Backyard Biology

Nature stories from my backyard and beyond

Nature Stories

Hitchhiking the River
A Tale of Survival

Friday, August 5, 2011

 

Consider for a moment - the freshwater mussel....

At first glance we have a lump of fleshy stuff with no moving parts, surrounded by two drab shells that spends its life underwater half buried in the muck.

At first glance, the freshwater mussel doesn’t command much consideration. But a closer look reveals a fascinating creature that has had to overcome some very daunting survival problems.

To take a closer look, we first must find and collect some mussels. Finding them is fairly easy. Just look along the banks of the river for their empty shells. A lot of shells on the bank indicate a flourishing population of living mussels in the water. Now to collect some living specimens to learn their secrets. This we will leave to the scientists who study the mussels. Of the twelve species of freshwater mussels in Massachusetts, three are on the state’s endangered species list and four are on the special concern list. Therefore, unless you can identify the different mussel species, it is best to leave them be!
The easiest way to view them without disturbing them is with a mask and snorkel or an underwater viewer. This can be made by gluing a clear piece of Plexiglas onto the bottom of a plastic bucket that has the bottom removed. By placing the bucket in the water - plexi side down - and the top of the bucket above the water’s surface, you can look through the bucket underwater and see the living mussels - along with other creatures. On a recent trip to the local river, I was able to spy mussels, crayfish and a number of baby eels swimming upstream.
For the mussels living in the Indianhead River, or any river for that matter, the biggest problem they face is the current. Currents tend to carry things downstream, and unless the mussels have a way to combat the current, they will all end up in the ocean, a not very pleasant place for a freshwater animal! The mussel’s main line of defense is its foot. Mussels, like all bivalves (clams, mussels and oysters) have only one foot - no legs - just one muscular foot. This foot is inserted into the bottom substrate, and then anchored by flaring out the tip. Once the foot is anchored, the mussel retracts the foot into its body. However, since the foot is anchored, it stays put while the mussel is pulled down under the substrate. Once half buried, the mussel is safe from the current and is ready for business.

The first order of business is breathing. The main organs for breathing are the gills. The gills are covered with tiny hairs or cilia. These cilia beat back and forth in unison to create a water current flowing through two tubes or siphons that the mussel can extend out past the shell into the water. The incurrent siphon brings oxygen-rich water into the mussel past the gills while the exhalent siphon directs the oxygen-depleted water out of the mussel.

The second order of business is feeding. The main organs for feeding are also the gills. Suspended in the water entering the incurrent siphon are tiny plants and animals called plankton, along with other organic debris. The plankton sticks to mucus on the gills and this mucus/plankton concoction is passed by the cilia in conveyor belt fashion across the gills to the mouth.

The third order of business is reproduction. Once again the gills play an important role. While some types of mussels are hermaphrodites - both male and female at the same time-most of the mussels in our watershed including our most common, the Eastern Elliptio, are either one or the other.
When it is time to reproduce, the male releases sperm from the exhalent siphon. The female will suck in some of the sperm through her incurrent siphon where the sperm will fertilize the eggs. The gills, as well as used in breathing and feeding, now are used as a brood pouch for the young larval mussels. These young larvae, called glochidia, grow rapidly in the brood pouches until they are released into the water through the exhalent siphon. Left to their own devices, these glochidia would soon be washed downstream by the current. Over a very short period of time, the mussel colonies would find themselves migrating closer and closer to the ocean, leaving the upstream areas devoid of mussels. To prevent this, they spend their young life as hitchhikers with as many of them catching a ride upstream as well as downstream!glochidium
The glochidia clamp onto the gills, fins or scales of fish. Some mussel species will attach to only one or two types of fish while others are not so choosy. The glochidia will become encased in the fish tissue and will receive nourishment from its host; however, these infestations are seldom large enough to harm the fish.
With the use of a plankton net and a microscope, these glochidia are easy to view. Since the glochidia are attracted to the fish by the disturbance of the water when the fish pass by, they are also attracted to the water disturbance caused by a plankton net moving over the river bottom. Once caught, they are easy to see with a low-powered dissecting microscope.

During their short hitchhiking period, the glochidia change form into miniature mussels. After about a month, they drop off the fish and are ready to settle down to their sedentary adult life. If chance favors them, their fish will have carried them to a suitable location with just the right bottom conditions for growth where they may live for fifteen years or more. If not, they die.

Because of their unique lifestyle, mussels are good indicators of the health of our rivers. Since they rely on a continuous supply of clean water for breathing, feeding and sex, as well as a healthy fish population for hitchhiking, the mussel shells littering the upper parts of a river is a comforting sight indicating clean water.