Green is where Unionids were found in research.
Red is where I collected Unionid shells.
[Figure derived from those at link.]
As we wandered around the river edge, we found numerous large mussel shells. I collected a few, with intentions of identifying the species that made them at some later time.
Fast forward a few years and I'm digging through some boxes in the basement. I'm not sure what I was looking for, but the shells grabbed my attention. It was time to figure out what they were.
I had collected two pairs of shells. One pair was thinner and the other was thicker. One of the thinner shells broke while in storage. The remaining shell is 13.8 cm long, 8.0 cm tall, and 2.6 cm deep. After some looking around at various documents, I realized I could identify these thinner shells as a specimen of a Unionid species called the Great Floater (Pyganodon grandis). The species seems to get this name because of the penchant for their shells to float away when one has died and has begun to rot.
|Thick-shelled Unionid. Lower-right|
is a closeup of growth ridges.
Through the process of trying to identify these shells, I accidentally identified a shell I had found in central Texas when I was in highschool. This shell is from another Unionid that is called the Threeridge (Amblema plicata). I had long ago given up on finding the name of this shell, so this was a cool bonus.
Unionids have all sorts of interesting biology. Like most bivalves, they make their living filter feeding water as they hide buried in the sediment. The live in freshwater river systems worldwide, with the most diversity present in North America. Adult Unionids can only travel very slowly by shifting their foot, so you would think they'd have a difficult time traveling up rivers. The Unionids have developed a very special trick to get around this limitation. They use fish to transport their babies.
Unionid larvae (called glochidia) spend some time as a parasite in the gills of fish. The fish can travel upstream or downstream, much further than the adult could ever crawl. After some interval, the glochidia drop from their fishy host and start living the traditional life of a c.
|Images from unionid.missouristate.edu|
These species are long-lived, but sensitive to environmental disruption. They can't survive in a river that dries up and are they're unable to get out of the way when water quality is impaired by human activities. Because of this sensitivity, there are legal restrictions on their harvest (in every state I've checked).
All the shells I have were collected on dry land, which sidesteps the legal restrictions designed to protect the live animals from harm. Frankly, I couldn't imagine collecting living animals to get their shells. You have to respect your elders, even if they happen to be living on the bottom of a river.
- Video of the river: www.youtube.com/watch?v=WMf6tSUlKso
- Great Floater (Pyganodon grandis):
- Aging shells: www.fishwild.vt.edu/mussel/PDFfiles/Eval_of_tech.pdf
- Amblema plicata:
- Unionid info:
- Legal stuff:
- Minnesota: www.dnr.state.mn.us/rsg/filter_search.html?mussel=Y&allstatus=Y&action=doFilterSearch
- Michigan: www.eregulations.com/michigan/fishing/it-is-unlawful-to/
- Texas: tpwd.texas.gov/regulations/outdoor-annual/fishing/shellfish-regulations/other-aquatic-life-fresh-saltwaters
- Nebraska: digital.outdoornebraska.gov/i/621072-fishing-guide-2016