Why is octopus grouped in cephalopods

The zoological class of Cephalopods (Cephalopoda) is a group of animals that belongs to the molluscs (Mollusca) and occurs only in the sea. There are both pelagic, i.e. free-swimming species, and benthic species, i.e. those that live on the ground. There are currently about 1000 known species alive today and over 30,000 fossil species. [1]

Outline of the body

Cephalopods have a body that consists of a body part (with an intestinal sac), a head part with attached arms and a pocket-shaped coat on the belly side. The orientation of the body structure does not correspond to the preferred direction of movement. In cephalopods, the front part of the foot is developed into a funnel and eight, ten or over 90 tentacles. The cavity of the mantle, the so-called mantle cavity, usually contains two (in the Nautiloidea: four) gills and opens out through a tube (called a hyponom or funnel). The mouth is surrounded by extensible tentacles. In recent species there is a parrot-like beak with an upper and lower jaw and a rasp tongue (radula) on the mouth.

Median section through a housing of a pearl boat (nautilus) with living room and chambered phragmocone

Shell, hard parts and floats

Scheme of the soft tissues of a nautilus

The more original species, such as the nautiloids and the extinct ammonites, have a calcareous housing made of aragonite, which gives them protection and support as an exoskeleton. The shell of this housing has three layers: under the outer periostracum, the membrane made of the glycoprotein conchin, lies the outer prismatic layer (ostracum) made of prismatic aragonite. The inner layer, the hypostracum, consists of mother-of-pearl like the septa. The housings are divided into the actual living chamber and a section with gas-filled chambers (phragmocone). With the help of this gas-filled part, the housing can be kept floating in the water column. Today's nautilus (Pearl boat) cannot use the housing for ascent and descent in the water column, since too little water can be pumped into or out of the housing (approx. 5 g), but it moves with the help of the recoil principle of the funnel ( also vertically in the water column). In the case of the extinct ammonites, however, it has recently been discussed whether this group was able to ascend and descend in the water column by changing the gas or water volume in the chambers. However, the internal structures of the shells of Nautiloidea and Ammonoidea are also very different.

In the case of the inner-shelled cephalopods, such as cuttlefish, belemnites, spirula or some squids, the hard parts are enclosed by the coat. The extinct belemnites have gradually reduced the living chamber down to a dorsal spoon or stick (Proostracum). In contrast to ammonites and nautilus, spirula has a lime spiral which is oriented towards the belly and is divided into individual gas-filled chambers. In addition, the cuttlefish have placed the original septa, which were originally approximately perpendicular to the longitudinal axis of the case, at a steep angle and converted them into a schoolp, which, however, still has a buoyancy function. The squids, on the other hand, have reduced the original calcareous casing to a horny, elongated strip (gladius) in the mantle, with loss of mineralization and thus of buoyancy, which only supports the body. In the case of the octopods, the former housing is reduced to cartilage-like relict structures or even completely reduced. Squids and octopods have developed alternative buoyancy systems (ammonia, oily substances, etc.). In today's seas, the inner-shell cephalopods (Coleoidea or Dibranchiata) dominate.

The outer-shelled cephalopods are now only made up of the five or six species of the genus nautilus represents. Some researchers also consider a species to be a representative of its own genus (Allonautilus) understood. From the fossil record, over 10,000 species of extinct nautiloids (pearl boat-like) have been described. The number of extinct ammonites has not yet been precisely recorded, but is likely to be in the range of around 30-40,000.

Nervous system

The nervous system of the cephalopods has a highly developed brain and is characterized by giant axons that are as fast as vertebrates. The arms also have their own autonomous nerve centers, so severed arms are able to approach food. There are powerful eyes on the head: With Nautilus the eyes function according to the perforated chamber principle, with the other more recent species according to the principle of lens eyes (everse eye), which are constructed analogously to the vertebrate eyes (inverse eye) and represent a classic example of convergent evolution.

Furthermore, cephalopods have statocysts, which are located on the side of the brain and have gravitation, acceleration and noise[2][3] can perceive.

Cephalopods, especially octopuses, are some of the most intelligent invertebrates. Learning experiments show that they are more intelligent than, for example, reptiles. Octopuses are able to take objects out of closed jars with screw caps by unscrewing the lid.

