help with nautilus and their shell!!?

Question

why do you think nautilus have retained their external shell while other cephalopods have a reduced internal shell or no shell at all?

Answer 1

To shorten the excellent answer of the person above me, it is all about their buoyancy. They use the different chambers of their shell and the differential amounts of gases in each to maintain their buoyancy in the water column. This was also used by ammonites and has been a very successful life strategy since at least the Devonian (more than 350 million years ago).

And if it ain't broke, don't fix it!

Answer 2

Nautilus (mollusk), a primitive marine coiled shell mollusk. The nautilus is the only remaining example of a nautiloid, which was the largest predator in the seas of the Ordovician period, around 450 million years ago. Today's species inhabit the warm waters of the Indian and eastern Pacific oceans.

Unlike other cephalopod mollusks, the octopus and squid, the nautilus has simple, lensless eyes that operate on the principle of the pinhole camera. It also has two pairs of gills instead of one pair, and it has up to 90 tentacles arranged in two rings around the mouth. Its most familiar characteristic is its smooth, coiled shell, up to 28 cm (up to 11 in) in diameter. The shell is lined with mother-of-pearl and is separated into a series of progressively larger compartments, the most recent of which is inhabited by the animal. The walls (septa) dividing the chambers are pierced by a tube (siphuncle) connected to the nautilus. Gas and liquid exchange occurs through the siphuncle walls, by means of which the nautilus can regulate its buoyancy.

The nautilus apparently rests on the ocean bottom during the day, at depths ranging to 600 m (2000 ft). At night it swims about by forcing water through a primitive funnel; it eats diatoms, shrimp, and algae. When the animal is feeding, its tentacles are extended to catch whatever swims into them; the tentacles are small, contractile, and adhesive but have no suckers. Little is actually known about the behavior and life cycle of nautiluses.

Many people believe that the paper nautilus is a type of nautilus. It is actually a type of octopus of the order Octopoda.

Scientific classification: Nautiluses make up the genera Nautilus and Allonautilus of the subclass Nautiloidea, class Cephalopoda