Molluscs 2BIbDF09 10/11

De biorousso
Aller à la navigation Aller à la recherche

What are molluscs?

Molluscs are invertebrates animals that live in water or in moist environmnent, and mostly in marine environment. There is an estimated number of 100'000 species in the phylum Mollusca. Molluscs appeared in the earliest Cambrian period 545 million years ago. They have a bilateral symmetry and are soft-bodied animals, but most are protected by a hard shell. Some molluscs have external hard shells, like Nautiluses, Snails or Clams, but other have reduced or internal shells, like Cuttlefish or Squids. The rest just lost it during evolution, like Octopuses or Slugs. A shell is not an exoskeleton; it is in fact a protection against predators or the dangers of their environment. If it was an exoskeleton, molluscs couldn't live without it but they can, so they have either a hard shell or an internal skeleton.

We can mainly define four different groups of molluscs (cf. appendix molluscs 2); the Gastropods (cf. appendix molluscs 3), the Bivalves (cf. appendix molluscs 4), the Polyplacophores (cf. appendix molluscs 5) and the Cephalopods(cf. appendix molluscs 6).
All of them have the same body plan separated in three parts. The first part is the foot which is the strong, muscular portion used for locomotion. The second one is the visceral mass, a soft-bodied portion that contains internal organs and the third one is the mantle, that covers and protect the visceral mass and, for those who have shells, secrets the shell. (cf. appendix molluscs 1)

Molluscs, as they are separated in groups, don't have the same diet. Some are carnivorous, others herbivorous, filter feeders or detritus feeders. Their diet is bounded with their live style. Cephalopods are carnivorous, gastropods are herbivorous and most bivalves are filter feeders. They mostly have a radula, which is a rasping organ that molluscs use to scrape up food: for example scrape algae off rocks or brake some shells. (cf. appendix molluscs 9) The food is then digest by two organs, like the human digesting cycle(cf. appendix molluscs 1).

Molluscs' circulation is basically open. The blood and body fluid circulates in the mantle cavity around the organs: they don't have veins.

Molluscs generally breathe thanks to gills. However, there are some exceptions like sea slugs which have reduced gills, and snails and slugs whose gills have been reduced in the evolution, in the adaptation to land life and who therefore have developed a snail lung.

Classification of Molluscs

  • Domain: Eukarya
  • Kingdom: Animals
  • Phylum: Molluscs
  • Classes: Bivalves, Cephalopods, Gastropods, Polyplacophores (cf. appendix molluscs 7)

What are the main features of each group?

The Polyplacophores

Polyplacophores are molluscs with many shell plates. We know approximatively 750 species of them. This class is the closest related to the molluscs' common ancestor. Chitons are the best known Polyplacophores. They can live up to 3000 meters depth, in marine environments. Unlike most molluscs, chitons do not have a calcareous shell. As we said, chitons are closely related to the molluscs' common ancestor but they have differences with it. For example, the chiton's cuticula of the mantle, which protects a small part of their back, is only located on its back sides, unlike the molluscs' ancestor. Their whole back is in fact protected by eight calcareous shell plates. (cf. appendix molluscs 12)

A chiton is flattened and elongated, and can attach itself tightly to rocks or something hard thanks to its strong foot. It usually attaches by a combination of muscular contraction. They can cling hard to the ground with their powerful foot. Chitons have a strong longitudinal muscle, that enables them to roll in like a woodlouse. (cf. appendix molluscs 13)

Chitons have no mantle cavity containing gills. Instead, they are provided with a groove running almost all around the animal between mantle and foot.

Most chitons are herbivorous and thus live on algae that they rasp from rocks or mussels. However, some species have change their diet to a carnivorous one. They lift their frontal part from the ground and wait for prey. They have small tentacles around their mouth that detect the prey. The chiton falls back on its prey and consumes it.

Like they live constantly with their back up, in the evolution they adapt to this characteristic: that is why they have light sense organs located on their shells. They enable the chiton to recognize a threat coming from behind without exposing its belly.

The majority of chitons have separate sexes. The pair of sexual organs called "gonads" usually are grown together forming one organ leading into the mantle groove. The fertilization occurs in the surrounding water, where the larvae metamorphoses itself into a small young chiton.

The Bivalves

The class of Bivalves is a big group too, as we know 10'000 species of them. Bivalves exclusively live in water, in sand or mud in marine and freshwater. Bivalves, "bi" meaning two, have shells divided into two valves hinged together. These valves are held together on the anterior part by a ligament, which can open the shells when relaxed. The mollusc protects itself by closing thanks to a very strong muscle that enables the mollusc to keep the shell shut. An other weak muscle enables the closing motion. So it is easier to stop the muscle from closing than opening it when already closed.

How do they breathe?

Mussels and most bivalves in contrary to all other molluscs live exclusively on filtration. Their huge gills enables them to filter water either for respiration and feeding. Ingestible and indigestible particles are separated when filtered. The indigestible particles are swept out with the rest of water leaving the mussel. These particles fall then to the ground with other sediments. Thus mussels in fact keep clearing the water. It is an extremely important capacity for the water's ecology. Indeed, one blue mussel filters up to 20 litres of water each day.

How do they reproduce?

Most bivalve have got separate sexes, though there are some hermaphroditic groups, such as oysters living in colonies. They change their sexes when the number of males is not sufficient to increase the population. For some bivalves, the fertilization occurs in the surrounding water when for others (in freshwater for example) it occurs on the female's mantle cavity. At a parasitic larval stage, the embryo has to infect a fish to be able to develop itself into a young mussel. For marine mussels, this stage is not obligatory; the larva develops in the water.

How do they move?

