Mussels live private lives, sunk in streambeds or hidden in the murky bottoms of lakes and ponds, quietly filtering food and cleaning the water. Mussels are misunderstood and underappreciated, but they are so important they are worth getting to know!
Some mussel species have already gone extinct, and many are endangered. Mussels matter, so take some time to meet the different mussels that live in our watershed.
Mussels are distinguished by their hard outer shells that are mirror images of each other. Safely tucked inside the hard exterior are the soft body parts. They have a circulatory system powered by a heart, plus a digestive system that takes in nutrients and excretes waste, just like us! Mussels also have two pairs of gills with three important jobs: breathing dissolved oxygen; filtering out the food and delivering it to the mouth; and, in females, it is where the larvae incubate until they are ready to be released in hopes of finding a host fish.
Words to know
Distinguished: To recognize something as different.
Excrete: To separate and expel waste.
Incubate: To keep warm to bring to hatching.
Murky: Dark and dirty.
Truncated: Shortened or without its top or end section.
There are also more soft body parts, each with an important job to do. But to learn what kind of mussel it is, all the evidence we need is on the shell. There are clues on both the inside and outside that can help us determine the species of a mussel we are observing.
First, we need to know the common language to talk about mussels. The two shells of mussels are called valves. There is a right and left valve. They can be orientated in this way:
The dorsal is the side where the beak and hinge are found.
The ventral is the opposite side from the dorsal.
The anterior is the back side. The beak is always closer to the anterior side. The anterior side is the side buried in the bottom.
The posterior is the front side. The lateral teeth are on the posterior side.
Look at the other labeled parts on both the inside and outside of the shell. By asking the right questions and looking at the clues, we can discover the names of the mussels we observe. Now it is time to investigate.
Questions to ask
1. What is the overall shape? Do the anterior and posterior ends have different shapes?
2. What color or colors is it? Are there rays or bands of color spreading across the shell?
3. Is it smooth? Can you see or feel ridges? Are there large waves and folds in the shell? Are there knobs or bumps on it?
4. How close or far apart are the growth lines?
5. What is the overall size and shape of the beak? What shape or pattern is visible if the beak is inspected more closely?
6. How thick is the shell?
There are more clues on the inside of the shell that can help us identify a mussel.
The nacre is the whole pearly layer covering the inside. It can be white, pink, purple or blue.
The beak cavity is the hollow space formed by the beak. It can vary from shallow to deep.
The anterior and posterior adductor muscle scars are the scars from the muscles that mussels use to open and close their shells when they are alive. The shape and pattern of the scar they leave behind is another clue.
The lateral teeth and pseudocardinal teeth are not truly teeth but part of the hinge mechanism of the mussel. These teeth can be distinctly detailed in size, shape and number or completely absent.
The interdendum is the space between the two sets of teeth. Its size and shape are different in different species.
The palial line is the scar left by a soft body part called a mantle. The mantle’s job is to secrete the calcium that forms the shell.
Meet some mussels
All native mussel species in Illinois belong to the family Unionidae. In Will County, we can find representatives from three subfamilies of the Unionidae family. Let’s meet a species from each and use the investigative tools we just learned to observe their characteristic features.
The washboard mussel got its name because of the bumps and folds on its thick shell. A washboard is a board with metal bumps and folds used to clean laundry. They are still used today, but as musical instruments instead of for cleaning. Many species in the subfamily Ambleminae have wavy shapes or bumpy spots on them. The washboard mussel is large and darkly colored. The anterior side is rounded, while the posterior side is more square shaped. The beak cavity is deep, and the outside of the beak has depressions making an M or W shape.
Washboard mussels can grow up to 11 inches in length. They often live for more than 10 years, and some may live to be more than 100! Their favorite habitat is large rivers with mud, sand or gravel at the bottom.
The creeper is a member of the subfamily Anodontinae. They will grow up to 4 inches in length. When young they can look very different and easily be misidentified. They are oval or elliptical shaped. The shell is smooth and shiny and green, dark brown or black in color. The shell is thin when young but will thicken as it ages. Its pseudocardinal and lateral teeth are poorly developed. These mussels like to nestle into mud, sand or gravel in small to medium-sized streams.
The fat mucket belong to a group of muscles in the subfamily Lampsilinae. All members of this group share some neat traits! They are all light or tan colored with brightly colored rays or bands. Also, in this group, the males and females look different from each other on their posterior ends. The males are slightly pointed, and the females are either square shaped or truncated.
Here are some observable features more specific to the fat mucket: Their nacre is white to blueish white. Their lateral teeth are long, thin and straight to slightly curved. Their pseudocardinal teeth can vary in shape, but there are two in the left valve and one in the right.
Fat muckets can grow to 5 inches in length. Lakes and small to medium streams with muddy or sandy bottoms are the fat mucket’s favorite habitat.
There are so many more cool and interesting species of mussels and so many more amazing facts about how they live and grow and contribute as important member of their habitat! Four Rivers Environmental Education Center in Channahon has just opened a brand-new exhibit space with information about mussels. There is also a new 2,000-gallon tank with many host fish species that are important to mussels. Come to Four Rivers and check out these new exhibits. While you are there, ask a naturalist to see our freshwater mussel shell collection!
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