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Freshwater Mussels: Engineering Ecosystems One Shell at a Time
(Released August 2011)

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  by Natalie Abram  

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What Is a Mussel?

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Freshwater Mussel
Freshwater pearl mussel (Margaritifera margaritifera).

It’s a ROCK!  It’s a CRAG! It’s a STONE! No, it’s a MUSSEL! If you have ever visited a body of water such as a river, a lake or ocean, chances are that you have been near a mussel. They are burrowers and could resemble rocks due to their hard shells and slow mobility. In actuality, mussels are shell-covered invertebrate animals and share the taxonomic phylum Mollusca with oysters, snails, octopi, and squid, among others. Most Mussels are filter-feeding planktivores, but can be parasitic during their larval stages. While naming scientific mussels ( taxonomy) is an exact science, they are known by many common names within the class Bivalvia as: clam, oyster, bivalve, mollusk, unionid, scallop, shellfish, cockle, abalone, conch, and seashell, and of course mussel. These amazing species are persistent subjects of science and biology. Mussels naturally filter an enormous amount of water daily, and steadily remove pollutants, microparticles, and organic compounds from the water (Baker and Levinton 97). This ecological service is free of charge to human residents living near the water source! Freshwater mussels in the tidal Hudson River in New York filtered nearly 5.3 million gallons of water per day, approximately equal to the daily freshwater discharge of the Hudson River during the summer (Strayer et al 243).Despite the biofiltering value mussels provide, their population is decreasing rapidly.  Williams, et al stated, “During the past 30 years, numbers both of individuals and species diversity of native mussels have declined in the United States and Canada. Freshwater Mussels (as well as other aquatic species) are imperiled disproportionately relative to terrestrial species” (6).

Mussels are found in all types of water including brackish, marine and fresh (Mozley 10-11). The diverse habitats make their adaptations even more remarkable, “They live from the Arctic to the Antarctic, in oceans and in brooks, rivers, ponds and lakes on every continent. They have been discovered far above the tree line on high Himalayan peaks, and in bleak, sandy or rocky deserts. They flourish in tropical jungles and on sandy beaches as well as in shallow lagoons and water as deep as 21,000 feet” (Rehder 11). Each species is uniquely adapted to their endemic conditions. Their specialized anatomy, physiology and morphology allow them to thrive in diverse water environments and are studied by Malacologists.

Mussels do not have a head, arms, legs, tentacles, jaws, or eyes. They do, however, create their own armor in the form of a shell. The shell is bilaterally symmetrical with two valve sets (posterior and anterior) connected by a protein hinge ligament (Morton and Yonge 4). The hinge ligament joins the two sides of the shell together like a spine joins two book covers together. Since the shell is secreted by the mussel, the chemical composition is an organic molecule,   crystalline calcium carbonate, which makes it very strong and rigid (McMahon and Bogan 331). The coloring or nacre inside of the shell is very unique across pearly mussel species. The iridescent coloring gives the nickname ‘pearly mussels’. This outer protection is the mussel’s first defense against predators and changing environmental conditions. It also allows for the body mass and the internal organs to become sealed from the external water column precisely when the shell shuts, so the mussel has the ability to be water-tight. The elliptical shape of the shell allows for easy maneuvering in the soft bottoms or benthos of the water body. Surprisingly, the shell remains intact even when the mussel dies. Empty shells are studied by Conchologists.

Bivavle Shell
Dorsal view of a bivalve shell by Muriel Gottrop, 2005

While the mussel has few muscles, the foot is its most noticeable. This feature, in conjunction with the nervous system, is responsible for mussel movement. The nerve cord receives a signal from a ganglion to push the foot outward (McMahon and Bogan 344). When the foot flexes out of the shell in front of the mussel, it pushes into the sediment, and then contracts, pulling the rest of the body behind it (Wetzel 691). This slow process takes time and an enormous amount of energy. Since mussels can move both vertically and horizontally, the exact distance travelled is difficult to track. Experimental studies have shown that larger adult mussels move less frequently than juveniles and cover smaller distances (Uryu et al 327). Each species has specific abilities that enable them to adapt to the environment.

The next important feature of the mussel is the mantle. This is the piece of flesh closest to the inside of the shell which is also responsible for secreting the calcium carbonate that creates the shell. In some mussels, the mantle can even be exposed outwardly with elaborate colorings and shapes to assist with reproduction as a lure (Voshell 223). The mantle has another feature that can reach beyond the shell. Two tube-like siphons, can be extended out of the shell or retracted inward (219). These siphons represent the site of water exchange between the mussel and the water column. Sometimes, the mussel is so far beneath the ground, only the siphons show. The water can contain dissolved oxygen, algae, phytoplankton, zooplankton, dirt, other dissolved gases and molecules. One siphon sucks water inside, while the other expels other waste the mussel produced to the surrounding sediment and water column (Howard 460). Fortunately, mussel waste is consumed by other invertebrates living in the sediment. Microbes and bacteria can further digest the waste. Mussels can also shift the waste away from them, “Bioturbation of sediments during mussel burrowing activities increases sediment water, nutrient and O2 content, thereby improving invertebrate habitat….If bioturbation also stimulates microbial metabolism in freshwater sediments, then it might increase bacterial food resources for mussels and co-occurring invertebrates” (Vaughn et al 413). Some excrement is strewn into the water column to be broken down later, or in some cases flows away with the current. The ammonia waste (NH3) from the excretory system naturally enters the water. Carbon cycling and therefore waste cycling is an important question for ecologists when constructing trophic levels of ecosystems (409). More experiments tracing waste deposits and ecosystem cycling in mussel beds are still needed to fully understand the complex biogeochemistry.

Mussel Water Flow
Water Flow in a Eulamellibranch bivalve (clam; phylum Mollusca), Heather Kroening/A. Richard Palmer/Bio-DiTRL, © 2000

All water entering the siphon travels to the gills. Gills are the sorting station of the mussel and distinguish between food particles and waste. This process is known as filter-feeding because the mussels are filtering the water entering the siphons and consuming anything edible (Ruppert et al 273). Food leaves the gills to enter the mouth and then is transported to the digestive system. Any excretory waste produced is first filtered by the kidney and then exits the mussel via the siphon (289). The gills also gather oxygen molecules for respiration and transportation to the vessels and aorta. Even though mussels have an open-circulatory system, they do not possess blood with hemoglobin, but an oxygen-loving liquid called hemolymph (McMahon and Bogan 336). Any carbon dioxide produced by cellular respiration will also exit the mussel through the siphon.

No space is left unfilled in the body plan of the mussels. The current ranges of freshwater mussel adult size can vary from 3mm up to and including 30cm (Cummings). The mussel contains a compact body within the confines of its shell which includes the nervous system, open-circulatory system, respiratory system, digestive system, excretory system and reproductive system. All of these systems function together so that mussels flourish in an underwater setting.  

Go To Mussel Differences

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