Photo Credit: Chris Barnhart, MSU
Freshwater mussels are an unassuming group of animals. They look more like rocks at first glance; they cannot possibly be all that interesting. Let us take a closer look.
Freshwater mussels are mollusks, the phylum that includes other bivalves (think oyster, clams, scallops, etc.), gastropods (snails and slugs) and even the cephalopods (octopus and nautilus). The is the second largest animal phylum, second to only to Arthropoda (insects). What relates all mollusks to one another is the presence of the mantle, the dorsal (or back) covering of the animal which encloses all its insides (the visceral mass). In the snails and bivalves, the mantel secretes calcium carbonate to form their characteristic shells. In the cephalopods the mantle also creates the siphon, a body part that cephalopods use to move around. It propels their body through the sea. In the snails and bivalves, the siphon is used to aid in respiration (breathing), In freshwater mussels and other bivalves, the siphon is involved in a number of tasks including filtering food, removing excretion and even reproduction.
Diagram of Mollusk Phylum from www.exploringnature.org
Freshwater mussels in the order Unionidae are one of the few taxa that have their highest diversity in temperate areas. Other taxa that also have their highest species diversity in temperate zones include salamanders and freshwater turtles, while most other taxa have the highest species diversity in tropical regions. In fact, North America has the highest number of Unionid mussels with nearly 300 species. These mussels are adapted to living in rivers and lakes, and they play a huge role in maintaining the health of these ecosystems. Mussels filter out material in the water and deposit it as waste, which can be used by other critters as a source of food, thereby cleaning the water. Mussels bury into the river substrate which can stabilize the river bottom, and thereby maintain river or stream habitat for other species. They are also a source of food for native wildlife and have cultural significance to Native American communities. They were also of commercial use in the late 1800’s and early 1900’s as a source of material for making buttons, a multi-million dollar industry at the time.
OK, so far mollusks and the mussel seem like a nice group, rather more diverse that you would have thought. That is pretty neat that they are most diverse in North America, you can’t say that about most other North American taxa (except salamanders and turtles). Anything else I should add?
Well, they have some funny common names.
Here are some we came across in a river here in Mid-Missouri one day during field work.
Pimpleback (Quadrula pustolosa)
Others are named elephantear, fawnsfoot, monkeyface, heelsplitter, fat mucket and spectaclecase...you get the idea, mussels have funny names. Malacologists (folks who study mussels) must think themselves to be very funny.
I also came across some freshwater mussels during a seiche on Lake Erie several years ago, The seiche caused the water level near Toledo to drop quickly, so we went out to look for mussels, because according to my friend that is what you do on a cold, windy day in November.
The picture above is a mussel that is moving, probably wondering where all the water went. You can see the trail it left behind. Adults generally don't move, but when they need to they certainly well.
A funny looking animal with a funny name, how quaint. Oh, did I mention that many freshwater mussel species are parasitic. Yes! The larva (also called glochidia) latch onto the gills of host animals, usually fish. That fish is than infected with thousands of tiny baby mussels finding purchase onto the fish’s gills, feeding off the host’s nutrients for a few weeks before dropping off, finding their own little plot of river sediment, and becoming an adult mussel filtering water to find its food. Yikes, that is crazy! Check out the two diagrams below that demonstrate the freshwater mussel lifecycle. They show the same process, I just could not decide which one I liked most.
Freshwater mussel life cycle. Credit: NC State University
Diagram of mussel lifecycle. Credit: Freshwater Pearl Mussel | Freshwater Pearl Mussel Ireland | Pearl Mussel Project
So…uh…how do these little, tiny baby mussels find the correct fish or host to latch onto to complete its lifecycle?
Leave it to the mother. Females of many freshwater mussel species have elaborate lures as part of their mantle, in some cases these lures mimic or look like a fish. By moving the lure, the mantle fish appears to move, like that of a normal fish, or perhaps a tasty worm. Other larger predatory fish decide to take a closer look, thinking they have found some tasty food, and then BAM! The female mussel might grab the fish and cover the fish with its little larvae like the Snuffbox mussel, or it might just spit out its larvae in the poor unsuspecting fish’s face like some of the muckets. Check out the videos!
Natural Fish Lure | Lampsilis Mussel and Bass - Bing video
Snuffbox captures logperch - YouTube
Darter capture by Epioblasma capsaeformis - YouTube
Many species of mussels are very specific about what species they use a a host for their larvae (or glochidia), others are more general and will use a number of different hosts. One mussel even uses salamanders (mudpuppies) as a host.
And for the nerdy geeky genetic types out there, another cool thing about freshwater mussels, most species have double uniparental inheritance of their mitochondrial genome. That means there are two types of mitochondrial genomes rather than just one, a female type that is found in the body tissue of both male and female mussels as well as the gonads of female mussels, and a male mitotype that is found in the gonads of male mussels. Current research is investigating whether the proteins these different genomes produce may play a part in sex determination in Unionids. Interestingly a majority of the Unionid taxa have this inheritance system and also have separate sexes. However, a few Unionid species are hermaphroditic, with one individual having both male and female reproductive organs. This double uniparental inheritance might be the first case in animals of sex determination being linked to differences in mitochondrial genomes. Frontiers | Putative Mitochondrial Sex Determination in the Bivalvia: Insights From a Hybrid Transcriptome Assembly in Freshwater Mussels | Genetics (frontiersin.org)
This is all cool and great, but as a reminder approximately 70 % of the unionid mussels in North America are threatened with extinction and as a group freshwater mussels are the most endangered taxon in the United States. So if you ever come across one while floating or playing on the river, consider their incredible unique biology, and give them a little respect.
USFWS: America's Mussels
Freshwater Mussels | Missouri Department of Conservation (mo.gov)
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