external image Clams.jpg
By Manal Cheema (MC)


TABLE OF CONTENTS


1. Classification/ Diagnostic Characteristics
2. Relationship to Humans
3. Habitat and Niche
4. Predator Avoidance
5. Nutrient Acquisition
6. Reproduction and Life Cycle
7. Growth and Development
8. Integument
9. Movement
10. Sensing the Environment
11. Gas Exchange
12. Waste Removal
13. Environmental Physiology
14. Internal Circulation
15. Chemical Control
16. References



CLASSIFICATION/ DIAGNOSTIC CHARACTERISTICS


Classification for the Hard-Shelled Clam
Kingdom: Animalia
Phylum: Mollusca
Class: Bivalvia
Order: Veneroida
Family: Veneridae
Genus: Mercenaria
Species: M. mercenaria

clams_sm.jpg(18) (JF)

Mollusks, the most diverse group of lophotrochozoans, have about 100,000 species that inhibit an array of terrains and aquatic environments. They are divided up into four clades: chitons, gastropods, bivalves, and cehpalopods. Clams belong to the bivalve group, which gets its name from their two hinged shells, which can be tightly closed.
Among clams, oysters scallops and mussels are all familiar bivalves. There are 30,000 species. Bivalves have a very small head and a hinged, two-part shell that extends over the sides of the body as well as the top.
Clams have a foot, visceral mass, and a mantle.
  • The molluscan foot is a muscular structure that was an organ of locomotion and support for internal organs.
  • The heart and the digestive, excretory, and reproductive organs are concentrated in a centralized, internal visceral mass.
  • The mantle is a fold of tissue that covers the organs of the visceral mass.
  • Shell is made of calcium carbonate embedded in organic matrix secreted by mantle (7)(KG).
Clams are bivalves that have two shells closed by two adductor muscles. They can also be identified by their muscular burrowing "foot." They can range in size from .1 millimeters to 1.2 meters (giant clam). They are often found buried in sand in shallow depths of water, which protects them from surrounding currents. (4) (SJ)



RELATIONSHIP TO HUMANS

Clams are often eaten by humans, and the shellfish industry is huge in clam-populated areas like the Northeast.

Commonly eaten clams include Pacific razor clams, littleneck clams, and cockles. While usually harvested commercially in bulk, these edible clams can also be legally harvested recreationally at public beaches as long as they are at least 1.5 inches wide. Clams that have burrowed under the sand and mud can be harvested using shovels, rakes, digging forks, or even bare hands. (3) (JLev)

Recently, the growth increments in clam shells have been studied by researchers to learn more about the history of oceans. Because clams can live up to 240 years and add growth lines to their shells year after year, the shell increments are much like sensors at the bottom of the ocean. Thus, researchers have started using old clams to help record information about the ocean including past conditions, temperatures, and circulation patterns. (5) (JF)

Native Americans used clams for hundreds of years in various ways. The hard clam shells were often carved and turned into jewelry and beads. Also, the shells were also used to make fish hooks to catch larger fish in shallow water. These clams became so valuable to them that by the seventeenth century the shells were used as money. (20) (WSS)

Numerous edible marine bivalve species live buried in sand or mud, and respire by using siphons. In the United States, these clams are collected by "digging for clams".(21)(Shwetha)

external image 205555_10150157946349881_7710080_n.jpg
Clams being eaten (CC)

HABITAT AND NICHE

Clams are found in both marine and freshwater environments.
Most clams burrow into the sand, mud, and other substances on the sea floor. They can be found in creeks, rivers, and along the beaches in the oceans and dig a home for themselves on the bottom surface. Their burrowing patterns vary with the rise and fall of the tides in the oceans, and they can be found in both salt water and freshwater aquatic habitats all over the world. (6)(BBV)

PREDATOR AVOIDANCE


The foot is a burrowing organ for clams, where the clam buries itself in mud and sand. The clam's hard shell helps protect it from the outside.
Clam predators include gulls, otters, and fish, as well as filter-feeding fish and invertebrates when they are still larvae. Not only do clams protect themselves with their burrowing and their hard shell, some clams can sequester toxins from the phytoplankton that they eat. (22) (DM)

A pearl is formed by a clam as a defense mechanism. When irritated by sand getting inside its shell, the clam forms a pearl around the sand as a defense mechanism.(TM)


NUTRIENT ACQUISITION


Bivalves feed by taking in water through an opening called an incurrent siphon and filtering food from the water with their large gills, which are also the main sites for gas exchange.

