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Bivalves exclusively live in water, only it has happened several times in evolution that bivalve groups managed to adapt to life in fresh water, which is why today there are several fresh water mussel groups that are not closer related to each other than all being bivalves. One of these are the large fresh water mussels also called naiads Unionacea. The main characteristic of mussels can easily be seen from outside: A mussel shell is divided into two shell valves hence the class name Bivalvia. Usually they are large enough to cover and protect all of a mussel's body.

The two valves of a mussel shell are held together on one side by an elastic band called the ligament. In a relaxed state it opens the shell. Antagonists to the ligament are the mussel's strong muscles. The mollusc has to keep the shell closed by active muscle action. To make this easier for the mollusc, its closing musculature consists in fact of two muscles: One relatively weak muscle for the actual closing motion and one very strong locking muscle to keep the shell shut.

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So it is quite easy to stop a muscle from closing its shell, but very hard to open it with the mollusc holding tight from inside. Mussels breathe exclusively with gills.

In different bivalve groups the gill construction can be so different, that the gill construction type is a major part of bivalve systematics. The gills not only serve for respiration, but also for feeding the mussel. Though there are some few species actively collecting food around their place, mussels generally sieve food particles out of the water current caused by respiratory action. Ingestible particles are separated from indigestible ones, the latter swept out by the same water current leaving the mussel. As those particles are clustered inside the mussel, they become to heavy to float and after leaving the mussel fall to the ground and are added to the sediment.

So the mussel has got an important part in clearing the water. Blue mussels for example can filtrate up to 5 litres of water per hour, oysters even manage 25 litres. Most bivalve species have got separate sexes, though there are some hermaphroditic groups. Especially bivalves living in colonies, such as oysters Ostrea can change from females into males, when the number of males in the colony is not sufficient to ensure the population's number.

While fertilisation takes place in the surrounding water among marine bivalves and some of the fresh water groups, there are also fresh water species the already mentioned naiads or Unionaceans , among which fertilisation takes place inside the female mussel's mantle cavity. They then develop past a parasitic larval stage, the Glochidia, that have to infect a fish to be able to develop into young mussels. In contrary to those, most marine bivalves' development happens past a free-floating larval stage, comparable to a trochophora , and later a veliger type of larva.

A bivalve's soft body is protected on both sides by the mantle lobes. The space within is called the mantle cavity. Usually both mantle lobes are partly grown together, leaving three openings, two, through which water enters and leaves the cavity, providing the mussel with food and oxygen, and a third opening for the foot to leave the shell.

Usually the foot is the only part of the mussel's soft body to be seen outside of the shell. On its rim the mantle is laid in three folds, which each have different tasks: The fold furthest outside contains cells producing shell and shell skin periostracum , the middle one has got sensory tasks, and the innermost fold regulates the water flow into and from the mantle cavity. Because most bivalve species rarely move from their place there was no need for evolution to provide them with a head to concentrate sense organs such as eyes.

That is why C uvier in named bivalves Acephala - the headless molluscs. There are, though, bivalve species that can swim and do so by flapping their shell halves, such as scallops Pecten and Lima.

The mantle rim of those bivalves is full of simple eyes ocelles. The mantle rim of giant clams is also inhabited by symbiotic algae zooxanthelles , that are protected by the mollusc and in exchange provide it with nutrients produced by photosynthesis. In the many bivalve species digging and drilling in the ground, the respiratory openings of the mantle rim are elongated to form tubes, which are then called siphons. So the bivalve, though dug deeply into the ground, is provided sufficiently with water and food. As there are two siphons, they are distinguished as ingestive leading in and egestive leading out siphon.

They may be grown together to form one double tube, and in erected state may be longer than the bivalve itself. That way, on one hand, soft shelled clams Mya arenaria , living dug deeply into the soil of the wadden sea, feed themselves and breathe over the siphons. If they are flushed out of the ground, they will die.

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The blue mussel on the other hand always lives on the soil surface and therefore has no elongate siphons. If it is covered with soil, it will die. The gills in a bivalve's mantle cavity serve not only respiration, but also nutrition. Their composition may be different and thus is used to divide the Bivalvia class into sub-classes:. As in most molluscs, the bivalves' blood circulation is mainly open.

The heart has three chambers, two antechambers atria and one heart chamber ventricle. The primordial bivalves from the Protobranchia subclass actively collect food like protozoa, larvae, eggs and detritus from the surrounding substrate. The food, collected by tentacles, is then transported to the mouth in a ciliate groove.

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Other than that, most other, more highly developed, bivalves feed by filtering their respiration water. Cilia located in the mantle cavity produce a water current from the ingestive siphon into the mantle cavity and out through the egestive siphon. Digestible particles are collected and transported to the mouth opening. Feeding by filtration mussels get into contact with a very large amount of water, including the matter dissolved in it. That makes them especially susceptible to harmful substances in the water. So bivalves especially are harmed by the growing extent, to which water is polluted by industrial wastes and agricultural fertilisers.

For man it is especially important, that mussels accumulate harmful substances they filter from the water. So the poisons, that are introduced into the water by industry, might be found again in the mussels and oysters we eat.

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A bivalve's foot is adapted to how it lives and moves. So there are different types of feet in bivalves, from beam-shaped, tongue-shaped or worm-shaped feet.

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Swimming mussels, as well as completely sessile mussels, often have a largely reduced foot. Some species, such as Mytilus , the blue mussel, Arca , the ark shell, Pecten , the pilgrim's scallop and Pinna , the pen shell, have got a byssus gland at their foot's end, producing a thread that hardens in water and can be used to fasten the mussel to the ground. Later the thread can be severed Mytilus or thrown off Pinctada. Blue mussels also use their byssus thread to defend themselves by tying down attacking sea snails, such as the netted dog whelk Hinia reticulata.

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Against a full grown common whelk Buccinum undatum , though, this defence will not help. Bivalves generally are known as creatures that almost never move. That is not entirely accurate. Many mussel species that are absolutely sessile as adults, may move around as juveniles. Even the adults, though usually sessile, may move, when necessary. To do so, the foot is inserted into the ground and then pumped up with blood, so that it serves as an anchor, after which the mussel can pull body and shell over the ground.

The same way mussels dig themselves in the ground see picture above.

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Like Rick Stein , the famous sea food chef, had to discover when he tried to pull a razor clam out of the soil in an episode of "Food Heroes of Britain - Another Helping", such a clam can give somewhat of a struggle! Blue mussels Mytilus also use their byssus threads to move themselves over the ground: The byssus is thrown out until it catches ground, then the mussel gradually shortens the thread, thus moving itself towards the point of attachment. There are also some bivalve groups Nut shell, Nucula ; Dog cockle, Glycymeris , Tellin, Tellina ; Venus clam, Venus , that possess a real crawling foot, like gastropods do.

Some bivalve species, such as the pilgrim's scallop Pecten see above , and the flame shell Lima are able to swim through open water on their own. The bivalve jerkily presses together the two shell valves and ejects the water contained within. With this bivalve-type rocket propulsion the mollusc is then propelled backwards. Those bivalves usually have simple eyes on their mantle rim, so they can collect information about light and shadow in their surrounding.