Freshwater Mussels: The Livers of the River

The creek was up and it ran murky and hard.  I didn’t expect to see much. It has been a summer of episodic heavy rains which tend to put a lot of mud in water and makes our streams run silty. 

I’m a freshwater snorkeler so I do what I always do: I stuck my face in anyway. 

I’ve been fooled by surface appearances before.  There’s usually an amazing world just beneath the surface.

The swift current tried to peel me from the bottom so I dug my fingers and toes further into the cobble. Minnows fed in the slick of material I inadvertently churned up. I couldn’t see much more than haze, like looking through a fogged windshield.  I could make out shapes, and colors stood out.

A small school of rosyfaced shiners frantically snatched insects I flushed off the bottom.  But I didn’t see much else.  I held against the current for a while, until my arms started to twitch like sewing machines.

Bracing against flow like this is like holding a continuous plank, and my muscles can only last so long before they fatigue. Even if I only caught a glimpse of these small fish with their red cheeks, it would have been worth the submersion.

Colorful Filters

Freshwater mussels are bivalves like their marine clam and oyster counterparts, but unlike the marine blue mussels we are used to getting served steamed with drawn butter at restaurants, these freshwater species are better for river filtration than eating.  They are filter feeders. 

They draw river water in through an incurrent siphon and filter out planktonic algae and diatoms which they send to their mouths and digestive tract.  They spit all the inedible material they filter from the water, like sediment, to the bottom as pseudo feces.

The bright spots that seemed to glow from between the rocks are the incurrent and excurrent siphons.  The question is why are they so colorful? What purpose does the color serve?

Maybe the colors serve no purpose. Maybe they are colorful just to be colorful.  Why does everything have to be utilitarian?

Freshwater mussels certainly perform critically important ecosystem functions like filtering 8-15 gallons of water per day per mussel which has led them to be called the livers of the river.

But I think their beauty and importance transcends the what have you done for me ecosystem service human-centric value we place on them.  This was evident as I peered at a garden of siphons – yellows, oranges, creams, and indigos, solid color and tiger striped pattered, poking through the fuzzy olive drab of the surrounding cobble.

Mussel Reproduction is Weirder than You Think

One explanation of the colors that makes as much sense as any other is reproduction.

Male mussels eject sperm into the water. Females take the sperm in through their incurrent siphon where it fertilizes eggs. Fertilized eggs, call glochidia, need an intermediary host in order to complete their development and turn into a juvenile mussel.

That host varies with species of mussel.  Most use fish, some use crayfish and a few even use aquatic insects.  Some mussel species aren’t very particular; any kind of fish will do.  Others are very discerning and only a specific kind of darter can serve as the host. 

Regardless, female mussels need to infest the host with her glochidia.  If she were to just spew them into the water, their chances of finding the right host aren’t great. 

So female mussels develop strategies to lure host fish into their marsupial-like gills, the pouches where the glochidia are stored. A possible explanation for the ornate colors is to attract fish in as part of a complex reproductive strategy. 

I saw some motion on the bottom and thought a rosyfaced shiner was back playing in my slick.  Except this fish was stationary as it twitched.

Then I realized I had been fooled.  This was a female Lampsilis mussel working her lure.