The cilia of the clam's gills catch particles of food that are drifting in the incurrent water and bring them to the clam's mouth. Clams' main source of food is plankton, so they don't have to work hard for their food. It is easy to acquire plankton because they are tiny water organisms that live right in the water, so when there is an incoming current of water there is usually plankton for the clam as well. (4) (PS).
Once the plankton has entered the clam, it is transported down the neck into the stomach lined by a chitinous gastric shield, this rotates and releases enzymes to break down the plankton which are ground up by the rotating stomach. (LC) 14

REPRODUCTION AND LIFE CYCLE


In clams, water and gametes exit through the excurrent siphon. Fertilization takes place in open water in most species of mollusks.
In the spring when the water temperature rises above 50°F, clams come out of "hibernation" and begin to feed on available phytoplankton. In late spring into early summer when the water temperature rises above 68°F the adults begin to spawn. Some clams begin their adult lives as males. During successive years they may change their sex and produce eggs.(HSC)
Once the eggs are fertilized, they develop into microscopic larvae. Water currents carry these larvae around for a period of 3-4 weeks (depending on water temperature) before settling on and attaching to gravel, shell, or sand grains and burrowing to the bottom. Clam larvae can end up miles away from their original parents. (2) (JLev)

aquatic clams have a type of larva called trochophore larva which then forms into veliger larva which then can grow into an adult clam. The trochophores are small, translucent, and free-swimming. A ring of cilia around its middle is what allows the trochophore to swim. Below this ring of cilia is a tiny stomach, anus, and mouth and above the ring is a sensory plate, a second clump of cilia, and a simple eye called an ocellus. In the next stage, the veliger stage, the ring of cilia expands into larger ciliated lobes called velum, which help with swimming, feeding, and gas exchange. Also the first development of a foot and a shell begins in this stage. Once the clam begins metamorphasis into an adult it will lose its velum lobes. (15-17) (BH)

Giant%20clam%20life%20cycle.jpeg (ES)


GROWTH AND DEVELOPMENT

Clams have bivalves and paired gonads in the upper part of the foot region. Paired gonoducts release gametes into water flow before it enters the exhalent siphon. Egg fertilization leads to the development of larva that swims and feeds in plankton for several weeks. This becomes a pediveliger, which is the settling stage of the clam. The development of the clam is temperature-dependent.8 (JLau)
The clams shell is always growing. The growth of the shell begins at the umbo (see integument) and moves progressively moves outwards twords the shell margin (the edge of the shell where the clam opens) like a rings on a tree. It is believed that one ring equals one year so you could possibly tell the age of the clam by counting the rings but new studies doubt the validity of this. It is more likely that the ring represents a longer period of time or a stressful event. ([1] ) (DA)

INTEGUMENT


The mantle secretes the hard, calcareous shell which is typical of most mollusks.
Clams have two-part shells (bivalves), which are joined together at the top. They are held closed by adductor muscles. If these muscles become relaxed, the shell opens because the two halves are pulled apart by ligaments on either side of the umbo (a central part at the top center of the clam's anatomy) ([2] ) (E.S.S.).

MOVEMENT


The molluscan foot is a muscular structure that was an organ of locomotion and support for internal organs.

While adult clams may not need to move for extended amounts of time, they are capable of doing so. By extending the muscular foot into the ground and enlarging the end of the foot by pumping blood into it, the clam is capable of anchoring themselves in the sand. Then, by contracting its foot, the clam pulls itself closer to the anchored position, thus moving themselves in the direction they desire. Through this anchor and pull method, clams can either move laterally or burrow themselves in the sand.[3] (FZ)

The foot uses two muscles, the anterior and posterior foot muscles. Clams are unique because of their retracting muscles that allow for greater control of the foot. Other than using their foot, clams can move through water currents, allowing them to travel from one region to another without much control. [4] (SM)


SENSING THE ENVIRONMENT


Clams do not have a brain. They have a nervous system, the organism's perception and control center, that consists of ganglion cells (a system of neurons, transmitter cells). Tiny tentacles located on the clam contain chemoreceptor cells (cells that receive stimulus from the environment) that are sensitive to the taste and movement of water. The ganglion cells receive this information from the chemoreceptor cells and perceive the motion that has just occurred.(7) (AY)


GAS EXCHANGE


In many mollusks, the mantle extends beyond the visercal mass and forms a mantle cavity. Within this cavity are the gills that are used for gas exchange.
The cilia on the gills beat, creating a current of water, which the highly vascularized gill tissue take oxygen from and release carbon dioxide. Many mollusks use their gills as filter-feeding devices; others feed using a rasping structure known as the radula to scrape algae from rocks.

