Why do cephalopods have a beak



Cirrus octopus or small octopus (Eledone cirrhosa) from the North Sea.
Photo: Erling Svensen.
classSpecies number
Snails (gastropoda)43.000
Mussels (Bivalvia)10.000
Barnacles (Scaphopoda)600
Single scarf (Tryblidia)20
Beetle snails (Placophora)750
Furchenf├╝├čer (Solenogastres)230
Shield-footed (caudofoveata)120
Molluscs (mollusca)55.400

The cephalopods are the largest marine invertebrates on earth. Their nervous system is much more developed than that of other mollusks and so they are capable of complex behavior that makes this class of molluscs appear comparable to vertebrates.

Their blueprint reveals the cephalopods as molluscs. They have no skeleton, neither inside nor outside. As with other molluscs, the body of the cephalopod is divided into head, viscera and foot, with the coat protecting the viscera.

Most recent cephalopods do not have an outer shell like the rest of the molluscs. Only the most primitive recent cephalopods, the genus nautilus, still swim through the ocean with a permanent outer shell. The fossil cephalopods, such as ammonites and belemnites, also had an outer shell.

The other modern (or recent) cephalopods have shells that are reduced to different degrees:

The so-called "squids" from the order of the cuttlefish have an inner shell, which is called a Schulp. Sepia shells are known to bird lovers as limestone for caged birds. In addition to their spawning packages, the shulps are usually the only components of cephalopods that wash up on the shore and that beach hikers can find.

Squids (Loligo), on the other hand, instead of the large Schulps, only have a narrow part of the case, which is known as a gladius. The largest and most highly developed cephalopods are the eight-armed octopuses of the genus Octopus. They are feared predators not only because they can see very well (they have eyes that can match those of vertebrates) and have very sophisticated behavioral patterns. Thanks to the fact that they do not have an obstructive shell, they can follow their prey into the smallest corner.

Small cephalopods, for example some octopuses, sometimes wait until the next high tide only in tide pools. Octopuses can even leave the water for a short time to get from one tide pool to another. However, cephalopods never live on land for long.

Body type of a cephalopod.


Already Alfred Brehm wrote about the cephalopods, "The eyes are extraordinarily large, not only in relation to the body, but also in themselves.". He was not the only one to make this point in his time. And in fact, the eyes of the cephalopod are among the most highly developed visual organs of the invertebrates.

Just like the vertebrates, lens eyes developed in the cephalopods. However, if one compares the fine structure of the cephalopod eye with that of a vertebrate eye, one notices similarities, but also clear differences:
Both eyes share a light-sensitive layer of sensory cells on the fundus - the retina. While in vertebrates the sensory cells are turned away from the incidence of light, so the light must first penetrate several cell layers before it reaches the light sensory cells, the light sensory cells of the retina of a cephalopod are turned towards the incidence of light. The eye of a cephalopod is called evers, the vertebrate eye is called inverse.

The reason for this different alignment of the retina is that the eye is created in different ways in vertebrates and molluscs: The eye of a cephalopod is created by the folding of the embryonic outer skin. Only then is this ectodermal eye cup supplied with nerves by the endodermal brain. The sensory cells therefore point outwards.

The vertebrate eye, on the other hand, is created by a protuberance of the interbrain. Firstly, like the optic nerve, the eye cup is created endodermally, and secondly, the sensory cells point inwards, as tissue is initially everted from the brain and then turned inside out to form the eye cup.

The lens is a special education. One can show a development path from the pit eyes of primitive snails to more highly developed pinhole camera eyes (nautilus), in which the opening of the pit has been made smaller at the expense of the light intensity of the image, but enables a sharper image, right down to the lens eyes of the highly developed squid.

Mollusc eyes


The scientific name Cephalopoda means cephalopod in Greek. It stems from the fact that all cephalopods have multiple arms or tentacles on their heads. The number and shape of these arms can be different. Octopus for example, it is so called because it has eight tentacles with suction cups. The squids (sepia) and squids (Loligo) have two long arms with suction cups on their widened ends. Eight shorter tentacles lead the prey, which the squid catches with the long arms, to the mouth opening. Squids have sensory cells around the suction cups that provide them with information about the nature of the object being held.

