Mollusca


Table of Contents

1. Diagnostic Characteristics
2. Acquiring and Digesting Food
3. Sensing the Environment
4. Locomotion
5. Respiration
6. Circulation
7. Osmotic Balance
8. Metabolic Waste removal
9. Self Protection
10. Temperature Balance

The Roman Snail
The Roman Snail


chiton

A picture of Chiton, a common mollusk. (EK)




Diagnostic Characteristics

The phylum Mollusk (derived from the Latin mulluscus), meaning soft) contains more than 150,000 known species of invertebrates and can be divided into eight classes; the four most prevalent being Polyplacophora (chitons), Gastropoda (limpets, snails, whelks), Bivalvia (clams, oysters, scallops), and Cephalopoda (octopuses, squid). Each of the classes has a characteristic shell shape and body form. (17 DC).




external image pic%20of%20mollusk%20evolution.gif (4 Major Classes of Mollusks) (20 VK)





Although mollusks generally appear to be different, most mollusks exhibit bilateral symmetry and have similar body plans. This includes a muscular foot for movement, visceral mass, which houses most of the internal organs, mantle, a tissue that folds over the visceral mass and secretes a shell (usually constructed of calcium carbonate- CaCO3), and at least two pairs of main nerve chords (EK8). All mollusks have a "soft" body, but some forms have lost their hard protective shell through evolution (such as slugs), and some have a reduced/internalized shell (such as squid) (CS 17). The space in between the visceral mass and the mantle is called the mantle cavity, which contains the gills, anus, and excretory pores. In general, Mollusks are soft-bodied animals protected by a shell.

Cephalopods such as squid have an internal mantle, while most octopi lack a mantle altogether.




Acquiring and Digesting Food

In general, mollusks have a strap-like rasping organ called a radula that scrapes up food, which then passes directly into the esophagus, the beginning of the alimentary canal (tube extending between a mouth and an anus).Most molluscs have muscular mouths with radulae, "tongues" bearing many rows of chitinous teeth, which are replaced from the rear as they wear out. This is primarily designed to scrape bacteria and algae off rocks(7DO). This food then travels to crops for storage and to the stomach where extracellular digestion takes place. The stomach is lined with ducts that lead to digestive glands called cecae, which release digestive enzymes and break down food. After being processed in the stomach, the food enters the intestine, where nutrients are absorbed into the blood (hemolymph). Once nutrient uptake has been completed, the food then passes through the anus and exits the organism.

the basic anatomy of a mollusk in relation to acquiring and digesting food (9 J Stein).
the basic anatomy of a mollusk in relation to acquiring and digesting food (9 J Stein).


Most of mollusks in the class Bivalvia (clams, oysters) are suspension feeders, the absorption of small food particles from the water. Most bivalves have gills that are coated in mucus that trap these food particles, which are then conveyed to the mouth by cilia (sweeping finger-like structures).

Mollusks in the class Polyplacophora (chitons) use their radulas to cut and ingest algae. Similarly, mollusks in the class Gastropoda use their radula to graze on plants; however, some gastropods act as predators capable of consuming other mollusks or fish, and their radulas can form poison darts.

Most Cephalopods, such as the squid or octopus, have a mainly carnivorous diet. Using their long tentacles and beaklike jaws, cephalopods are adept at capturing and consuming prey. Some cephalopods are also capable of injecting a toxin to immobilize their prey.
Cephalopod’s beaks are highly specialized and effective. The two parts are not actually connected by a joint, but held together by the buccal mass. The buccal mass is a system of muscles, connective tissue, ganglia, and the beak that can be extended or retracted and swiveled to varying degrees (DPOD 19).

The radula produces a extremely fast vibration, which can stun small fish and is painful to touch. Some species of whelk, including the knobbed and channeled whelk, protrude their radulae to catch prey. When fish come over and bite the often peach colored wormy looking radula they are momentarily stunned, and then consumad by the whelk (JE).




Range of eye complexity among mollusks: The top picture illustrates the rather simplistic eye found in limpets, while the bottom picture shows the almost human eye of the octopus.
Range of eye complexity among mollusks: The top picture illustrates the rather simplistic eye found in limpets, while the bottom picture shows the almost human eye of the octopus.
Sensing the Environment

As shown in the diagram to the right, the complexity of eye structure and function varies greatly between the types of mollusks: from clusters of photoreceptors in limpets, to camera-like eyes of cephalopods. This difference can largely be accredited to differences between lifestyles and evolutionary needs of the different organisms. Limpets are perfectly capable of being successful with a simple eye structure due to their less intensive lifestyle; however, cephalopods, such as a squid or octopus, require complex eye structures due to their predacious lifestyles and need to capture fast-moving prey.
The cephalopods have well-developed brains, composed of many groups of ganglia, which maintain their highly developed sensory system and lively, predatory nature. These large brains are covered with cartilaginous cases. The rapid responses of the cephalopods are due to a bundle of giant nerve fibers that control the muscles of the mantle or outer region of the brain. (4 AW)





Locomotion

In general, mollusks are capable of moving by swimming, digging and crawling, while others are sessile and are anchored to a certain spot. Most mollusks have a muscular foot used for movement. Chitons (in class Polyplacophora) have a foot that acts as a suction cup, allowing them to attach to and creep along rock surfaces in the intertidal zone. Gastropods move via the rippling motion of their elongated foot; most produce a slimy mucus that helps them move along the surface. The largest portion of gastropods' energy expenditures consist of mucus production; it exceeds that of the crawling motion. (AR 4) Most mollusks of the class Bivalvia contain a hatchet-shaped foot that is extended when the bivalve opens and is used for digging or anchoring. Cephalopods are more skillful swimmers. For example, the squid moves by drawing water into it's mantle cavity and then fires a jet of water acting as propulsion.




