This past weekend I spent a perfectly lovely day at Hirundo Wildlife Refuge. Just 10 miles from the University of Maine in Orono, this private refuge straddles Pushaw Stream and showcases forested wet lands, open sedge meadows and miles of canoeing and hiking trails enjoyable by all. I was there as a teaching assistant in a class studying fresh water ecology, and we looked at trees and unique shrubs particular to wetlands, canoed on the stream in 28 foot peace canoes, seined for fish, and hunted for elusive fresh water bryozoans. As surprised as I was to learn that there was such a thing as a fresh water bryozoan, I was even more surprised to learn how many different fresh water mussels there are, and how different they are than the ones we have in the ocean.
Fresh water mussels! Who knew? As some one who specializes in the marine environment, moonlights with terrestrial botany and occasionally goes a little nuts over aquatic insects, I didn’t. We have lots of mussels in the ocean of course, mainly the ones we eat, blue mussels Mytilus edulis, and their deep water cousins, Modiolus modiolus, the big sub tidal horse mussel. Compared to these, fresh water mussels are mussels in name only. Taxonomically fresh and salt water mussels are both in the class Bivalvia, a group of Molluscs that has a common trait of having two shells. They then split off into two separate subclasses; the salt water mussels fall in the evolutionary line that also includes scallops and oysters. Freshwater mussels are out alone in their own subclass, whats called a monophyletic group; they are the only type of organism on that branch of the tree of life.
Unlike marine mussels which live as epifauna, strongly adhered to hard surfaces by anchors called byssal threads, freshwater mussels are infauna, they live buried in the sediment more like clams. Their relatively short siphons and weak feet ensure that while they are in the sediment, they cannot go deep into the substrate and will be found near the surface of the bottom. Like most bivalves marine or fresh, fresh water mussels make a living filtering organic matter out of the water. These food items can be algae, bacteria, or detritus. They have an inhalant siphon and an exhalant siphon (sort of like us, except in mussels the inhalant and the exhalant apertures are located directly next to one another, think about that for a minute.). To eat, which they do continuously, they suck water in and it flows over their large gills, which perform the dual function of not only absorbing oxygen from the water but also filtering out bits of food, and just about anything from the water. Food items that don’t agree with their palates are trapped in mucus and ejected before they can be digested. I had no idea that filter feeders could do this. All this time I was under the impression that they ate indiscriminately, and thus were always a net bonus for water quality. Little did I know that they can reject food, which in regions with the invasive Zebra mussel, is actually creating a water quality issue as the Zebra mussels preferentially eat planktonic algae but spit out cyanobacteria, leading to a large change in the planktonic composition in these lakes and waterways.
Besides living it the mud or sand, and not having byssal threads, the biggest difference between marine and fresh water mussels is reproductive strategy. Marine mussels simply release their sperm and eggs into the water column, employing fully external fertilization. If the sperm and eggs meet in the water column, free floating larvae are formed, which go through several stages of development as plankton before they settle out as what we would recognize as tiny little mussels. Fresh water mussels don’t leave it so much to chance. Males release sperm into the water but the females then capture that sperm with their incurrent siphon (think about that for a minute), it simply comes in with everything else they filter out of the water. The females fertilize the eggs internally and hold the eggs in special pouches on the gills for a period of days to months depending on species. When the larvae are big enough the female expels them into the water. Here is where it gets really interesting. The larvae have to find a fish, and not just any fish, to act as a host for days or months, again by species. If they find their proper fish, and there are many strategies for doing so, including stringy mucus lures meant to attract unsuspecting fish, the larvae attach themselves to the fish, either to its skin and fins or in its gills, again, depending on the species. They grow and develop and actually get encysted in the fish’s tissue, apparently at no cost to the fish, though at least one reference refers to the relationship as parasitic, which implies harm. At the end of their period of fishy development, they drop off and if they are really lucky, they drop off into appropriate sediment substrate, and don’t get eaten by a hungry predator.
So all this time, this has been going on in fresh water streams and rivers, and ponds all over the state, and I had no idea. I feel like some one just told me about another planet in our solar system. Somewhere I knew, in a dimly lit corner of my head, about the existence of what I thought of as fresh water “clams”, from a child hood of fresh water swimming, but it was a part of my brain I never funneled any light or energy to. Its nice to know that surprises exist, and not just on the frontiers of science, but right in your own back yard. Seek them out, you will be glad you did.
Hirundo’s website: http://www.hirundomaine.org/
Maine’s info page about threatened fresh water mussels
Nice site from Virginia Department of Game and Inland fisheries: http://www.dgif.virginia.gov/wildlife/freshwater-mussels.asp
The American Museum of Natural History has an extensive website about the freshwater mussels of the New York and New Jersey area, many of which are the same ones we have here in Maine http://www.amnh.org/our-research/center-for-biodiversity-conservation/research/species-based-research/invertebrate-conservation/freshwater-mussels
NOAA document on zebra mussels and blue green algae (cyanobacteria) and changes to Great Lakes aquatic ecosystems