Mollusks arthropods # Moluscos

Clases de molusco # Gastropoda. Polyplacophora. Bivalvia. Cephalopoda. Sexual reproduction

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There are over 50,000 known species of Mollusks, which makes them second only to the Arthropods in invertebrate phylum size. Among the Mollusks are some of the most well known of invertebrate sea creatures, like snails, clams, mussels, squid, and octopus? Although one might not see an obvious physical relationship between a snail and a squid, they are remarkably similar in construction.

In general, mollusks have 3 body regions: a head, a visceral mass, and a "foot." The head contains the sense organs and "brain," while the visceral mass contains the internal organs. The "foot" is the muscular lower part of the body, which is in contact with the substrate. Mollusks usually have a shell (although some do not). Mollusks also have an extension of the body wall called the mantle. This portion of the animal's anatomy is responsible for secreting the shell. The mantle encloses the mantle cavity, which contains the Ctenidia (gills), anus and excretory pores.

Many mollusks have a radula, a tongue of sorts, which is rough like sandpaper and is used to rasp away at food. The radula is made of a hard material called chitin, the same material of which Arthropod exoskeletons are made. In addition, some Mollusk radulae are impregnated with magnetite to give them superior wear characteristics. Mollusks have well developed body organs (nervous system, circulatory system, respiratory system, etc.) but lack body segmentation.

There are seven classes of Mollusks: Monoplacophora, Polyplacophora, Aplacophora, Gastropoda, Scaphapoda, Bivalvia and Cephalopoda. Of these, Monoplacophora, Aplacophora and Scaphapoda are rare and/or extremely deep-water creatures, and will not be discussed.

The Class Gastropoda

The class Gastropoda (meaning "stomach-foot") contains about 70% of the Molluscan species (around 35,000). These are the familiar snails, limpets, nudibranchs and abalones. The snails, limpets and abalones have a shell while the slugs and nudibranchs do not. There are a small number of lands Gastropods such as some snails, slugs, etc.

Big Gastropod

The conch (pronounced "konk") is a big snail. There are two eyes on stalks peeking out from under the shell. The points on the shell protect it from other animals. This conch weighs more than a pound!

Colorful Gastropod

This is a nudibranch, basically a snail without a shell. The word "nudibranch" means naked gill. The tuft of yellow on the right side is the animal's gill cluster. Nobody is sure why some nudibranchs are so brightly colored. Some believe that the coloration is aposematic, meaning that it tells other animals

"Danger! I don't taste good!"

The Class Polyplacophora

Members of the class Polyplacophora (meaning "many-plated") are more commonly called the chitons (pronounced Kie-tuns). These creatures have eight plates and outwardly resemble the "pill-bugs" which are common under logs and rocks in the forest (they are not related, however). Chitons crawl along rocks and forage for food (mostly algae) using their radula to scrape it off of the substrate. Magnetite is used for hardening the teeth of the radula in chitons, so they can scrape coralline algae off of rocks. These teeth are so hard that they can etch glass!

These creatures are extremely slow moving. In a year, a chiton may not move more than ten feet. They can detect the presence of light with primitive eyes embedded in the shell plates.


A chiton is stuck to a rock. It may not look like much, but getting it off the rock is nearly impossible!

The Class Bivalvia

The bivalves (meaning "two-shells") are perhaps the most well known mollusks simply because of their history as a source of food. Clams, mussels, oysters and scallops are all bivalves. There are about 15,000 known species of bivalve, with about 80% of them being marine (the rest are found in fresh water).

The bivalve may either burrow through the bottom, or attach itself to the substrate with glue-like strings called "byssal threads." Some scallops do not attach themselves because they prefer to be able to swim away with a squirt of water forced from the mantle.

Most bivalves feed by filtering organic particles from water, and therefore do not have a radula. The gills are used in feeding by means of a mucous coating, which traps food particles as water passes through them. Some of the giant clams found in the Pacific have symbiotic algae in their mantle tissue, and can utilize the light of the sun to make food, although they still filter feed as well.

