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Common name: nautilus
HABITAT AND GEOGRAPHICAL DISTRIBUTION
The nautilus is a family mollusk Nautilidae belonging to the genus Nautilus, within which we find only five species of which the Nautilus pompilius it is the most widespread and known (note 1).
The peculiarity of the genus is that it includes molluscs that existed millions of years ago, so much so that they are commonly called "living fossils". In fact some fossils of Nautilus they have been dated about 500 million years ago, therefore related to the end of the Cambrian period (Paleozoic) period in which life on the mainland was not yet developed but there was a lot of it inside the water.
Today the different species of the family Nautilidae they live in the eastern Indian Ocean and the western Pacific Ocean. The most widespread species, the Nautilus pompilius, which lives in the seas of the Philippines, Indonesia, the Andaman Islands, Papua New Guinea and Australia, near the great coral reefs, as well as in the Solomon Islands and as far as the Fiji Islands. The Nautilus belauensis is known only in Palau (West Caroline Islands) while the Nautilus macromphalus it is found exclusively in the waters of New Caledonia and the Nautilus stenomphalus it has only been reported on Australia's Great Barrier Reef.
In zoology they are very important as they are the only living cephalopods tetrabranchiati (ie provided with four gills).
Unfortunately it must be said that even today many aspects of the life of the nautilus are unknown. For example, its eggs have never yet been found in nature, it is not known where and how the young specimens live, the exact size of the populations is not known and there are still many doubts about their classification ... In short, these incredible animals are still a great mystery.
Although belonging to the order of the cephalopods, the Nautilus it is the only genus with a shell characterized by a spiral shell. The shell is composed of layers of aragonite (mineral formed by neutral calcium carbonate), alternating with layers of a protein substance similar to the composition of our nails that the animal processes from the coat. in the fossil state.
The nautilus lives inside this shell and has two large, crystalline-free eyes that work with the darkroom principle. Therefore the sight is not very developed unlike the sense of smell with which it identifies the prey and orientates itself.
It can have up to 90 filiform and retractable tentacles, without suction cups, but equipped with thin grooves that allow it an excellent grip. The tentacles are arranged in two concentric circles around the mouth and used to locate and grasp prey, and are also equipped with cells sensitive to various chemicals and therefore useful for locating prey. The mouth is formed by a horny beak with which the animal shatters the shells of the crustaceans (especially hermit crabs) which it feeds on.The rest of the body is formed by a sort of sac that contains the different organs (heart, digestive system, reproductive system, etc.). This sac is enclosed within a mantle (a tissue that secretes the shell). Between the sac and the mantle are the four gills (hence the name tetrabranchiati).
In its ventral part there is a sort of funnel thanks to which the water of the mantle with the four gills is expelled: the water is rhythmically aspirated and expelled by means of rhythmic contractions of the retractor muscles of the body, causing the animal to move. . In this way the Nautilus swim in a hydrojet. This system allows it to reach a maximum of 0.2 m / sec.
The shell, of primitive structure, is formed by numerous chambers of increasing size, arranged in a spiral, separated by transverse septa but all connected to each other thanks to a tube called a siphon, which is located in a central position with respect to the shell. Its purpose is to fill or empty at will the various chambers that contain a gas of composition similar to air and a liquid called "chamber liquid", which provide the thrust the animal needs to maintain itself at the desired depth, in practice they function as floats for vertical displacements. In practice, the nautilus is able to regulate the amount of gas and liquid inside the chambers, thus varying its specific weight and therefore its position in the water. It has been observed that the gas pressure inside the chambers is always about one atmosphere, even when the nautilus dives to great depths. Despite this, the shell implodes only at a depth of about 800 meters (i.e. with a pressure of 81 atmospheres or withstanding a pressure of about 81 kg per square cm!).
In the last chamber, the largest and most external one, the mollusk lives. The shell is generally white on the outside with numerous red-brown streaks while it is pearly on the inside. The shell opening is covered with a sort of hood when the body is retracted inside.
They are quite large molluscs we have in fact that the Nautilus pompilius, the largest species, has a diameter of 20 - 27 cm while the Nautilus macromphalus, the smallest species, has a maximum diameter of 16 cm.
CHARACTER, BEHAVIOR, SOCIAL LIFE
The Nautilus it lives mainly at great depths, around 300-500 meters, especially during the day. At night it tends to swim to the surface to feed.
They are long-lived animals and can live for more than 12 years.
They feed on crustaceans but also on dead fish if necessary.
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- All the images shown in the article are from Nautilus pompilius;
- Attribution: Profberger (English Wikipedia). Image licensed under Creative Commons Attribution-Share Alike 3.0 Unported;
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