As we already said, bivalves are mostly sedentary, but it doesn't mean they do not move. They move when they need to hide under sand for example. They insert their foot into the ground and then pump it up with blood and use it as an anchor which enables it to come over the floor. Mussels dig themselves into the ground by the same way. Mussels or oysters are sedentary species. However, not all bivalves are. Some species like the Pecten and the Lima are able to swim into the water. Those species jerkily presses the two shells together and ejects the water they contain. They usually have simple eyes on their mantle so they can have a few information about their environment. They only see the differences between light and shadow. Some of them have tentacles too, that help them to feel their environment.

The Gastropods

The Gastropods form the most important group of molluscs (over 40,000 species). This group include snails, whelks, limpets and slugs. We call them Gastropods because it means stomach-footed. All the gastropods are oviparous animals.

We can find them nearly everywhere in the world because they live either in water or in moist environments. Their anatomy differ a bit with other molluscs, because of their color or their way of respiration, for example.

Most gastropods are herbivores or detritivores. They have a unique structure in their mouth called a radula, which allows them to eat easier.

The radula is a rasping organ which scrape up algae plants cell or other type of food on rocks for example. It is usually covered with teeth. Sea butterflies are gastropods with a reduced or absent shell but with a large winglike foot that flaps and power swimming movements.

How do gastropods move?

Gastropods move by muscle contractions, thanks to their ventrally flattened foot. That is why gastropods that live on land are very slow.

Gastropods can have a shell, like snails.

What are the differences between the Land gastropods and the sea gastropods?

The shell of snails change. Land snails have thinner shell than marine snails. This shell is in a shape of spiral, which is used for protection.

The nudibranchs are marine gastropods which have lost their shell in evolution. We also call them sea slugs.

snails: The snails are hermaphrodite. This means that every snail have a female part and a male part. The female part can produce eggs and the male part produces sperm. It is impossible for only one snail to reproduce itself, the snail has to meet an other one. However during the mating the snails can change their sexual part when the two snails have their eggs fertilized, so they can leave and lay some eggs after fertilization.

The snail's body is kept moist by mucus which helps this animal to glide along as foot ripples. The mucus which is produced by the snail helps it to move and reduces the risk of wound. When snails pull their bodies into their shells, they can close the operculum, which is like a door to protect themselves. They have two pairs of tentacles of which the longest has eyes in its extremity.

Nudibranchs: Limpets are the most primitive organisms of all the gastropods. they are found in marine environments. Limpets have the same body plan as other molluscs.

The Cephalopods

Cephalopods differ from gastropods and bivalves. Unlike bivalves and most gastropods which have shells, most cephalopods, like the Octopus, miss it. Only a few cephalopods have a heavy external shell (like the Nautilus) or a small and internal shell (like the Squids), which is a cuttlebone; it is neither a bone or a skeleton, but a shell rest. (cf. appendix molluscs 10)

They are carnivorous predators made for speed and agility; cephalopods are know for their intelligence and ability to catch their fast-moving prey. They are found in all oceans of the world. All species of cephalopods have tentacles that they use to catch and hold prey; mostly fishes. The tentacles seem to be arms that help them to feed themselves and cling to surfaces, thanks to suction cups on the tentacles. Cephalopods are great predators thanks to their speed movement, camera eye-vision and their intelligence.

Their complex behaviour makes them comparable to vertebrates rather than other invertebrates.

Locomotion

They move by jet propulsion, which is a complex muscular motion:

The seawater is first drawn into the mantle cavity over the gills. When the animal wants to move, it quickly contracts the mantle wall. The pressure of water within the mantle cavity is increased because there is only one orifice not sealed, the funnel or siphon, by which the water goes out, propeling the animal fast through the surrounding water. (cf. appendix molluscs 8)

Sight

The cephalopods need to see quite well so that they can catch prey easily, thus they have what we call a "camera eye vision", which is a great sight they have thanks to their developed eyes, that don't differ so much with human eyes. Indeed, the eye of a cephalopod is composed of a two chamber system, separated by a lens, with a retina that catch light and a pupil with an iris. The structure of the retina show similarities too, but the human eye is more complex. In both, the pupil protects the retina from the amount of light. The octopus's eyes is probably the highly developed light sense organ in the animal's kingdom.

Intelligence

Cephalopods are considered like the most intelligent invertebrates. They have a develop nervous system and an extremely complex brain, which is an advantage being predators. Thanks to their intelligence, they are able to adapt to their environment and Squids have the ability to "disappear", using camouflage as the main protection against predators. They can control their pigment cells (chromatophores) and thus are capable of changing their colour, to protect themselves against predators or hide ambushing prey. (cf. appendix 11)

Reproduction

The Squid for reproduction uses a special arm called hectocotylus, which enables it to fertilize the female into its mantle cavity. It then lays the eggs in the water, which hatch and let squid larvae out. Unlike Squids, which lays their eggs, females' octopuses keep them to take care of them. It doesn't eat when it guards the egg, that is why the female octopus generally dies after its offspring's hatching, because it's too weak to defend itself.

What are the main features of animals?

Animals are eukaryotic and multicellular organisms which live on earth, in all environments; water, underground, aboveground, in the air, in plants, etc. They are heterotrophic: they need to ingest food to generate energy and survive. They can move and differ from plants, algae and fungi in lacking rigid cell walls and having a different embryos development.

Are molluscs animals?

Molluscs are eukaryotic and multicellular organisms. They mostly live in water or moist environments on earth. They need to ingest food to survive; either other animals or plants and algae. They can move, even if some molluscs are sedentary and have this particular embryos development which is a characteristic exclusive to animals. Thus, we can confirm that molluscs are animals.

References

back to Animals_2BIbDF09_10/11
back to Accueil

Récupérée de « https://edutechwiki.unige.ch/bioroussowiki/index.php?title=Molluscs_2BIbDF09_10/11&oldid=51152 »