The mantle from the inside generates the shell. Clams, oysters and mussels make pearls when a grain of sand or other small irritant becomes painfully stuck in the mantle of the creature. The bivalve coats the irritant with the same material, which is secreted to produce the inner lining of the shell. This makes the irritant smooth, and theoretically, less painful to the bivalve. Although many people think of pearls as coming only from oysters, most bivalves can produce pearls, as well as some snails, like the conch.

The scallop

This is the scallop that most people never see. This bivalve has hundreds of tiny eyes to keep a lookout for predators (those are the tiny black dots around the opening). The animal feeds by filtering food from the water.

The Class Cephalopoda

Although we usually think of Mollusks as benthic (bottom dwelling), the cephalopods have taken to a nektonic (swimming), rather than benthic, existence in the ocean.

Squid, octopus, cuttlefish and nautiluses are all members of the class cephalopoda, meaning "head-footed." This term stems from the way a cephalopod's body is constructed. The "feet" (usually called arms or mistakenly called tentacles) are attached to the part of the body containing the eyes (the "head") while the rest of the body is out in front of the head. Thus, the body does not connect directly to the arms.

Caribbean Reef squid

This is the Caribbean Reef squid, an animal capable of amazing color changes. Using chromatophores in its skin, the squid can go from white to blue to gold in the blink of an eye. It can jet off at high speed by squirting water though its nozzle, or it can hover in one place with its fins.

The cephalopods may not seem very closely related to the other mollusks, but physiologically, they are similar in internal construction. Perhaps the most obvious difference between most cephalopods and other mollusks is the apparent lack of a shell. The octopus do not have shells at all, and the squid have a small chitinized internal shell. Nautiluses are the only Cephalopods with an external shell. Nautiluses are found in the South Pacific and Indian oceans.

Cephalopods have the most well developed nervous systems of all mollusks, as well as the most well developed eye. The cephalopod eye is one of the most notable examples of convergent evolution in the entire animal world, because this eye evolved from a completely different direction than the mammalian eye, yet it turned out to function in almost the exact same way. Cephalopods, therefore, have extremely good eyesight.

This eyesight is well suited for finding prey. The cephalopod then grasps the prey firmly with its arms and eats the prey with a mouth located between the arms. Cephalopods also have a beak, very similar to that of a parrot, used to help bite into prey.

The Blue-Ringed octopus of the South Pacific has a salivary gland that secretes venom to subdue prey. The venom is so powerful that the bite of this octopus is almost always lethal to a human. Fortunately, it takes a lot of provocation to get an octopus--any octopus--to bite a person.

Common Caribbean octopus

The Common Caribbean octopus on the prowl at night, looking for food

While the octopus has eight sucker-equipped arms, the squid actually has ten. Eight of the squid arms are of the same length, while the other two are extra long, and are used for helping to grab prey. These two additional arms are called the tentacles.

Cephalopods do share many characteristic molluscan traits with the rest of the creatures in the phylum, such as the presence of a mantle and mantle cavity, a radula in the mouth and a U-shaped digestive tract (useful for a creature in a conical shell, but not necessary for a Cephalopod).

Octopuses or Octopi?

Many people get confused about the proper plural for the word octopus. Octopus is frequently given a Latin plural, yet the word derives its second root from Greek, not Latin (the english "foot" root from Latin is "pod" or "ped", while from Greek it is "pus"). What is the proper plural, anyway? Many say "octopi."

The singular looks like a Latin masculine singular, ending in "us," but it really isn't, so pluralizing it to "octopi" is not correct. Most people also mispronounce it as well. If it were "octopi," it would be pronounced "octo-pee" not "octo-pie," and that sounds silly! So, the correct plural is "octopuses," believe it or not. I prefer not to use either of them.

Every time you say "octopuses" someone will correct you, and you end up in an argument. So I like to use "octopus," which is the proper plural of the taxonomic order Octopoda, containing the eight-armed cephalopods.

The banded snail

Typically the banded snail has a brown lip around the rim of the shell, where this is white in all other species. It is one of the most common snails in Britain. The shell is known to be up to one inch in diameter and shows a variety of brilliant colors. In most cases the banded snail will have five brown or black bands running parallel to the whorls of the shell. These snails are widespread in Europe where they are regularly eaten and have more recently been introduced to North America. They can be found in woodlands, copses, hedges and open country. Some are even found in sand hills around the coast where they will emerge at night to feed. Like the common garden snail, the banded snail has a definite roost where it always returns after feeding.

The favored food of this creature is stinging nettles, but they are also known to feed on other plants and dead earthworms. The banded snail eats by rasping tissue off its food with its file like tongue. There are many different predators that prey on the banded snail. Some of these include rats, voles, hedgehogs, field mice and even rabbits. Thrushes consider the banded snail one of their favorites, holding the shell in their beak and hammering them against a stone to get at their prey. In areas of sand dunes the glowworms will prey on banded snails and man tends to favor this creature as food.

Since snails are hermaphrodites, but cross-fertilization occurs, each snail fertilizes the other. Their courtship is most extraordinary method as they stimulate each other to full breeding. Each banded snail has a chalky spicule that it forms in a special pouch. During the courtship this love dart is ejected into the partner's body as a very forcible stimulus for mating. Mating takes place in the morning hours. One snail will follow another's slime until the two meets. Rearing up the pair will bite each other and the more active partner ejects his dart, followed by the other snail's dart. The mating process takes several hours and eggs are laid from the end of May to the beginning of August.

Banded snails will often lay up to four clutches of eggs unless the weather is very dry, in which case they may not lay any eggs. The sizes of the clutches vary, depending mostly on the time of year and the color of the parents. Banded snails with five bands have been known to lay twice as many eggs as other varieties. The snail digs into the ground forcing its way down with its muscular body to lay its eggs. When it is almost buried it will lay eggs every 10 to 30 minutes. After approximately 3 days the snail leaves the nest and covers the eggs. Young snails dig their way out once they hatch and go in search of food. Maturity is reached in 2 to 3 years.

Biologists have been collecting snails for some years both from anvils and the surrounding ground where thrushes have foraged the creatures, hoping to learn more about their habits. The sizes of the shells appear to vary based on availability of food and weather changes. Interestingly, it was found that the thrush tends to pick banded snails of a specific color. A result of this study demonstrates to some extent, Darwin's theory of natural selection, which is popularly called today, survival of the fittest.

Banded Snail

Mollusks arthropods # Moluscos

Sexual Reproduction in Populations of

Wild Animals and Plants

Amongst the populations of wild animals and plants sexual reproduction also produces offspring that are varied. This is important because it can help them to survive in an environment that is always changing. This will become clearer if we study an example.

Mollusks arthropods # Moluscos

The banded snail is a common snail that is found all over Europe. The shell of the banded snail may be of different colors: yellow with brown bands around it, all yellow, all pink or all brown. This snail lives in woodland, open grassland and in hedges. Biologists know that the colors of the snail's shell are inherited from its parents. Also amongst a population of snails one may find all the different types of shell.

In different habitats some types of shell color are more common than others. In open grassland you will find more yellow-shelled snails. In woodland there are more brown-shelled and pink-shelled snails. Amongst the hedgerows the snails often have banded shells. Biologists think that one of the reasons for this difference in the numbers of shell colors is camouflage.

Mollusks arthropods # Moluscos

A stone used by a thrush to break open snail shells. Can you see there are some shells from banded snails amongst them?

Birds, rats and mice eat these snails. The yellow-shelled snails are more difficult to see in grassland but they are very easy to see amongst the dead leaves of a forest. The brown-shelled snails are less easy to see in woodland but they will stand out in grassland. The predators of these snails will eat those snails which are easy to find.

Mollusks arthropods # Moluscos

The example of the banded snail, however, is not as simple as it seems at first. Other factors seem to influence the number of the different types of shell colors. In warm regions of southern Europe yellow-shelled snails are more common. In colder regions, such as Germany, the brown-shelled snails are more common. This could be because dark objects warm up more quickly than pale objects, which reflect heat. Therefore, in cold climates the brown-shelled have an advantage, whilst in hot climates the yellow-shelled snails have the advantage. You can see that the explanations for the variations amongst organisms are not always easy to find.