A special task of the cephalopod arm is that of the Hectocotylus (see: Chapter Reproduction). This redesigned arm is used by the male cephalopod to transfer sperm cells in a package called the spermatophore to the female. The tentacles of the cephalopods can also be used separately from the animal's body to a limited extent, as they have a powerful, partially autonomous, nervous system. Some paper boats (Argonauta), for example, separate the hectocotylus and allow it to find the female's mantle cavity on its own. Some severed squid arms have already been mistakenly described by naturalists as separate species because they were not identified as part of an animal's body.

In contrast to the higher cephalopods, the primitive pearl boats (nautilus) a significantly higher number of tentacles - up to 90 tentacles without suction cups distinguish Nautilus and its relatives from the rest of the cephalopods. Another common scientific name for the class of cephalopods is Siphonopoda. It describes the fact that the sipho and tentacles of the cephalopods are homologous to the characteristic foot of the molluscs.

Food intake and digestion

Like other mollusks, cephalopods have a radula, or rasp tongue, which they use to dissect the soft parts of their prey. In addition to this, they have developed the upper jaw into a horny parrot-like beak. Most cephalopods live on at least crustaceans, among other things. They dissect the shell of their prey with their beak in order to eat the soft parts of their body with the radula.

Octopuses often digest their prey externally. After cutting a hole in the prey's outer shell with their hornbills, they inject saliva into the wound. The saliva contains a neurotoxin that paralyzes the prey. It also contains various enzymes, of which chitinases dissolve the shell of the prey and proteases liquefy their internal organs so that the octopus can then suck them up.

Gills and breathing

Cephalopods are gill breathers, they belong to those groups of molluscs that have never left the water in the course of their evolution.

The recent cephalopods have two gills. Only the primitive pearl boats (nautilus) have four gills and are therefore referred to as four-gill (Tetrabranchiata or Nautiloida) in contrast to the two-gill (Dibranchiata or Coleoida). Four gills and the fact that Nautilus still has an ancient shell make it a living fossil, along with other ancient features (such as its ancient lensless pinhole camera eyes).

Cephalopods have multiple hearts. A main heart takes over the transport of the blood in the largely closed blood circulation, two gill hearts have a supportive effect. In contrast to the open blood circulation of the other mollusc classes, the closed blood circulation represents a further development in the context of the evolution of the cephalopods.

Locomotion and Defense


Some squids (e.g. the octopus) migrate on their tentacles along the ocean floor. Others, like the sepia, move with the help of their fin seams, which are located on the edge of the mantle. Especially when they have to flee, but in the case of the squid also for normal locomotion, squids squeeze water out of the mantle cavity and use the recoil to move backwards, which is considerably faster than locomotion with the fin fringes. The cephalopods can use the siphon as a control organ by directing the flow of water out of the mantle cavity.

As a defense, the squid drop a cloud of ink into the water. This confuses the attacker, also hinders his olfactory organs, so that he cannot orient himself and then flee from the danger zone. Some squids also use their ink glands to confuse their prey and then ambush them.

Some cephalopods are also poisonous: The poison of the blue-ringed octopus, for example, is primarily used to paralyze the prey. However, it is also very useful for defense. Blue-ringed octopuses, which are found around Australia, are as poisonous as snakes. Unlike these, however, they do not produce their own poison. They use symbiotic bacteria that can produce tetrodoxin, a poison that puffer fish and poisonous snails such as the side-gill snails also use.

Color change and camouflage

In this picture there is an octopus (sepia) hidden.

Many squids can also change color. Since they can control individual pigment cells (chromatophores), they are able to generate different patterns. For example, squids ready to mate have a different color during the breeding season than when they are not ready to mate. Of course, squids use their ability to change color, also for camouflage from enemies and when hunting. An octopus placed underwater on a chess board replicated the pattern in order to hide. What is particularly interesting is that octopuses cannot see colors, even though their eyes are very sophisticated.


