Female argonauts make a laterally compressed, calcareous eggcase in which they live. The eggcase curiously resembles the shells of extinct ammonites. It is secreted by the tips of the female's two greatly expanded dorsal tentacles (third left arms) before egg laying. After she deposits her eggs in the floating eggcase, the female herself takes shelter in it, often along with the male's detached hectocotylus. She is usually found with her head and tentacles protruding from the opening, but she retreats deeper inside if disturbed.
These ornate, curved white eggcases are occasionally found floating on the sea surface, sometimes with the female argonaut still clinging to it. It is made of calcite, with a 3-layered structure The argonauts may have evolved to use ammonite shells for their egg laying, eventually becoming able to mend the shells and perhaps even make their own.
The egg case also contains a bubble of gas used for buoyancy similar to shelled cephalopods, although it does not have chambers as seen in other shelled cephalopods.
"Argonauta argo" is the largest species in the genus and also produces the largest eggcase, which may reach a length of 300 mm. The smallest species is "Argonauta bottgeri", with a maximum recorded size of 67 mm.
The beaks of "Argonauta" species are distinctive. They are characterised by a very small rostrum and a fold that runs to the lower edge or near the free corner. The rostrum is 'pinched in' at the sides. This makes it much narrower than in other octopuses, with the exception of the closely allied monotypic genera "Ocythoe" and "Vitreledonella". Argonaut beaks are most similar to those of "Ocythoe tuberculata" and "Vitreledonella richardi". They differ in 'leaning back' to a greater degree than the former and because they have a more curved jaw angle than the latter.
Feeding mostly occurs during the day. Argonauts use tentacles to grab prey and drag it toward the mouth. It then bites the prey to inject it with poison from the salivary gland. They feed on small crustaceans, molluscs, but also jellyfish and salps. If the prey is shelled, the argonaut uses its radula to drill into the organism, then inject the poison.
Argonauts are capable of changing their color. They can blend in with their surroundings to avoid predators. They also produce ink, which is ejected when the animal is being attacked. This ink paralyzes the olfaction of the attacker, providing time for the argonaut to escape. The female is also able to pull back the web covering of her shell, making a silvery flash, which may deter a predator from attacking.
Argonauts are preyed upon by tunas, billfishes, and dolphins. Shells and remains of argonauts have been recorded from the stomachs of "Alepisaurus ferox" and "Coryphaena hippurus".
Male argonauts have been seen living inside salps, which are planktonic tunicates. Little is known about this relationship.
The genus "Argonauta" contains up to seven existing species. Several extinct species are also known.
The extinct species "Obinautilus awaensis" was originally assigned to "Argonauta", but has since been transferred to the genus "Obinautilus".
Protection
Protection means when any group of people or things try to defend the people or things that they care about (so that they would not get hurt). Protection can also mean other things, for example:
Minor
Minor can mean:
Camouflage
Camouflage is a visual disguise. Without it, an animal would be recognised easily. If the natural colour of an animal makes it look like its surroundings, that is camouflage. A tiger's stripes in the long grass, and the battledress of a modern soldier are examples.
Camouflage is a form of deception. The word camouflage comes from the French word "camoufler," which means "to disguise".
In nature, most animals blend into their environment or conceal their shape. They are very hard to see. This way they survive, and if they survive, then they can reproduce. There are exceptions: animals which are dangerous to eat (e.g. wasps) advertise with warning colouration.
Prey animals hide from predators. Predators must search for prey without being seen. Natural camouflage is one way to do this: an animal can blend in with its surroundings. Another way is for the animal to disguise itself as something harmless.
Some camouflaged animals also copy movements in nature, e.g., of a leaf blowing in the wind. Other animals attach natural materials to their body for concealment. A few animals change color in changing environments. Seasonally: (many Arctic animals, such as the arctic fox, or arctic hare). Or quickly, like the chameleon and the cuttlefish. Some herd animals, like zebra, have a pattern which makes it difficult for the predator when they are running.
Mimicry is a special kind of camouflage, where an animal or plant looks like another, usually one which is unpleasant to eat or dangerous.
Most animals are dark on top and light underneath. With light coming from the sky, this "countershading" makes them less visible. That this is the correct explanation is proved by cases where the animal lives upside down. A famous example is the Nile catfish, which swims belly uppermost. In this case, the belly is dark, and the back is light in colour.
Transparent or partly transparent animals are common in the pelagic layer of the sea. These are the layers into which light penetrates. The effect of transparency or silvering works is better under water than above it. This is because the amount of light which penetrates is less and less as the depth increases. Also, the proportion of the light reflected is much less under water (0.001–0.6%) than in air (2–5%).
Extant
In biology, an extant group is one which has survived to the present day, and so it is a living group. This distinguishes it from an extinct group.
The term 'extant' can also be used to describe species or higher groups (such as genera or families) which are still in existence (living).
Example: in the group of molluscs known as the cephalopods, there are about 600 extant species and 7500 extinct species.
Echinoderm
Echinoderms are a successful phylum of marine animals. They include sea stars, brittle stars, sea urchins, sea cucumbers and their relatives.
Echinoderms have these features:
Echinoderms live in all parts of the ocean, but mostly on the sea floor. Some are filter feeders, and others (starfish) are important predators of molluscs and other shell-fish. They are extremely common near the shore, and on reefs.
They have a long and abundant fossil record. This phylum appeared in the early Cambrian period; it contains about 7,000 living and 13,000 extinct species. The four or five main groups are called sub-phyla by some authorities, and classes by others.
Echinodermata is the largest animal phylum which is entirely marine: no animals in this group live on land or in fresh or brackish water.
For many purposes it is better to use the following classification. It is easier to learn and understand, and is based more on the living forms.
Only with the crinoids is it necessary to deal with the fossil forms, because the stalked crinoids were so important on the Palaeozoic sea floors.
All echinoderms have fivefold radial symmetry in their body at some stage of life. The holothurians have bilateral symmetry as adults, and do not have so much of a skeleton as other groups.
Echinoderms have a hydraulic water vascular system. This network of fluid-filled canals moves and feeds the animal, and allows gas exchange. There is also a complete digestive tube. They have a simple radial nervous system that consists of a modified nerve net. There are nerve rings with radiating nerves around the mouth extending into each arm. The branches of these nerves coordinate the movements of the animal. Echinoderms have no brain, although some do have ganglia. They usually move with their tube feet, but brittle stars use their arms to pull against rocks. Some feather stars and sea cucumbers can swim.