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Biology of Mussels & Clamps

Introduction:

Mussel is the common name used for members of several families of clams or bivalvia

mollusca, from saltwater and freshwater habitats. These groups have in common a shell

whose outline is elongated and asymmetrical compared with other edible clams, which

are often more or less rounded or oval.

They have two shells connected by a hinge-like ligament. Around the world, mussels live

in a variety of freshwater habitats but are most prevalent in stream and rivers. They

vary in their adult sizes from those as small as a thumbnail to others as big as a pie

plate.

The common name "mussel" is also used for many freshwater bivalves, including the

freshwater pearl mussels. Freshwater mussel species inhabit lakes, ponds, rivers,

creeks, canals, and they are classified in a different subclass of bivalves, despite some

very superficial similarities in appearance.

Fig: Marine blue mussel, Mytilus edulis,

Importance:

By dooing this assignment we can know about:

The internal anatomy of mussels

The external feathers

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The reproductive system

The distribution

The life cycle

The food & feeding behavior

Classification:

Kingdom: Animalia

Phylum: Mollusca

Class: Bivalvia

Subclass: Pteriomorphia (marine mussels), Palaeoheterodonta (freshwater

mussels), Heterodonta (zebra mussels)

Order: Mytiloida

Family: Mytilidae

Subfamily: Mytilinae

Genus: Mytilus

Species: M. edulis (Blue mussel)

Three key characteristics of Mussels:

1. These include a two velvet shell to contain soft body.

2. A muscular foot often seen extended from between the two velvet.

3. Its aids the mussels in locomotion, burrowing & positioning in the river bottom.

General Anatomy:

The mussel's external shell is composed of two hinged halves or "valves".

The valves are joined together on the outside by a ligament, and are closed when

necessary by strong internal muscles.

Mussel shells carry out a variety of functions, including support for soft tissues,

protection from predators and protection against desiccation.

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The shell has three layers. In the pearly mussels there is:

An inner iridescent layer of nacre (mother-of-pearl) composed of calcium

carbonate, which is continuously secreted by the mantle;

The prismatic layer, a middle layer of chalky white crystals of calcium carbonate

in a protein matrix; and the periostracum, an outer pigmented layer resembling a

skin.

The periostracum is composed of a protein called conchin, and its function is to

protect the prismatic layer from abrasion and dissolution by acids (especially

important in freshwater forms where the decay of leaf materials produces acids).

Fig: External Morphology of Mussels

Different organs:

Fluting – repeated ridges and valleys alternately arranged

Hinge – the edge of the shell where the two valves are physically connected

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Lateral teeth – the elongate, interlocking ridges on the hinge line of each valve

Mantle – the fleshy tissue that is attached to the nacre and envelops a mussel’s soft

parts

Nacre – the pearly interior of a mussel shell that may vary in color

Pallial line – the indented groove on the inner shell surface, roughly parallel to the

ventral edge, that marks where the mantle was formerly attachedPeriostracum – the outermost external layer of a shell

Pseudocardinal teeth – the interlocking tooth-like structures located near the umbo

Pustule – a small bump or knob

Rays – a solid or broken stripe on the periostracum that usually radiates from the umbo

Sculpture – raised portions on the shell exterior that form lines, ridges or pustules

Sulcus – a narrow shallow shell depression extending from umbo to ventral margin

Umbo – the area of the shell first to form (sometimes called the beak)

Valve – one of the halves of a shell

Wing – a thin posterior extension of the shell most notable on heelsplitters

Internal Morphology:

This simplified illustration shows the arrangement of the soft tissue body parts

of a freshwater mussel.

The adductor muscles function to close the two halves of the shell.

The mantle surrounds the visceral mass and covers the interior surface of the

shell. The mantle also manufactures the shell itself, which is mostly composed of

calcium carbonate. Water enters the mantle cavity through the incurrent siphon.

Mucus secreted by the gills traps food which moves to the palps, is passed to

the mouth and is digested in the stomach.

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The intestines continue to the anus, where waste is carried out the excurrent

siphon. Water circulation also provides oxygen exchange in the gill tubes and on

the gill’s outer surface.

Mussels have two pairs of gills; one pair rests in each shell valve.

Other organs of the mussel include the hepatopancreas, gonad, kidney and a 2-

chambered heart.

The nervous system consists of ganglia. The foot extends from between the two

shell halves and is used for movement and to anchor the animal in the substrate.

Fig: Internal Morphology of Mussels

Distribution:

Marine mussels are abundant in the low and mid intertidal zone in temperate seas

globally. Other species of marine mussel live in tropical intertidal areas, but not in the

same huge numbers as in temperate zones.