Circulation system

Cephalopods are the only molluscs that have a closed circulatory system. In coleoids, the blood is pumped to the gills through two gill hearts that are located at the base of the gills. This leads to high blood pressure and rapid blood flow, and is necessary to support the relatively high metabolic rates of the cephalopods. The blood is enriched with oxygen at the gills. The now oxygen-rich blood is pumped to the rest of the body through a systemic heart.


Gills are the primary respiratory organs of the cephalopods. A large gill surface and a very thin tissue (respiratory epithelium) of the gill ensure an effective gas exchange of both oxygen and carbon dioxide. Since the gills are located in the mantle cavity, this type of breathing is linked to movement. In squids and octopods, although a smaller part of the respiration was attributed to the skin[4] As with many molluscs, oxygen is not transported in the blood of the cephalopod by iron-containing hemoglobins (as in vertebrates, among others), but by copper-containing hemocyanins. In addition, hemocyanins are not located in special cells (like hemoglobins in red blood cells), but are free in the blood plasma. When hemocyanins are not loaded with oxygen, they appear transparent and turn blue when they bind with oxygen.

Diet and Digestion

With the exception of the detritus-eating vampire squid, cephalopods are active predators whose diet consists exclusively of animal food. The prey is perceived visually and grasped with the tentacles, which are equipped with suction cups. With squids, these suction cups are provided with small hooks. The cuttlefish and nautilus feed mainly on small invertebrates that live on the ocean floor. Squid prey includes fish and shrimp, which are paralyzed by a bite in the neck. Octopods are nocturnal hunters and primarily hunt snails, crustaceans and fish. To effectively kill their prey, octopods have a paralyzing poison that is injected into the prey. After ingestion, the food enters the muscular digestive tract. Food is moved by peristaltic movements of the digestive tract and is digested mainly in the stomach and appendix. After passing through the intestine, undigested food leaves the body through the anus and, when the water is expelled from the mantle cavity, gets out through the funnel.


As active predators, cephalopods rely primarily on locomotion after the recoil drive. The space between the head and the mantle wall is closed by the contraction of the circular muscles of the mantle, thereby reducing the volume of the mantle cavity. The resulting pressure forces the water out through the funnel and pushes the body in the opposite direction. The direction of travel can be varied by changing the position of the funnel. The side fins serve for stabilization in squids and for cuttlefish, whose side fins encompass a large part of the mantle rim, they “float” and move through wave-like fin flapping. Octopods are associated with the seabed (benthos) and crawl with the help of their tentacles. However, they also use the recoil drive when escaping.


Many cephalopods have pronounced sexual behavior. After extensive foreplay, the male usually releases his sperm, which is packed in spermatophores, into the female's mantle cavity with one arm, the hectocotylus. In paper boats, however, the hectocotylus detaches from the male and actively swims in the female's mantle cavity, attracted by chemical messengers (chemotaxis). The egg cells of the female are fertilized when they emerge from the fallopian tube and can be laid in bunches (cuttlefish, octopus) or in tubes (loligo), which contain a large number of eggs. The female lays voluminous and extremely yolk-rich eggs. During embryonic development, the embryo feeds on the energy stored in the yolk. Female octopods clean the laid eggs with their tentacles and bursts of water.

The cleavage during embryogenesis is partially discoid and causes the developing embryo to grow around the yolk. Part of the yolk mass is shifted inwards (inner yolk sac); an often larger part of the yolk mass (outer yolk sac) connected to the inner yolk sac remains outside the embryo. Hatching occurs after or before the outer yolk has been used up. The inner yolk serves as a food reserve for the time between hatching and the complete switch to independently preyed food. After hatching, adult cephalopods do not care for their offspring.

The cephalopods are the largest living molluscs. The largest specimen found so far belongs to the giant squid and was 13 m long. Ammonites reached a housing size of up to two meters.

Physiological peculiarities

Cephalopods have special skin cells called chromatophores. These contain a pigment (dye) and are surrounded by tiny muscles that adhere to them. When these muscles are tensed, a chromatophore cell expands and changes color in this part of the body. The selective expansion and contraction of chromatophores enables the color and pattern of the skin to be changed. This plays an important role in camouflage, warning and mating behavior, among other things. For example, in stressful situations, cuttlefish let stripes of color run across the body like waves and can adapt to a chessboard in color and pattern.

With the help of brown or black ink (made up of melanin and other chemical substances), cephalopods can scare and deceive their predators. The ink gland lies behind the anus and releases the ink through the mantle cavity and further out through the funnel. Furthermore, e.g. Sepia officinalis inked the many layers of the egg shell, thus camouflaging the embryos.

