|Cuvier , 1797|
The zoological class of cephalopods ( cephalopoda , from ancient Greek κεφαλή kephalē "head" and ποδ- pod- "foot") is a group of animals that belongs to the molluscs (Mollusca) and occurs only in the sea. There are both pelagic (free-swimming) and benthic (bottom-dwelling) species . There are currently about 30,000 known species that are extinct and 1,000 today. The cephalopods are the largest living molluscs. The largest giant squid found so far was 13 meters long. The extinct ammonites reached a housing size of up to two meters.
The name “Cephalopoda” was introduced by Georges Cuvier in 1797 and replaced the older term “ polyps ” ( πολύπους polýpous “wolverines”) handed down by ancient authors such as Aristotle and Pliny , which Réaumur had transferred to the cnidarians in 1742 and which was used in modern zoology is used exclusively in this sense.
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. The orientation of the body structure does not correspond to the preferred direction of movement. The front part of the foot is for cephalopods to the hopper and to eight, ten, or more than 90 catch arms ( tentacles developed). The cavity of the mantle, the so-called mantle cavity , usually contains two (in the case of the Nautiloidea: four) gills and opens out through a pipe (called a hyponom or funnel). The mouth is surrounded by stretchable tentacles. On the mouth of recent species there is a parrot-like beak with an upper and lower jaw and a rasp tongue ( radula ).
Shell, hard parts and floats
The more original species, such as the nautiloids and the extinct ammonites, have outer shells : a calcareous housing made of aragonite gives them protection and support as an exoskeleton . The shell of this case has three layers. Under the outer periostracum , the membrane made of the glycoprotein conchin , lies the outer prismatic layer ( ostracum ) made of prismatic aragonite. Like the septa , the inner layer, the hypostracum , is made of mother-of-pearl .
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, because too little water can be pumped into or out of the housing (about five grams), but it moves with the help of the recoil principle of the funnel away (also vertically in the water column). In the case of the extinct ammonites, however, there has recently been a discussion as to whether it was not possible for this group 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.
The outer-shelled cephalopods are only represented by the five or six species of the genus Nautilus . Some researchers also consider a species to be a representative of its own genus ( Allonautilus ). 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 30,000–40,000.
In the case of the inner-shell cephalopods, the hard parts are enclosed by the coat:
- The extinct belemnites have gradually reduced the living chamber 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 housing, at a sharp angle and converted them into a Schulp , which, however, still has a buoyancy function.
- The squids , on the other hand, have reduced the original chalky shell to a horny, elongated strip ( gladius ) in the mantle , with the loss of mineralization and thus of buoyancy , which only supports the body.
- In the octopus , the former housing is reduced to cartilage-like relict structures or even completely reduced.
Squids and octopuses have developed alternative buoyancy systems ( ammonia , oily substances, etc.). In today's seas, the inner-shell cephalopods (Coleoidea or Dibranchiata) dominate.
The cephalopod nervous system is the most efficient of both molluscs and invertebrates. This is especially true of the modern cephalopods (Coleoidea), in which the large nerve nodes (cerebral ganglia, pedal ganglia, pleural ganglia) are fused into a complex structure that can be called the brain . The nervous system of the ten-armed squid (Decabrachia) is also characterized by giant axons , the speed of which is as high as that of the axons of vertebrates. Experiments with separated octopus poor showed that the arms above their own, autonomously acting nerve centers have, by certain reflexes may be triggered, for example when foraging or hunting, independent of the brain. The pair of eyes on the side of the head work in the nautilids according to the pinhole camera principle . Modern cephalopods have everse (equipped with light- facing photoreceptors in the retina ) and ontogenetically turned-in lens eyes analogous to the inverse (with light-turned-away photoreceptors in the retina) and ontogenetically turned-up lens eyes of vertebrates , which is a classic example of convergent evolution . The cephalopod brain has large optical praise for rapid processing of the optical stimuli.
Cephalopods are considered to be the most intelligent invertebrates. Behavioral experiments show that the cognitive abilities of octopuses partly approach that of dogs. They are capable of abstraction (e.g. counting up to 4 or differentiating between different shapes) and solving complex problems (e.g. opening the screw cap of a jar to access the contents, although their anatomy also serves them well).
As active predators, cephalopods rely primarily on locomotion using the recoil principle . Here, the space between the head and the mantle wall, and thus also the volume of the mantle cavity, is reduced in most representatives by the contraction of the circular muscles of the mantle. As a result of the overpressure in the mantle cavity, the water is forced out through the funnel, which causes the body to move in the opposite direction. The direction of movement can be varied by changing the position of the funnel. The side fins serve to stabilize squids and to “float” and propel cuttlefish, whose side fins line a large part of the mantle, 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 to escape.
