By Dr. David Wartinbee, for the Redoubt Reporter
Freshwater mussels can be found in a number of streams and lakes on the Kenai Peninsula. However, most of us never see them because they are found exclusively on stream beds and they are almost completely submerged in the substrate.
On top of that, their outer shell covering, called the peristracum, is usually about the same dark color as the surrounding substrate. So, unless you are scuba diving and looking carefully, or happen upon a broken shell in a stream, you may not have a clue to their presence.
These mussels prefer slower-moving, meandering watercourses like the Swanson River or any number of smaller feeder streams. And they can be found embedded in the bottom sands or ooze in a number of local lakes.
One of the species found on the peninsula has the common name Yukon floater. We can understand the Yukon part, since it is a northern species, but what’s with the “floater” part of the name? It turns out that a number of the freshwater mussels found in Alaska are in the genus Anadonta and they have very thin shells.
Because of their relatively thin, and thus very lightweight, shell, they can remain on top of fairly soft sediments. If they had a thicker shell, like a number of their eastern relatives, they might sink into the thick ooze and be lost. In a way, they “float” on the top of the silt. With that thin shell, they can also use their foot to pull themselves around in the soft substrate to hide from predators.
Almost all clams and mussels feed on fine particles being washed along the bottom. The fine particles used by our local freshwater mussels are the remains of chopped-up leaf fragments, drifting algae or tiny zooplankton.
When a mussel starts to open its shell, a pair of tubelike openings project out that allow water to come into the shell. The second of the paired openings is used to force that same water back out. The length of these tubes, called the incurrent and excurrent siphons, will vary according to the particular species of mussel. Those found in our area have relatively short siphons that just barely project above the upper edge of the shell. When feeding, they start taking in water and remove edible particles before squirting the water back out.
Quite interestingly, it is the gills that do the capturing of the food materials from the water passing through. The gills have fine grooves that run in differing directions. Very fine cilia are able to force the food particles toward the mouth, while sand or other inedible materials are pushed toward the excurrent siphon. If you watch a mussel feeding, every once in a while it will squirt water out the excurrent siphon so as to clean out the sand and inedible materials.
This filtering of fine particles by mussels or clams can actually clarify the stream or lake water. When the alien zebra mussel grew in massive numbers in Lake Saint Clair, near Detroit, the water was clarified so much that the predominant fish species changed. Walleye and perch that normally prefer slightly turbid water were replaced by largemouth bass and pike that prefer clearer water.
Since there is a constant movement of fine particles in streams and along lake bottoms, mussels can thrive in one area for long periods of time. In a pinch, mussels will actually move from one place to another. You can imagine they don’t move very fast, nor do they move very far. Although, while adults aren’t able to move far, young larval mussels have the opportunity to travel great distances.
Male mussels release sperm cells into the water column and local females pick up the sperm through the filtering action of their gills. The female’s eggs develop and grow within the gills after fertilization. When the eggs mature, they become Glochidia larvae.
These larvae look like miniature clams with one enlarged, very sharp tooth in the middle of each side of their shells. When a fish passes nearby, the glochidia larvae are released by the parent so they can hitch a ride. Then the larvae snap shut on a fine filament of the fish’s gill or on a thin section of the tail or fin and become a parasite. The mussel larva survives on nutrients taken from blood and other fluids from the fish tissue. The fish targeted as a host by glochidia larvae in our area are mostly sticklebacks, chinook salmon and rainbow trout. However, sculpins, char and cohoe salmon are also believed to be possible host fish.
The larva may stay attached to the gills for only a few days, or as long as a month. A fish with its parasitic hitchhikers can end up in waters that are far from where the mussel parents were found. When the larvae give up their joyriding and free lunch, they settle out on the bottom substrate materials.
They adopt their parents’ filter-feeding style and soon become recognizable as small mussels. During this time of their young life, they can be chosen as a food item by some species of fish. But once they get a little larger, they are fairly safe from most predators, except larger mammals like otters or muskrats.
Under ideal situations, our local freshwater mussels are able to live about 10 to 15 years. That is much longer than most freshwater invertebrates like insects or worms.
While rarely seen and often completely unknown to casual outdoor visitors, freshwater mussels can actually make up a large portion of the animal biomass in a local stream or lake. These creatures are a great example of the idiomatic phrase, “There is more than meets the eye.”
David Wartinbee, Ph.D, J.D., is a biology professor at Kenai Peninsula College’s Kenai River Campus. He is writing a series of columns on the ecology of the Kenai River and Cook Inlet watershed.