How do cephalopods eat

Octopus (Octopodidae)


Suckers of a giant octopus (E. dolfleini)
Image: Sami from kelpdiver.com.
 

The term octopus, by which an octopus is also known in common parlance, comes from the Greek. He describes the characteristic that most obviously distinguishes octopuses and their relatives from other cephalopods: octopuses and paper boats have eight instead of ten tentacles, in contrast to squids and squids.

The number of their arms is not the only characteristic in which the octopuses differ from the other cephalopods: In the octopus the tendency to regression of the shell, which can already be recognized in the other cephalopods, is complete: they lack any sign of an internal one The shell, which is designed as a "Schulp" or "Gladius" in the other cephalopods, can be seen from the ancient pearl boats, which even have an outer shell.

Without an inner shell, octopuses are highly flexible and can squeeze through the tightest of openings. Octopuses often hide in the most interesting containers, such as the young of Octopus vulgaris in the beer can shown on the right.

In contrast to the other cephalopods, octopuses mainly live benthically, i.e. on the ocean floor. Like the Scylla from Homer's Odyssey, they like to hide their soft abdomen in a crevice in the rock and reach with their eight tentacles into the surrounding water to catch crabs, snails and mussels, but also other octopuses.

Since, like the rest of the cephalopods, they breathe through their gills, octopuses cannot survive on land permanently. For a limited time, however, octopuses are able to leave the water and migrate on land as well. At low tide, octopuses often hide in tide pools on the coast, where they catch crabs and snails. However, some octopus species also swim through open water, for example to get from one reef to another. They propel themselves by expelling water from their mantle cavity. The Sipho is used to determine the direction. Octopuses do not use the eight tentacles for swimming, but rather pull them behind them in their wake.


Breathing octopus with clearly visible cloak cavity and siphon.
Photo: Jim Cosgrove.
 

Swimming blue-ringed octopus on flickr.com.

Octopuses use their tentacles to catch their prey, much like other cephalopods. After the prey is grasped, it is pulled towards the mouth opening where the powerful horned beak makes a hole in the shell or armor of the prey - most octopus prey - crabs, crabs, snails, clams, and others. - have either an outer armor or a shell - bites. The octopus then dissolves the prey's internal organs with its digestive juice and soaks up the protein juice.

 
Giant octopus (Enteroctopus dolfleini) eats a crab.
Image: Sami from kelpdiver.com.

The hiding place of an octopus can often be recognized by the remains of its prey (shells and empty tanks) lying around.

Some octopuses can distribute sensitive bites when they have to defend themselves, with some, such as the aforementioned Australian blue-ringed octopus, the digestive juice is highly toxic, so that every year (sometimes fatal) accidents occur due to careless divers or beach walkers who find an octopus in a tide pool and try to pick it up.

Especially when attacked, octopuses can use another weapon - their ink gland. This bladder-shaped organ is a side gland of the rectum and produces a dark liquid that the octopus can release into the water in the form of a cloud through the mantle cavity and siphon. With added mucus, the octopus can even shape the cloud and, while the attacker is attacking the cloud, retreat after it has dyed itself in the surrounding colors to camouflage itself.

Octopuses only live one to two years. Their life usually ends with the mating and the subsequent rearing of the young. While the male usually dies after mating, female octopuses sometimes survive considerably longer.

During mating, the male octopus transmits a spermatophore to the female with a tentacle that is converted into a spoon-shaped end. This converted tentacle, which is also found in other cephalopods, is called a hectocotylus. The female octopus can store the contents of the spermatophore in a blister-shaped organ, the receptaculum seminis (cf. also the genital apparatus of the Roman snail), until fertilization occurs separately from mating and under favorable conditions for egg-laying and the development of the young . The female octopus looks for a suitable cave to lay eggs and lays the eggs there in strings or packages. During the entire development of the young octopus, the female octopus ensures that the eggs are clean, fanning them with oxygen-rich water and defending them against enemies. During this entire development period, the female octopus never leaves the clutch and therefore eats very little. After the young octopuses hatch, the female usually dies of exhaustion or is killed by enemies against whom she can no longer defend herself. After the mother's death, the newly hatched octopus larvae are left to their own devices for the first two months, in which, in contrast to the benthic way of life, they live pelagically. The expected losses are offset by the average number of up to 150,000 eggs per clutch of an octopus.

