Clam

Page 8 Clam

1. Classification/Diagnostic characteristics

Clams are bivalves, a type of mollusk along with chitons, gastropods and cephalopods, all of which are types of lophotrochozoans. The various types of mollusks all differ in their morphology, but share major features: a foot, a visceral mass, and a mantle.
 * The foot is a muscular structure that serves as a form of locomotion and a support for internal organs. In cephalopods the foot forms arms and tentacles along with various sensory organs on the head. For bivalves, the foot is instead used as a burrowing organ. For some mollusks the foot is used much less.
 * Most of the internal organs, including the heart and the digestive, excretory and reproductive systems are concentrated in a centralized internal visceral mass.
 * The mantle is a fold of tissue that is used to protect the visceral mass. The mantle secretes the shell that many mollusks have. This feature of the clam is a soft, retractable organ attached to both sections of the inside of a clam's shell and is directly responsible for creating and growing the clam's shell. The mantle processes the calcium deposits that it has stored over weeks or months. The clam opens its shell and extends its mantle to the edges of both sides. The mantle releases the glue-like calcium compounds along the edges of the shell.

Bivalves are a type of mollusk that are characterized by a small head and a hinged two-part shell that covers the entire body.

2. Relationship to humans Clams are often used to test for water quality, as they are very sensitive to changes in water composition of their surroundings. Humans also eat clams, in very large quantities.

Clams were also used for money and crafted into cylinders and beaded in the seventeenth century.

3. Habitat and niche Clams typically inhabit aquatic environments, such as rivers, lakes, and oceans. In those environments, bivalves are usually first or second order consumers, consuming plankton and other small organisms.

Clams burrow in the ocean floor near the coast at depth around 10 cm. Clams are filter feeders,and eat plankton and toxic algae. They act as natural purifiers for coastal water by maintaining the populations of plankton and toxic algae to a safe level. Clams are prey to fish, shore birds, and starfish. [] .

Predator avoidance Clams rely on their shells, made of extremely hard calcite, and burrowing in mud or dirt to protect them from predators. Additionally, the dark outer perostracum of the clam's shell blends in well with the bottom of the body of water, effectively camouflaging the clam with its surroundings.

5. Nutrient acquisition Clams have two siphons, round opening in the clam’s mantle cavity. Water goes in from one siphon and exit the other. When the water is inside the clam it passes through the gills. Clams feed by using their gills to filter food from the water. The food is then moved to the digestive system through the mouth, then is digested by the stomach and absorbed into the body in the intestines.

6. Reproduction and life cycle Some bivalves are hermaphroditic- approximately 4%, although clams reproduce between differentiating sexes.Their reproductive systems are in the upper part of their foot region which in close proximity to their digestive systems, and some areas are interwoven. Clams expel gametes through their gills, and fertilization occurs when the gametes mix with gametes from another nearby clam.

7. Growth and development After fertilization, embryonic clams undergo several stages of development to become adult. First, the embryo, a single cell, quickly divides many times to form a blastula, a hollow ball of cells. Then, the now larval clam develops into a trochophore. During this stage, the larva develops cilia that help it swim, rather than sink to the bottom as it does as an adult. Then, the larval clam becomes a veliger, in which it develops a large organ that extends beyond the shell, called the velum. The velum's purpose is to obtain food and assist in movement. In the final part of the veliger stage, called pediveliger stage, the larva develops its foot and begins to sink to the bottom. These stages of development take only 18-48 hours to complete before the clams become juvenile. The amount of time it takes for juvenile clams to become adults vary depending on species, but it is often a matter of years. Clams also need to grow their shells with them, and so a tissue adheres to the mantle, or inner surface of the old shell, and muscular tissue at the mantles edge then deposits new shell material. Like trees, the rings of different shell layers can be used to determine the age of a clam.

8. Integument Bivalves get their names from their two-shell hinged integument, which form a relatively hard to penetrate armor. The two valves are composed mainly of calcite; and the shell is attached to the rest of the organism by a series of small mantle retractor muscles attached to the shell along the interior. Additionally, the outside of bivalve shells, called the periostracum, is made primarily of an organic protein called conchiolin. One of the main purposes of the periostracum is to provide a surface on which additional calcite can be secreted to increase the size of the shell. The shell is secreted by the mantle of the bivalve and is used both as an attachment point for muscles as well as a means of protection from potential predators.

9. Movement Most clams use a muscular projection between the hinge of the two shells, called a "foot" to burrow through the seabed. This wedge-shaped foot expands and contracts on different surfaces, allowing the clam to burrow through different aquatic mediums, like mud and sand. Some clams, usually in their early stages, contain a byssal gland which produces silky fibers called byssus threads. These threads, located in the growing foot, help anchor the clams to rocks and sand particles.

10. Sensing the environment Most clams have fairly primitive sensory organs. All clams have a chemoreceptive sense organ that (using chemical stimuli) detects water currents entering the clam. With the exception of scallops, clams either do not have eyes or have simplistic photoreceptors to distinguish between being in light and in shadow. Processing little information does not require an intricate nervous system: the nervous system of most clams simply consists of two clumps of nervous cells called ganglia.

11. Gas Exchange The siphon protrudes above the body of a clam to take in water from above the muddy seabed where it lives. Clams use their gills, located in the mantle cavity to exchange oxygen and carbon dioxide with the water around them. The gills have a large surface area and a small epithelial lining, the greater surface area: volume ratio allows clams to readily absorb nutrients and expel waste. Cilia move water over the gills, the oxygen that is taken in will then pass through its circulatory system (1).

12. Waste Removal Waste removal is simple for clams, just expelling the waste material out the anal opening. Clams contain a mouth, a stomach, and an anus. Food enters the stomach and is then processed into the blood. Any undigested materials are packaged and released into the water. By utilizing this process, clams can ensure that the water passing through the gill is not contaminated.

13. Environmental physiology The clam uses a tubular structure, siphons, to send water from its surrounding environment over the gills to take in oxygen and plankton. If oxygen concentrations in the water are less than or equal to that of the inside of the clam, the organism will resort to anaerobic respiration and utilizes its energy without oxygen (1).

14. Internal Circulation Mollusks do not have a closed circulatory system. Blood and other bodily fluids all are mixed in a large body cavity called a hemocoel, which gives the combined fluid access to all of the internal organs, The fluids are pumped through this hemocoel by a heart.

15. Chemical control (Endocrine System)

Review Questions:

What is a clam shell made up of? What is the purpose of gills and how do they function? What are bivalves and what function do they serve?

Sources: [] (1) http://seagrant.gso.uri.edu/G_Bay/HabitatEco/Shellfishing/quahog_dev.html http://www.asnailsodyssey.com/LEARNABOUT/CLAM/clamRepr.php [] [|http://marinelife.about.com/od/glossary/g/byssalthread.] http://www.nefsc.noaa.gov/faq/fishfaq5.html [|http://www.britannica.com/EBchecked/topic/388398/mollusk/] [] [] http://animal.discovery.com/marine-life/clam-info.htm http://www.encyclopediavirginia.org/Fishing_and_Shellfishing_by_Early_Virginia_Indians http://barnegatshellfish.org/mercenaria01.htm