement in the cutgrass to my left, the rustle of an anhinga taking flight ahead, the plop of a frog into the water. I froze as my eye caught something shimmering on the dark bottom. And then, fortunately just before I plunged the imaginary spear-like bill on my face into the water, I shook myself out of my revelry.
Instead I fished down clumsily with my hands, wetting my sleeves in the process. I had to feel around a bit due to refraction, but finally my fingers closed around my target. It was the fragment of a bivalve’s shell and shone like burnished silver. When we think of swamp invertebrates our minds tend immediately to go to crayfish, insects, snails and such, but we seldom give much thought to the snail’s mollusk cousin: the freshwater mussel.
After a bit more probing I was able to come up with a whole shell, although it was empty. Its ebony exterior was striated by growth rings, some of which had eroded revealing the iridescent mother-of-pearl beneath. In the center was a neatly chiselled rectangular opening, probably the work of a limpkin’s bill.
This is the same mother-of-pearl that was used for over half century in button production until plastic came into being in the 1940’s. Today ground freshwater mussel shell is still favoured as a seed material for Japan’s cultured pearl industry.
However, freshwater mussels are not simply freshwater versions of the marine mussels we love to eat cooked with butter and garlic and washed down with a glass of white wine. True, they both are aquatic filter feeders, sort of resemble each other by having asymmetrical shells that are longer than wide, and are both bivalves, but freshwater mussels are members of an entirely different subclass (Palaeoheterodonta). In fact, marine mussels are more closely related to oysters and scallops than they are to their freshwater namesakes.
The biggest difference is in their life cycles. Marine mussels are sessile. They typically attach themselves in clumps with those stringy byssal threads we scrape off when preparing them for the pot. They reproduce by releasing sperms and eggs into the water in a rather hit or miss fertilization process. They then go through a floating planktonic stage before settling out of the water column to live in a fixed location.
On the other hand, freshwater male mussels release gametes into the water column. The gametes are then sucked into a female’s siphon, where her eggs are fertilized internally. The fertilized eggs develop in specialized brood chambers in her gills into tiny motile glochidia. This specialized larval form has an array of hooks, which allow them to attach to fish where they live as parasites for a period.
Many species are highly host specific. Some mussels even use mimicry to lure in host fish by packaging their glochidia to look like prey items such as fish eggs or insect larvae. Typically the glochidia attach to the fish’s gills, although some species also utilize the tail fins. This is an important adaptation to aid distribution. Otherwise, in freshwater streams and rivers distribution would be one in direction only, i.e. downstream. After a few weeks feeding on the host fish they complete their metamorphosis into juvenile mussels, then detach and fall to the bottom to begin their adult stage.
This is where a second huge difference comes in. Freshwater mussels have a powerful clam-like foot which enables them, through a series of muscular contractions and expansions to burrow, anchor themselves or move along the bottom. They seldom move more than a few hundred feet, but that could be critical to survival as water levels drop. Finally, they are known for their longevity. Some species live up to a hundred years.
The mussel I was holding in my hand appeared to be a Florida shiny spike (Elliptio buckleyi or to some taxonomists E. jayensis) – one of 365 North American, 62 Floridian and 14 lower-peninsula species. I say probably because many freshwater mussels are incredibly hard to identify without the aid of magnification.
The same species from different habitats often have variations in shell shape. There is so much speciation in freshwater mussels due to their intolerance to saltwater. This inhibits spreading from one river basin to another, isolating populations. It is therefore not surprising that 69% of Florida’s freshwater mussels are endemic, some to specific catchment areas.
To date, three host fish species have been identified for Florida shiny spike’s glochidia. These are Bluegill, Largemouth Bass and the Florida Gar, but in the incredible web of life about 40% of shiny spikes are in turn parasitized by several species of mites.
Freshwater mussels provide an important ecosystem function. As filter feeders they are constantly straining bacteria, algae and particulate matter from the water column, cleansing it in the process and reducing contaminant loads. They can filter as much as of 1-2 quarts per hour depending on the bivalve’s size. Their faecal matter and ejected particulates then provide food for invertebrate communities that in turn support fish populations.
They occur in almost every freshwater habit, except highly acid rivers which inhibit shell production. However, some 60 North American and 7 Florida freshwater mussel species are listed as threatened or endangered. This is due to habitat loss or alternation such as silting or pollution. So keep a look out for those telltale glittering fragments of mussel shells next time you’re in the swamp.
Oh, one final difference between marine and freshwater mussels: they are a prized food source of raccoons, otters, crayfish, and many wading birds. Also, they were consumed by Native Americans after much boiling, but apparently taste like old boot – something I haven’t put to the test yet!
Patrick Higgins is a National Association of Interpretation Certified Interpreter, Vice-President of the Friends of Fakahatchee and Project Manager for the development of the Boardwalk Master Interpretive Plan.