The nervous system of the octopus is very efficient in several ways, so that the brain of the octopus, an invertebrate mollusc, is superior to that of a fish, a vertebrate. The optical cerebral lobes and the control centers for the color cells are particularly well developed. The octopus eyes are the most developed visual sense organs of the invertebrates. They are lens eyes that can withstand comparison with the eye of a vertebrate. Since the eyes of the molluscs developed in a different way than those of the vertebrates, they are structured differently despite the same functionality. The retina of an octopus eye is created as part of the eye cups and is only supplied later by nerve fibers of the optic nerve. This is why the light-sensing cells in an octopus' retina point inside the eye. This is where the everse mollusc eye differs significantly from the inverse vertebrate eye, in which the light sensory cells of the retina are only secondarily created by the growth of the optic nerve into the eye cup and therefore point to the fundus.

Also noteworthy is the innervation of the octopus tentacles. The octopus-like Argonauts can, for example, detach their Hectocotylus and this finds its way into the female's mantle cavity on its own. Even involuntarily severed arms of the octopus (Argonauts are the only octopus-like that sever their Hectocotylus) move on alone after the separation from the rest of the body. In general, octopuses do not have to control all movements from the brain, but the neural patterns stored in the nerve nodes of the tentacles allow at least simple movement sequences.

Particularly noteworthy is the innervation of the suction cups, whose chemical sensory cells provide the octopus with information about the nature of the touched matter, as well as the neuronal control of individual color cells (chromatophores) in the skin, which allow the octopus to change its color. In addition to their flexible shape, octopuses, like other cephalopods, can control their color by controlling individual color cells in their skin. They use this ability to camouflage, but also to communicate with other members of their own species. The blue-ringed octopuses of Australia and Southeast Asia only show their coloring with the characteristic blue rings when they feel threatened. Otherwise their color is rather inconspicuous yellowish-brownish. However, the color of the octopus is not only a warning color, but above all also communication, especially of course with other species. In contrast to the squid, octopuses are solitary animals who defend their territory against other octopuses or eat smaller conspecifics as well as other molluscs.

 
Octopus in the maze.
Source: Quarks & Co. (WDR).

In addition to the performance of their nervous system, octopuses are very capable of learning. You can remember colors and shapes and you can also find ways out of complex labyrinths. It has been proven that octopuses can open sealed cans and uncork bottles to gain access to prey. The octopuses also learn by observing a process, i.e. without having tried the process themselves beforehand. In an experiment for the octopus to learn shapes, it was possible to prove that octopuses could combine colors with food that was subsequently served and that they could also distinguish simple geometric shapes. Other octopuses kept in the neighboring aquarium acquired this knowledge by observing the learning process of the experimental octopus through the panes of the aquariums.

The ability of octopuses to learn can be explained by their evolution in competition with other highly developed marine animals. Octopuses must therefore be able to learn particularly well and quickly, since they have no opportunity to pass on acquired skills due to their short lifespan (octopuses can reach a maximum of 4 years old) and the fact that the mother dies shortly after the young have hatched. So all skills have to be acquired again and again.

Octopuses are found in all seas on earth. They reach sizes from a few centimeters up to the Pacific giant octopus (Enteroctopus dofleini), which reaches spans of over 4 meters. They live in many different habitats between the intertidal zone and up to 7 kilometers deep. In the Mediterranean, for example, the common octopus (Octopus vulgaris) with up to 1 meter arm length and the slightly larger white point octopus (Octopus macropus) in front. Smaller species are cirrus octopus (Eledone cirrhosa) and musk octopus (Eledone moschata) with up to 40 cm arm length.

The largest known octopus is Enteroctopus dofleini, the (North) Pacific giant octopus found on the American northwest coast. It reaches an arm length of over 2 meters, according to unconfirmed information even specimens are known that had arms over 3 meters long and thus had a span of up to 7 meters.

In Australia and Southeast Asia comes the genus of the blue-ringed octopus (Hapalochlaena) in front. These small octopuses (arm length up to 10 cm) are not only known for their characteristic warning color but also for the high toxicity of their saliva, which is transmitted when they bite. The saliva of the blue-ringed octopus contains tetrodoxin, a neurotoxin that paralyzes the voluntary muscles and kills them within a few hours of being bitten.

A total of around 200 species of octopus are known, many of which, however, have not yet been clarified or are undetermined.