In order to reproduce, the male squid transfers packets of seeds (spermatophores) into the female's mantle cavity. Octopuses develop a special arm for this, the hectocotylus. Pearl boats, on the other hand, have a mating organ that consists of four fused tentacles and is called a spadix. It is located next to the mouth in the middle of the remaining 90 or so tentacles.

The female then lays a number of eggs, often in a solid package, from which ready-made small squids hatch. In more developed cephalopods, such as the great octopus, the female even guards their clutch.


The conger eel (Conger conger) lives mainly from
Cuttlefish. Photo: Dubois.

Cephalopods are both hunters and hunted in the sea. Her favorite prey are fish and crustaceans. The nutritious molluscs are also hunted by many other marine animals. Numerous fish, e.g. moray eels, conger eels, sharks, but also toothed whales, from dolphins to large whales such as the sperm whale, enrich their diet with cephalopods. Sperm whales dive to great depths on the hunt for the giant squids of the deep sea. Up to 18 m tall specimens of this squid group occur in the North Atlantic. Based on the scars left by giant squid's suckers on the head of sperm whales, conclusions have been drawn as to the size of these squids and the conclusion that there may be even larger specimens in the deep sea, in the sperm whale's hunting ground. Parts of tentacles have also been found in the stomach, which lead to similar findings. However, such results are only reliable to a limited extent, as whales grow naturally and in some cases their scars enlarge. And so you will probably never know exactly how big octopuses and other cephalopods really get.

Use of cephalopods by humans

Octopus (Octopus) on the Naschmarkt in Vienna.
Image: Robert Nordsieck.

Squids are mainly used as food for humans. Squids and other cephalopods play an important role in the cuisine of different countries by the sea. In addition, cephalopods are used as bait for fishing.

The ink liquid is also used in the cosmetics industry. Use, as in earlier times for coloring films (sepia brown), is no longer relevant today in the age of color film. However, the sepia ink is used to color pasta (Nero di Seppia).

The squid schoolp (sepia) is used as a whetstone and lime reservoir for caged birds.

Indeed, more research is being done in the field of cephalopod physiology. For example, there is an enzyme in the nervous system of squids that can render nerve gases harmless. In addition, cephalopods are important objects of investigation for neurobiology and behavioral research.


It has now been found that the severed arms of octopuses can continue to function autonomously because they have their own nerve nodes. Movement impulses are sent from the brain to the arm and trigger there autonomously stored neuronal reactions that control the movement of the arm. For humans, for example, these findings represent interesting prospects for robot science: While a human body is less interesting as a model for a robot due to its low flexibility and the few anatomically possible movements, an octopus, especially with the autonomous function of the arms, could present interesting opportunities.

Myths and Legends

The legend of the multi-armed sea monster that lurks in the depths of the sea for the careless sailor is probably as old as seafaring itself. Long before the Swedish Bishop Olaus Magnus first mentioned the octopus at the beginning of the 16th century Homer in his Odyssey in the 7th century BC processed the legend of Scylla, a multi-armed sea monster that, sitting with its abdomen in a rock, uses its arms to fish sailors from the passing ships. Over 2000 years later, in his Natural History of the Nordic Peoples, Olaus Magnus worked on the legend of the octopus, which the Norwegian fishermen he met on his travels told him about. Similar reports come from a later time by Egede and Pontoppidan.

Giant octopus (Enteroctopus) are well known to science today - the largest live on the American west coast.

However, one of the last unsolved mysteries of the earth is the giant squid, which is always a subject of discussion. How big does the giant squid really get? After all, one speaks of sizes up to 25 meters and several tons of weight! Recently, in any case, a giant squid was washed ashore on the Australian coast, "only" 15 meters tall.

It is also interesting here that the authors of the relevant articles still cannot tell the difference between octopus and squid. But that's only a small part of the secret ...