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 in the mantle cavity , this type of breathing is linked to movement. In squids and octopods, a smaller part of the respiration was attributed to the skin . 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 detritus -eating vampire squid are cephalopods active predators that live exclusively on animal food. The prey is perceived visually and grasped with the tentacles, which are equipped with suction cups. In squids, these suction cups have small hooks. Sepia and nautilus feed mainly on small invertebrates that live on the sea floor . Squid prey includes fish and shrimp, which are paralyzed by a bite in the neck. Octopods are nocturnal hunters and primarily hunt down snails, crustaceans and fish. To effectively kill their prey, octopods have a paralyzing poison that is injected into the prey. After being ingested by parrot-beak-like jaws (made from chitin, among others ), 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.
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 deposited in grapes ( cuttlefish , octopus ) or in tubes ( squids ), 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 connected to the inner yolk sac (outer 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 changeover to independently preyed food. After hatching, adult cephalopods do not care for their offspring.
Dyes and bioluminescence
Cephalopods have special skin cells called chromatophores . These contain a pigment (dye) and are surrounded by tiny muscles that adhere to these skin cells. 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 frighten 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, z. In Sepia officinalis , for example, the many layers of the egg shell are provided with ink, which thus camouflages the embryos.
Within the squid more than 70 genera with bioluminescence are known. 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 aid 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.
Beneficiary of human environmental changes
The population sizes and distribution areas of many cephalopod species have increased significantly over the past 60 years. Studies suggest that the cephalopods' short life cycle enables them to adapt quickly to environmental changes. This is presumably accelerated by accelerated growth phases due to rising sea temperatures in the course of global warming . The cephalopods also benefit from the overfishing of their predators and food competitors .
Classification of large groups of cephalopods (hierarchical)
- Cephalopods - Cephalopoda
- The following groups are also summarized under the term new cephalopods (Neocephalopoda).
- Ammonites - Ammonoidea (Ammon's Horns) †
- Bactrites - Bactritoidea †
Squids - Coleoidea
- Belemnites - Belemnoidea (thunderbolts) †
- Ten-armed squid - Decabrachia
- Eight- armed squids - Octobrachia / Vampyropoda
Phylogenetic system of large groups of cephalopods
The family tree ( phylogenetic system ) of the cephalopods has not yet been fully elucidated. Is reasonably sure that the squid (Coleoidea) which Ammoniten (Ammonoidea) which bactritida (Bactritida) and parts (the subclass of the herein the Geradhörner Actinoceratoida ) a monophyletic forming group as neocephalopoda is referred to (Neocephalopoda), while all remaining cephalopods as pearl boats i. w. S. (Nautiloidea i. W. S.) 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 already in the Lower Devonian. Within the cuttlefish, the extinct belemnites (Belemnoidea) on one side and the eight-armed and ten-armed cuttlefish on the other side face each other as sister groups. The latter two sister groups are also known as new squid (Neocoleoidea).
Further content in the
sister projects of Wikipedia:
|Commons||- multimedia content|
- Tree of Life - Cephalopoda
- CephBase In: thecephalopodpage.org
- Cephalopods in the Fossil Atlas – WiKi In: mineralienatlas.de
- Squid Archive TEUTHIS In: tintenfische.com
- Kopffüßer (Cephalopoda) In: weichtiere.at
On the phylogeny of the squid
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- L. Bonnaud, R. Boucher-Rodoni, M. Monnerot: Phylogeny of Cephalopods Inferred from Mitochondrial DNA Sequences. In: Molecular Phylogenetics and Evolution. 7. 1997, pp. 44-54.
- DG Carlini, JE 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, pp. 57-76.
- DB Carlini, RE Young, M. Vecchione: A Molecular Phylogeny of the Octopoda (Mollusca: Cephalopoda) Evaluated in Light of Morphological Evidence. In: Molecular Phylogenetics and Evolution. 21. 2001, pp. 388-397.
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- Gerhard Haszprunar, Klaus-Jürgen Götting: Cephalopoda, Kopffüßer. S. 352–362 in: W. Westheide, R. Rieger (Ed.): Special Zoology Part 1: Protozoa and Invertebrates. 2nd Edition. Spektrum Akademischer Verlag (Elsevier), Munich 2007, ISBN 978-3-8274-1575-2
- German Sumbre, Yoram Gutfreund, Graziano Fiorito, Tamar Flash, Binyamin Hochner: Control of Octopus arm extension by a Peripheral Motor Program. Science. Volume 293, No. 5536, 2001, pp. 1845–1848, doi: 10.1126 / science.1060976 (alternative full text access : Sensory Motor Performance Program, RIC ( Memento of the original from November 20, 2016 in the Internet Archive ) Info: The archive link was automatically inserted and not yet checked. Please check the original and archive link according to the instructions and then remove this note. ); see also Tom Clarke: Octopus arms strike out alone. Nature News, September 7, 2001, doi: 10.1038 / news010913-1
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- Marian Y. Hu, Hong Young Yan, Wen-Sung Chung, Jen-Chieh Shiao, Pung-Pung Hwang: Acoustically evoked potential in two cephalopods infered using the auditory brainstem response (ABR) approach. Comparative Biochemistry and Physiology, Part A: Molecular & Integrative Physiology. Volume 153, No. 3, 2009, 278-283, doi: 10.1016 / j.cbpa.2009.02.040 (alternative full-text access : ResearchGate ).
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- S. Hunt, M. