Backyard Biodiversity February 2, 2020 - Freshwater Mussels

Four Mucket mussels being used in an eDNA shedding experiment

So I haven't been keeping up with posts (since last year....). Here I will attempt to at least make a backyard biodiversity post once a month ... this one is supposed to be January's so as you can see, I am still behind....so lets get started.

At work we are currently developing environmental DNA detection methods for freshwater mussels.

What are freshwater mussels and why are we interested in them?  You are probably familiar with oysters or scallops or blue mussels found in the salty waters of costal areas, but there is also a large group of these bivalve animals that are found in freshwaters such as lakes and rivers. There are many groups of mussels that live in freshwater, and several families of these are found in the United States and North America. For today’s post however I am focusing on the mussels found in the taxonomic order Unionida or Unionids for short (though that is really not any shorter). Unionids are found worldwide in freshwater ecosystems. But what makes them so neat is that they are one of the few groups of animals (along with salamanders and a few other things) that have their highest diversity in terms of number of species, in the temperate regions of the world, such as in North America, as opposed to the tropics.

Unionid mussels burrow partially into the substrate of the river pumping water through openings in their body and filtering out phytoplankton and other small, microscopic organisms in the water. The soft tissue of these animals that contains their organs are enclosed in a shells made of two halves or “valves” (thus the name bivalve). The shell develops from material secreted by the soft body tissue and the two halves are connected together by a ligament tissue.  Unlike other mussels which attach themselves to the substrate with fibrous threads called byssal, Unionids have a muscular foot that allows them to move through the substrate and dig down.

Unionids also have a special way of reproducing. After males spawn and the released sperm is taken up by the female, tiny baby mussels or larvae called glochidea develop. The female releases these larvae onto a host fish, where the little bivalves clamp on to the fish’s gills and parasitically live on the fish until they reach a large enough size to live on their own. Then they fall off the fish and are now free living mussels, now capable of filtering water to obtain their microscopic food, rather than living off of the fish. If this wasn't  cool enough some species have elaborate ways to entice their host fish. Females of some species have specialized tissue lures that can trick a fish into thinking it is a tasty bit of worm or fish, thus drawing the predatory fish nearby, at which point the mussels can catch the fish in its shell and inoculates the fish with the baby mussel larvae. The fish is then let go and is left to unknowingly help feed the baby mussels. (Check out this video, it is amazing!  

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Freshwater mussels unfortunately are in danger of extinction. In fact 88 of the 298 describes species in North America are currently listed as federally endangered or threatened in the U.S. Because of their aquatic lifestyle and the fact that they filter water removing detritus, pathogens, and pollution from our drinking water, they are good indicators of water quality. It has been estimated that an adult mussel can filter 15 gallons of water a day. Their aquatic lifestyle, however, also makes them more prone to water pollution. Chemical spills in water ways can kill off healthy mussel’s beds and chronic water pollution can stress mussel populations, making them more susceptible to disease or other stressors. Other stressors are also suggested for reasons why populations appear to be declining, including a warming climate, changes to the water flow in a stream that may be caused by a dam, and loss of a host fish species. Recently large mussel declines have been observed, biologists are still trying to understand the cause. 
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Given their incredible diversity in form, unique natural history and importance to cleaning our waters we should all give Unionids a second look and help them out. Check out some more links:

https://www.nationalgeographic.com/animals/2019/12/freshwater-mussels-die-off-united-states/

​www.biologicaldiversity.org/campaigns/freshwater_mussels/
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A Spectaclecase mussel named for its resemblance to a case for glasses. Notice the large white foot protruding from its shell.

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A bed of Spectcalecase mussels. Spectaclecase are a federally endangered species.

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