Monday, January 2, 2012
Fresh Water Mussels
As you can see the kids were having fun diving for mussels in our pond. What a great way to spend a hot summer day!
There are hundreds of fresh water mussels in the World, and nearly 300 of them live in North America. In fact North America is home to the largest variety of mussels compared to anywhere else in the World. Many are in threat of becoming endangered, in fact the estimate is as high as 3/4 of the known species may be in peril. As many as 35 species have already gone extinct. The Midwestern portion of the United States contains the highest concentration of species, Missouri alone is home to some 65 species. Native Americans utilized mussels as an important part of their diet. Not only were they a valuable food source for tribes throughout the Midwest, but they also held value in other ways. The shells were used for tempering pottery and tools, they also made jewelry, and utensils. In the late 1800's "white man" recognized the potential value of the shells in the fashion industry. The button industry reached a boon during this time in history and many mussels were collected for the "mother-of-pearl" and sold to the button factories. The epicenter for the button industry was right in the good old Midwest.
As many as 200 button factories were in operation by 1912. Barge loads of mussels were harvested each season.This over harvesting of mussels drastically affected the population of mussels. It wasn't until the use of plastics came in vogue during the 1950's that the interest in mussels for buttons waned.
The livestock industry also benefited from the harvest of mussels, by manufacturing livestock feed from the soft parts of the mussels.
With the advent of plastics it seemed as if the mussels would be left to rebound depleted populations, but that was not to be the case. The Japanese discovered that mussels could be used to create cultured pearls. The mussels are boiled and cleaned. The shells are then formed into small beads and inserted into the shells of oysters as an irritant. The oyster will form a pearl around the irritant and the resulting pearl is sold as a "cultured pearl". Thousands of tons of mussels are collected, boiled, cleaned, formed into beads and shipped to Japan for this very lucrative trade. Fourteen states still allow the harvesting of fresh water mussels to be sold for this purpose. There is some indication that certain species of mussels are immune to many types of cancer. There is hope that the components that allow the mussels this immunity will be isolated and will lead to cancer curing medicines to be created.
An added problem mussels face is pollution. Many areas where fresh water mussels occur are heavily polluted. The mussels cannot survive in those conditions. Mussels are filter feeders and consume micro-organisms. In heavily polluted water they are also consuming the chemicals and debris that are present in the water. Because of this sensitivity to pollutants they are considered indicator species. This basically means that biologist can determine water quality by the presence, or lack thereof of the mussels in a given aquatic habitat. Depending upon the circumstances much can be determined about water quality based on these mussels. If a pond that traditionally held mussels, suddenly is mussel free that would "indicate" a severe problem with the water conditions. A sudden decrease of mussels can also indicate a problem in progress. Mussels that remain can be collected and their tissue can be tested for the type of pollutant, and the extent of the pollution that is causing the problem. Many times biologists are able to head off a disaster because of the data collected in the mussels. Mussels are an important part of the aquatic ecosystem, not only for their ability to indicate water quality issues, but because of their ability to filter the water. This filter feeding increases the water quality and creates an overall healthier water ecosystem.
They are also an important component in the food chain. Many animals will consume mussels including herons, and other shore birds, skunks, raccoons, foxes, minks and otters are all huge fans of mussels. Even muskrats that are traditionally plant feeders will utilize mussels as a food source when other food is not available.
The life cycle of mussels is very unique. Males release sperm directly into the water, and the females filter the water and sperm directly into their cavities. Once fertilized the female will carry the eggs and young larvae called glochidia within her gills. Some species will release their larvae the same year they are fertilized, other species carry the young overwinter and release them the following spring.
After being released the journey to adulthood gets interesting. In order for the mussels to complete their lifecycle they must attach themselves to the gills or fins of fish....yes that's right FISH! Who would have thought that mussels rely on fish to survive. The species of fish used is all dependent upon the type of mussel in question. Some mussels have very species specific requirements and will only use one type of fish. For instance Pocketbook mussels need to find small mouth bass. So how do they insure that the proper host is found?
They use a lure in the form of a soft tissue that they wiggle which resembles a small minnow, the small mouth bass is attracted to the "bait" and comes in to investigate. Instead of finding a meal, it finds itself a host to a mouthful of glochidia. Such deceit! Many mussels utilize any fish available, but have a wide variety of methods to attract them. Oyster Mussels open their shells and expose a blue mantle. The bright blue color attracts nearby fish. When the fish swims in closer to investigate the mussel will clamp its shell down onto the fishes head and holds tight. As the mussel holds on the glochidia swim into the fishes mouth. I am sure the fish is much relieved when the mussel finally releases it. The young larvae remain attached to the host fish for up to 7 months depending upon species. Once they have reached maturity they will drop off the fish and lead a sedentary life. Mussels are very limited in their ability to move which is why they rely on fish to spread their populations. Of the millions of eggs produced by females and fertilized by males, only about 1 in a million will survive to the juvenile stage. Mussels are long lived creatures and many species may live a century or more. Other species live for decades and like many long lived animals they do not reach sexual maturity until they are older. Some species may be ready to reproduce at 2 years of age, and others it may not be until a decade later.
Other factors can affect the over all survival of mussels, including habitat destruction. This can be caused from dredging, damming and forming channels. Mussels are vulnerable in other ways too, consider that they have a later maturation rate, very low juvenile survival ratio, susceptibility to pollution, inability to disperse in a sustainable fashion, and host specificity. All these things combined makes a person wonder how they survive at all. On top of all these issues, they also suffer from competition with invasive species. Zebra Mussels have wrecked havoc on many fresh water environments. Zebra mussels originated in Poland and the former Soviet Union. They made their way to North America in the ballast water of ships. They were first discovered in 1988 in Canada, and four years later were found in the United States throughout the Great Lakes. By 1991 they were discovered in the Mississippi River. Humans are responsible in most cases for the expansion of the zebra mussels range. The mussels attach themselves to boats and other water craft and then are carried to new locations. Zebra mussels unlike many mussels have a very sort reproductive cycle which allows them to colonize areas in greater numbers. The presence of zebra mussels causes many problems for native mussels, because of their habit of attaching themselves to natives species. This interferes with feeding, movement, growth and reproduction of native mussels. In Lake Erie alone it is estimated that Zebra mussels are responsible for the 90% reduction of native mussels. It is vitally important that individuals frequently utilizing the water in a recreational manner make sure that their water craft is free of any attached mussels before heading to other waterways. This can go a long way in reducing the spread of these invasive species.
Sometimes the most unassuming creatures perform some of the biggest jobs. While mussels move very little, and are rarely seen they perform a huge job in purifying water, determining water quality, providing product to the jewelry and fashion industry, and providing food for many other animals. We as humans should try to reduce invasive species, limit habitat destruction or alteration, and reduce harvesting especially in locations where native species area already struggling to maintain viable populations. We also need to continue to reduce chemical runoff which greatly impacts these fresh water mussels.
I feel very fortunate to have such a healthy and large population of these mussels present in our ponds. This indicates to me that the water quality is superior and all the right components are in place to allow them to survive and reproduce. Now if I could just figure out how to make them palatable I'd have it made.
Resources: Texas Parks and Rec; MDC; and NRCS