Over 70 genera with bioluminescence are known within the squid. In several genera this is produced with the help of symbiotic bacteria; in the other genera, however, by a reaction of luciferin and oxygen with the help of the enzyme luciferase. In this way, bioluminescent cells, so-called photophores, can be used for camouflage and for mating behavior (in deep-sea octopods). In addition, bioluminescent particles can be ejected with the ink.


Classification of large groups of cephalopods (hierarchical)

  • Cephalopods - Cephalopoda
    • Pearl boats in the broad sense - Nautiloidea in the broad sense; partly also called cephalopods (Palcephalopoda). This also includes parts of the straight horns (Orthocerida) and other nautiloids that are usually not or only slightly curled (Ascocerida, Oncocerida, Discosorida, Tarphycerida)

Phylogenetic system of large groups of cephalopods

The family tree (phylogenetic system) of the cephalopods has not yet been fully elucidated. It is reasonably certain that the squid (Coleoidea), the ammonites (Ammonoidea), the Bactriten (Bactritida) and parts of the straight horns (here the subclass of the Actinoceratoida) form a monophyletic group, which is also known as New Cephalopods (Neocephalopoda), while all remaining cephalopods are summarized as pearl boats iwS (Nautiloidea iwS) or also as cephalopods (Palcephalopoda). This second group, however, is probably paraphyletic, since the new cephalopods have with some certainty emerged from the cephalopods.

The ammonites arose in the Devonian from Bactrite-like ancestors. The cuttlefish (Coleoidea) are also derived from the Bactrites. The Bactrites are therefore a para- or polyphyletic grouping that would have to be dissolved.

The cuttlefish (Coleoidea) originated from Bactrite-like ancestors in the Lower Carboniferous, possibly as early as the Lower Devonian. Within the squid, the extinct belemnites (Belemnoidea) on one side and the eight-armed and ten-armed squids on the other side face each other as sister groups. The latter two sister groups are also known as new squid (Neocoleoidea).

Selection of literature on the phylogeny of cuttlefish

  • Berthold, T. & T. Engeser: Phylogenetic analysis and systematization of the Cephalopoda (Mollusca). In: Negotiations of the Hamburg Natural Science Association. new series, 29th 1987, 187-220.
  • Bonnaud L., R. Boucher-Rodoni, M. Monnerot: Phylogeny of Cephalopods Inferred from Mitochondrial DNA Sequences. In: Molecular Phylogenetics and Evolution. 7. 1997, 44-54.
  • Carlini D. G. & J. E. Graves: Phylogenetic analysis of cytochrome c oxidase I sequences to determine higher-level relationships within the coleoid cephalopods. In: Bulletin of Marine Science. 64. 1999, 57-76.
  • Carlini D. B .; R. E. Young & M. Vecchione: A Molecular Phylogeny of the Octopoda (Mollusca: Cephalopoda) Evaluated in Light of Morphological Evidence. In: Molecular Phylogenetics and Evolution. 21. 2001, 388-397.
  • Haas, W .: Trends in the Evolution of the Decabrachia. In: Berlin Paleobiological Treatises. 3. 2003, 113-129.
  • Vecchione M., R. E. Young & D. B. Carlini: Reconstruction of ancestral character states in neocoleoid cephalopods based on parsimony. In: American Malacological Bulletin. 15. 2000, 179-193.
  • Zheng X.D., J. Yang, X. Lin & R. Wang: Phylogenetic relationships among the decabrachia cephalopods inferred from mitochondrial DNA sequences. In: Journal of Shellfish Research. 23. 2004, 881-886.

see also

Cephalopod intelligence

Web links

Template: Commonscat / WikiData / Difference

Individual evidence

  1. ↑ Westheide, Wilfried, Rieger, Reinhard: Special Zoology Part 2, 2nd Edition, Spektrum Verlag (2007)
  2. ↑ Kaifu et al. (2008) Underwater sound detection by cephalopod statocyst. Fisheries science 2008; 74: 781-786.
  3. ↑ Hu et al. (2009) Acoustically evoked potential in two cephalopods infered using the auditory brainstem response (ABR) approach. Comp Biochem Physiol 153: 278-283.
  4. ↑ Madan and Wells (1996) Cutaneous respiration in Octopus vulgaris. J Exp Biol 199: 683 2477-2483