WASTE REMOVAL


From the heart, waste eventually passes into the kidneys where wastes are removed, producing urine (7) (KG).Wastes tend to exit the clam from the excurrent siphon in the form of ammonia. (9) (Shwetha)
The excretion of mollusca consists of an excretory system with an anus. The excretion first begins with a single pair of renal glands which allow a wide tube wall for wastes to expel. The nephridium then will dispose of wastes going through the anus. The excretion will have a strong concentration of ammonium chloride in its urine, where is mostly influenced by the environment they live in. As a result of the presence of urine, the primary process of blood filtration is attained, where salts are reabsorbed in the glandular. Urine flow for freshwater mollusks is 45% of its body weight. (AWC)[5]

ENVIRONMENTAL PHYSIOLOGY (TEMPERATURE, WATER, AND SALT REGULATION)


Clams take in and expel water for respiration and feeing through two tubes, the siphons, or the "neck". A siphon is a round opening in the clam's mantle cavity. The water is brought in by the movement of millions of cilia, hairlike structures, on the gills. Water then flows out of the clam through the other siphon (4) (PS)


INTERNAL CIRCULATION

Molluscan blood vessels do not create a closed circulatory system, therefore it is an open circulatory system. Who knew? Blood and other fluids empty into a large, fluid-filled hemocoel, through which fluids move around the animal and deliver oxygen to the internal organs. Eventually the fluids reenter the blood vessels and are moved by a heart. They retain vestiges of an enclosed coelem around their major organs.
The clam does have a heart and a few major blood vessels. Once blood is pumped out of the it goes through large arteries where is enters into large spaces and passes over the cells of various organs. The blood collects in a large internal space called the lacunae from which it passes through the kidney and the gills, returning to the heart to complete the cycle. (10) (AA)


CHEMICAL CONTROL (I.E ENDOCRINE SYSTEM)


The endocrine system is not well documented in clams. They do have some hormones related to determining sex such as testosterone and
oestradiol. Clams that were placed in a polluted marina (specifically with the chemical tributyltin) were found to me more likely male than a clam in its natural habitat. This suggests endocrine disruption occured. (19)(BS)


Review Questions
1.Through what body part(s) do clams get rid of water?(NC)
2. Differentiate between the excurrent siphon and the incurrent siphon. What is the function of each? (JF)

REFERENCES


  1. Principles of Life Textbook, Hillis (this is for every piece of information that isn't cited with a footnote)
  2. http://wdfw.wa.gov/fishing/shellfish/clams/
  3. http://wdfw.wa.gov/fishing/fish-local/how_to_fish_shellfish.html
  4. http://www.britannica.com/EBchecked/topic/119631/clam
  5. http://www.futurity.org/earth-environment/old-clam-shells-act-like-ocean-sensors/
  6. http://www.blurtit.com/q919153.html
  7. http://www.britannica.com/EBchecked/topic/67293/bivalve/35745/The-shell
  8. "CLAM: REPRODUCTION & DEVELOPMENT." CLAM: REPRODUCTION & DEVELOPMENT. N.p., n.d. Web. 11 Dec. 2012. <http://www.asnailsodyssey.com/LEARNABOUT/CLAM/clamRepr.php>.
  9. http://www.crestwood.sparcc.org/userfiles/867/Classes/9262/7%20-%20Mollusks.pdf

  10. Peck, Lisa. "Clams and Oysters: Circulatory." Clams and Oysters: Circulatory. Shorecrest.org, n.d. Web. 11 Dec. 2012.
  11. "Bivalve Anatomy." Bivalves. N.p., n.d. Web. 11 Dec. 2012.
13. http://www.ehow.com/how-does_4744976_a-clam-move.html
14. http://www.britannica.com/EBchecked/topic/67293/bivalve/35745/The-shell
15. http://universe-review.ca/R10-33-anatomy.htm
16. http://www.britannica.com/EBchecked/topic/606207/trochophore
17. http://www.britannica.com/EBchecked/topic/624839/veliger
18. http://www.pac.dfo-mpo.gc.ca/fm-gp/commercial/shellfish-mollusques/clam-palourde/bio-eng.htm
19. http://www.ncbi.nlm.nih.gov/pubmed/15093007
20. http://www.encyclopediavirginia.org/Fishing_and_Shellfishing_by_Early_Virginia_Indians
21. http://en.wikipedia.org/wiki/Clam
22. http://www.asnailsodyssey.com/LEARNABOUT/CLAM/clamPred.php


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  1. ^ http://www.conchologistsofamerica.org/articles/y1989/8903_jones.asp
  2. ^ http://www.assateague.com/nt-bival.html
  3. ^ http://molluscs.at/bivalvia/main.html
  4. ^ http://www.ehow.com/how-does_4744976_a-clam-move.html
  5. ^ http://www.forces-of-nature.net/topics/molluscum/Excretion_Of_Mollusca.htm