Octopuses are cephalopod mollusks characterized by having eight arms, no tentacles, and the internal shell lost or considerably reduced. There are over 100 species of octopuses (Genus Octopus) in the world as well as numerous species of deep-water and pelagic octopuses (Order Octopoda). Our knowledge of octopuses comes almost entirely from only a few species (Octopus vulgaris, O. bimaculatus, Enteroctopus dofleini, Eledone cirrhosa).

The species known as Octopus dofleini, the Giant Pacific Octopus has recently be re-classified as Enteroctopus dofleini (Hochberg 1998). The genus Enteroctopus includes the other giant octopuses of the world (E. dofleini in the north Pacific, E. megalocyathus off S. America and E. magnificus off southern Africa).

Enteroctopus dofleini is the largest species of octopus in the world. Although it is very unusual to find an individuals over 100 pounds (45 kg), one large individual captured just near Victoria, British Columbia in 1967 weighed 156 pounds (70 kg), and was almost 23 feet (7.5 m), from arm tip to arm tip. There are records that seem to be well referenced of a few individuals that were more than 300 pounds (136 kg) and one that was more than 400 (182 kg). (Ref: Dr. F.G. Hochberg, Santa Barbara Museum of Natural History).

Enteroctopus dofleini occurs on the continental shelf of the North Pacific Ocean, where its range extends from southern California, north along the coastline of the Pacific Northwestern Americas, across the Aleutians, and south to Japan. The species occurs at depths from the intertidal to 750 m.

Enteroctopus dofleini live 3-5 years. When mature, females' lay eggs on the inner side of a rocky den and may lay 20,000 to 100,000 eggs over a period of several days.

Eggs are tended, cleaned and aerated by females until they hatch. Incubation takes 150 days to seven or more months, depending on the temperature. Females do not feed while tending eggs and die when the eggs hatch or shortly thereafter. Many of the eggs will die if not tended by the female until hatching.

Following hatching E. dofleini swims toward the surface and spend 4-12 weeks drifting in the plankton until they reach a size of >14 mm mantle length (still under 5 grams.). The young then settle to the bottom, although not much is known about this settlement phase.

Dens are an important resource to octopus during all benthic life stages. Dens are used both as brooding chambers and as refuges from predators, including other octopuses, various fishes, and many marine mammals. Most dens of juvenile E. dofleini are naturally occurring spaces under rocks or in crevices or are an excavated cavity in sand or gravel under a boulder.

Octopuses consume mostly crustaceans and mollusks, most often-small crabs and scallops. Other bivalves, snails, fish and other octopus are also eaten.

Octopuses use three different techniques to gain entry to hard-shelled prey: they may pull it apart; bite it open with their beak or 'drill' through the shell. Preys that are difficult to pull or bite open are drilled: secretions from the salivary papilla soften the shell of the prey, and the softened material is then scraped away with the radula to create a tiny hole in the shell. Through this hole, the octopus secretes a toxin that paralyzes the prey and begins to dissolve the connective tissue. The prey is then pulled apart and consumed.

The third right arm of the male octopus is hectocotylized with a modified tip, called the ligula that is used during mating and in E. dofleini may be one-fifth the length of the arm.

Males may mate with more than one female and females receive fairly large spermatophores (up to 1m long) during mating. Eggs are laid some time after mating (one report of 42 days in captivity in Hartwick, 1983).

The mouth is located at the center of the arms. Octopuses have a strong, parrot- like beak contained within the bucal mass, the muscles which work the beak and tongue.

Mature female E. dofleini have 2240 suckers, 280 on each arm, but males have fewer because there are only about 100 on the hectocoylized arm. (Toll, R. 1988. "The use of arm sucker number in octopodid systematic (Ceph: Octopoda)" American Malacological Bulletin. 6(2): 207-211. 1988). In at least shallow-water species of octopuses there seems to be a species-specific number of suckers per arm that is reached at a size less than the maximum size, with the number on the hectocotylus also species-specific. In the deep-water species, this doesn't necessarily hold.

Mollusks arthropods # Moluscos