Respiration

Mollusks in aquatic environments generally use gills located in the mantle cavity as the primary method for gas exchange. These gills are surrounded by cilia, which create water currents for more efficient oxygen uptake of the counter-current exchange system. The gills work with tubes called siphons to bring in water and oxygen and release water and carbon dioxide (EG). In terrestrial mollusks such as land snails, the mantle cavity functions as a lung allowing for gas exchange with the air. Most mollusca have blood that contains a respiratory pigment called hemocyanin that becomes colorless when deoxygenated and bluish when oxygenated (17 SC).




Circulation

Most mollusks (mainly in the Gastropoda, Polyplacophora, and Bivalvia classes) have an open circulatory system in which blood bathes the organs directly. The term "blood" refers to the fluid, that is separate form interstitial fluid, contained within closed circulatory systems. (7 AL) Because there is no barrier between interstitial fluid and the blood, body fluid of mollusks is called hemolymph. The heart then pumps hemolymph through vessels and out into sinuses, which are spaces surrounding organs. One advantage to having an open circulatory system is that it renders a mollusk much less vulnerable to pressure. This can be beneficial for mollusks that live at great depths because it prevents the compression of their bodies (7 MB).
Because the mollusk has a discrete circulatory and respiratory system, they have a greater capacity for oxygen intake and distribution which allows them to be greater in body mass. For this reason, mollusca includes the largest and most advanced of living invertebrates. (J. Sun 20)

The class Cephalopoda, however, contains a closed circulatory system in which blood is confined to vessels and is separated from the interstitial fluid. The heart then pumps blood into vessels that branch into smaller ones that surround organs.
mollusk.png
The Open Circulatory system of a mollusk (7 AN)






Osmotic Balance

Because mollusks can be terrestrial, marine, or freshwater organisms, there are different strategies for maintaining an osmotic balance. Terrestrial mollusks have waxy cuticles that help prevent water loss, such as the shell of land snails. Marine mollusks, like other marine organisms, constantly face the threat of dehydration. Therefore, mollusks drink a lot of seawater, excrete excess salts by active transport out of the gills, and produce very little urine. Freshwater mollusks face the opposite problem of absorbing too much water and losing salts, therefore they regain lost salts via active uptake (and from food) and excrete very large amounts of dilute urine.




Metabolic Waste Removal

Mollusks contain tubular metanephridia, which function as kidneys. Excretory pores open into the mantle cavity. Aquatic mollusks excrete ammonia, while terrestrial mollusks excrete uric acid. Waste from digestion is expelled through the anus.

Example of camouflage of an Octopus
Example of camouflage of an Octopus





Self Protection

Most mollusks are soft-bodied creatures protected by a shell formed by the mantle. If a mollusk feels threatened, the animal can simply retreat into this shell. Mollusks in the class Bivalvia are made up of a shell divided into two parts that are hinged along the mid-dorsal line. Strong adductor muscles shut the two halves together, forming a defensive fortress for the mollusk. Members of the class Cephalopoda have either small internal shells- as is the case for sqid, or no shell- as is the case for octopuses. The only living cephalopod with a true shell is the chambered nautilus. (3RM)

Cone snails, a family of gastropods, possess a long tooth that has a poison gland containing neurotoxins. This tooth can be launched out of the snail's mouth like a harpoon. Usually, this mechanism is a way of obtaining food; it can, however, also be a method of self-defense. The snails' neurotoxins can be extremely effective, paralyzing an organism instantly. Several human deaths have been attributed to cone snails. (SW 20)

Cephalopods are famous for their ability to create a "smoke screen" out of ink to allow for a quick escape. Another interesting self-defense maneuver is cryptic coloration, or camouflage, which is caused by pigmented cells called chromatophores, allowing for the cephalopod to blend in with the environment.

(13RM)



Temperature Balance

Aquatic invertebrates are thermoconformers and have little control of their body temperature. However, temperature of the water, as well as other factors like the mollusk's diet, hormone feedback, and acidity, influence the formation of the mollusk's shell. (3 AS)

Review Questions
1) What does the mantle do for molluscs? (CH)
2) How does the eye of the molluscs gradually change in complexity? (11T2)
3) Why would it be an evolutionary advantage for many marine mollusks, given their habitats, to have an open circulatory system (7 MB)?
4) Why do mollusks' eyes have such a big range in complexity? (23 HL)


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