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Certain species of marine mussels prefer salt marshes or quiet bays, while others thrive

in pounding surf, completely covering wave-washed rocks. Some species have colonized

abyssal depths near hydrothermal vents. The South African white mussel exceptionally

doesn't bind itself to rocks but burrows into sandy beaches extending two tubes above

the sand surface for ingestion of food and water and exhausting wastes.

Freshwater mussels inhabit permanent lakes, rivers, canals and streams throughout the

world except in the Polar Regions. They require a constant source of cool, clean water.

They prefer water with a substantial mineral content, using calcium carbonate to build

their shells.

The diversity of freshwater mussels in the United States is unmatched. Of the estimated

1,000 species worldwide are found. In comparison to other countries and continents

like Africa with 96 species, China with 60 species, and Europe with a paltry 12 species,

the wealth of the U.S. becomes truly impressive. The lion's share of this diversity is

found in the southeastern drainages of the Ohio, Tennessee, Cumberland, and Mobile

rivers.

Habitat:

Mussels generally live half-buried in gravel, sand, or mud on the bottoms of

streams and lakes.

Some species prefer to live in large rivers, while others are adapted to small

creeks or to ponds or lakes with standing water.

Mussels that live in moving water often have shells that are heavy or are

sculptured on the outside. Weight and sculpturing help anchor mussels in river

beds and prevent them from being washed away.

Mussels that live in lakes or ponds tend to have thin, unsculptured shells.

Food & Feeding Habit:

Mussels are efficient suspension feeders. They feed by actively filtering particles

from the water, which passes into and out of the mantle cavity through the frilled

siphons. Breathing also occurs as this stream of water passes over the creature's

gill.

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Feeding both marine and freshwater mussels are filter feeders; they feed on

plankton and other microscopic sea creatures which are free-floating in

seawater.

Phytoplankton cells both living and dead constitutes the main source of food, but

other sources of carbon such as decomposed macrophytes or resuspended

detritus may also supplement their diet. As stocking density increases, the

demand for food eventually exceeds the supply, ultimately resulting in food

limitation, which in turn could reduce growth.

A mussel draws water in through its incurrent siphon.

The water is then brought into the brachial chamber by the actions of the cilia

located on the gills for ciliary-mucus feeding.

Lifecycle of Freshwater mussel:

Freshwater mussels belonging to the order Unionoida (also called naiads) are large

bivalves which live in the sediment of lakes and rivers around the world. The lifecycle of

freshwater mussels is unique among bivalves: larvae (called glochidia) are released into

the water where they attach to the gills and fins of fish. The glochidia remain attached to

the fish for between one week and several months, developing into juvenile mussels

before detaching from the fish host and dropping off into the sediment to grow. In this

way the mussels can be transported upstream by the host fish

Fig: Life cycle of M. eduli

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Spawning:

When sperm and eggs are fully ripe they are released from the follicles in each gonad

into a series of genital canals that gradually combine into a common gonoduct that

opens on the genital papill . Eggs and are liberated via the exhalent siphon directly into

the water column, where fertilization occurs. About 10,000 spermatozoa are shed for

each ovum spawned

Gametes and Fecundity :

The eggs are spherical and 68-70 pm in diameter; each possesses a vitelline coat 0.5- 1.0

pm thick, and contains numerous lipid droplets and yolk granules. Accurate

measurements of fecundity are difficult to obtain due to the experimental difficulties of

assessing an individual female's output over the entire breeding period. Estimates

obtained by inducing females to spawn in the laboratory suggest that a large adult

mussel (4-5 cm long) weighing 1 (tissue dry weight) may liberate about 8 x 10 6 eggs .

Spermatozoa swimming freely in the water column react on contact with the egg by

extruding an acrosome filament (Bayne 1976b) that enables the sperm to penetrate the

vitelline coat and initiate meiosis. At 18 OC the eggs hatch about 5 h after fertilization to

produce a ciliated embryo.

Reoproductive Strategy:

The large number and small size of eggs produced by the blue mussel are typical of the

planktotrophic reproductive strategy in which output is maximized but nutrient

investment per egg is small (for review see Bayne 1976b). This strategy is generally

considered to aid dispersal as it allows mussels to produce the maximum number of

larvae which require a prolonged pelagic development period. However, such a strategy

produces an egg with the barest minimum of nutrient reserves to enable the larva to

develop sufficiently to start feeding on plankton.

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Larval Development and behavior:

The stages of larval development and their durations are summarized in Table 1. It must

be emphasized that the larval stage may anywhere from 15 to 35 days and that the

duration is dependent on prevailing environmental conditions. Larvae of blue mussels

from Connecticut developed normally only within the temperature range of 15-20 OC;

at 15 OC they developed normally in salinities ranging from 15 to 35 ppt, whereas at 20

OC they could only developnormallybetween 20 and 35 ppt (Hrs-Brenko and Calabrese

1969). Bayne (1965) reported that the trochophore could develop successfully only in a

salinity of 30-40 ppt and at temperatures of 8-18 OC, and that within this temperature

range, the development rates were fastest at the highest temperatures. Larvae from an

oceanic blue mussel population grew fastest at 30-32 ppt and did not grow well at

salinities below 24 ppt .

Reproductive behavior:

The life cycle of the freshwater mussel is

one the most complex and interesting in the

animal world. Unlike other animals that can

actively search for a mate, the sedentary

mussel depends on the river current to

reproduce. The process begins with the

male releasing sperm, and the female

located downstream drawing it in through

her incurrent siphon. Numbering in the

100's to hundreds of thousands, the

fertilized eggs develop into glochidia within

her gills.

Once mature, they are released into the water column to begin the second part of their

lives-attaching to the gills, fins, or scales of freshwater fishes. At this point, the process

is further complicated because not only do the glochidia have to find a fish, but it has to

be one of a few specific fish species for the life cycle to continue. If a glochidium attaches

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to the correct fish species, it encysts into the fish's tissue and undergoes a short life as a

parasite.

Over several weeks, it begins to develop gills, a foot, and other internal structures to

become a juvenile mussel. The now fully transformed, but still microscopic, juvenile will

drop off the fish and begin its life on the stream bottom. Unbeknownst to the fish, it has

just served as a taxi transporting the young mussel into new habitat away from its'

parent. If the mussel is lucky enough to grow into an adult, it may live 20-100 years or

more depending on the species.

Fig: Lampsilis' display attracts host fish - Paul L. Freeman, The Nature Conservancy

Lampsilis ornata (Pocketbook) from Coosa River, Alabama

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Fig:

Glochidia of Lampsilis encapsulated on the gills of largemouth bass. Glochidia are

parasites but they usually do no harm to the host. The glochidium clamps on, and the

epithelial cells of the gill then migrate to form a capsule around it. This structure is

often erroneously called a cyst, but technically cysts are structures made by a parasite,

while capsules are made by a host.

Mussels and nutrient mitigation:

Marine nutrient bioextraction is the practice of farming and harvesting marine

organisms such as shellfish and seaweed for the purpose of reducing nutrient pollution.

Mussels and other bivalve shellfish consume phytoplankton containing nutrients such

as nitrogen (N) and phosphorus (P). On average, one live mussel is 1.0% N and 0.1% P.

When the mussels are harvested and removed, these nutrients are also removed from

the system and recycled in the form of seafood or mussel biomass, which can be used as

an organic fertilizer or animal feed-additive. These ecosystem services provided by

mussels are of particular interest to those hoping to mitigate excess anthropogenic

marine nutrients, particularly in eutrophic marine systems. While mussel aquaculture is

actually promoted in some countries such as Sweden as a water management strategy

to address coastal eutrophication, mussel farming as a nutrient mitigation tool is still in

its infancy in most parts of the world. Ongoing efforts in the Baltic Sea (Denmark,

Sweden, Germany, Poland) and Long Island Sound and Puget Sound in the U.S. are

currently examining nutrient uptake, cost-effectiveness, and potential environmental

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impacts of mussel farming as a means to mitigate excess nutrients and compliment

traditional wastewater treatment programs.

Factors affecting Mussel biology:

Temperature:

The adults are incapable of surviving in the waters, where summer water temperatures

exceed 27 0C (Wells and Gray 1960). Lethal temperature response of the blue mussels

depends on the animal's previous thermal history. In mussels acclimated to an

intermediate temperature (20 OC) and transferred to water at 27.5 OC, the "time to 50%

mortality" was 350 h, which was about 8 times as long as that of mussels acclimated to

10 OC.

Salinity:

The blue mussel is a euryhaline species that occurs in environments ranging from full

oceanic salinities (34 ppt) to mesohaline (5- 18 ppt) estuarine conditions (Bayne et al.

1976a). If the tidal variation in salinity is small (< 10 ppt), the blue mussel remains

active throughout the salinity fluctuation (Widdows 1985). In response to a rapid drop

in ambient salinity, such as may occur with the ebb tide in estuaries with a large input of

fresh water, the blue mussel first closes its exhalent siphon to stop the ventilation of the

mantle cavity, and then closes its shell if the salinity change is large enough (reviewed

by Davenport 1982). It can thus effectively isolate its tissue from up to half the change

in ambient osmotic concentration (Shumway 1977). Shell closure cannot be maintained

longer than about 96 h (Gilles 1972) because the animal must rely on stored nutrient

reserves and anaerobic metabolism to sustain energy demands.

Substrates and Current:

The mussel is an epibenthic species that as an adult lives in areas with substrates

ranging from rock to coarse gravel; even areas with mud and sand substrates can be

colonized provided that there is a firm surface, such as a stone or another mussel shell,

to which the byssus threads can be attached. The blue mussel requires sufficient water

flow to ensure larval dispersal and carry suspended food particles. It tends to attain a

larger size in sheltered environments than in more exposed open coast conditions (Seed

1976). This differential size is probably due to differences in food availability between

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habitats and the stunting effect of the extensive disturbance of feeding and exposure to

air in open coastal populations (Seed 1976). In addition, predation may be so severe in

the exposed sea coast populations that few mussels survive into their third year (Seed

1969).

Oxygen:

Oxygen diffuses into the blue mussel across the gill, where there is close contact

between the water and the hemolymph pumped through the gill filaments by the heart.

There are no oxygen-carrying pigments in the blood (Newel1 1979). Theede et al.

(1969) reported that under laboratory conditions the blue mussel survived 35 days in

water at 10 OC containing only 0.15 ml 0 2 . I-', but survival was reduced to 25 days

when hydrogen sulfide was present. In response to a gradual decline in oxygen content,

the blue mussel can increase its extraction efficiency for oxygen.

PH:

The upper pH tolerance limit of the zebra mussel (Dreissena polymorpha) has not been

established experimentally. This study was designed to test the effect of elevated pH on

the health and survivorship of zebra mussels. All zebra mussels acceptable ph level is

9.3 to 9.6 . The zebra mussels in the high-NaOH treatment moved and formed byssus

attachments less often and had lower mean dry body mass than zebra mussels in other

treatments. Dishes that contained zebra mussels had more algal genera than control

dishes. In this experiment, the upper pH tolerance limit of zebra mussels was between

9.3 and 9.6 and may have been dependent on the rate of pH change.

Economic value:

Freshwater mussels have a high ecological value. They are an important food source for

many other animals including muskrats, minks, otters, fishes, and some birds. More

importantly, they serve as indicators in that they are sensitive to pollutants such as

heavy metals, pesticides, agricultural nutrients, heavy loads of fine silts. These

pollutants will kill mussels and thus provides a warning of the particular waters

infested with these pollutants. It is inevitable that mollusks are going to be exposed to

water born chemicals. Once exposed uptake is very rapid and very easy. Examples of

these compounds include benzene hexachloride, chlordane, DDT, dieldrin, and

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toxaphene. There are other unnoticeable pollutants that are found in commercial

plastics such as Phthalate esters. Since mollusks are not very good at biodegrading these

pollutants pose a threat to their lives.

Uses of mussel as food:

Humans have used mussels as food for thousands of years and continue to do so. About

17 species are edible, of which the most commonly eaten are Mytilus edulis, M.

galloprovincialis, M. trossellus and Perna canaliculus.

Freshwater mussels nowadays are generally considered to be unpalatable, though the

native peoples in North America ate them extensively. During the second World War in

the United States, mussels were commonly served in diners. This was due to the

unavailability of red meat related to wartime rationing.

In Belgium, the Netherlands, and France, mussels are consumed with french fries

("mosselen met friet" or "moules-frites") or bread. In Belgium, mussels are sometimes

served with fresh herbs and flavorful vegetables in a stock of butter and white wine.

Frites/Frieten and Belgian beer sometimes are accompaniments. In the Netherlands,

mussels are sometimes served fried in batter or breadcrumbs, particularly at take-out

food outlets or informal settings. In France, the Éclade des Moules is a mussel bake that

can be found along the beaches of the Bay of Biscay.

Recommendation:

Due to chemical pollutants and zebra mussels, many of the unionids are declining in

population or becoming endangered. Conservation efforts have begun by two main

methods. Ecologists are focusing on mussel relocation to separate the native mussels

from the zebra mussels, or polluted body of water. While those in the gaming industry

are focused on killing the zebra mussels by a variety of different methods including

electrical. Nonetheless many of the native mussels are endangered and must be

protected, by mainly not polluting our freshwaters. In terms of species domination by

the zebra mussel, perhaps that plays in the role of natural selection, and unless there

are severe consequences, should be left alone.

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Conclusion:

Freshwater mussels are an essential component of our rivers and streams. By their

siphoning actions, mussels filter bacteria, algae, and other small particles, which make

them one of the few animals that improve water quality. Mussels also serve as a food

source to many species of fish, reptiles, birds, and mammals. So as a student of fisheries

we have to give more emphasis on it.

References:

Links:

http://en.wikipedia.org/wiki/Mussel

http://www.gpnc.org/shells.htm

http://www.museum.state.il.us/intro_habitat.html

http://molluskconservation.org/MUSSELS/Reproduction.html

http://www.museum.zoo.cam.ac.uk/bivalve.molluscs/bivalve.research/freshwater.mussels

http://www.dgif.virginia.gov/wildlife/freshwater-mussels.asp\

http://www.bio.umass.edu/biology/conn.river/reproduction.html

http://www.inhs.uiuc.edu/cwe/wwwtest/mussel/Pages/introduction.html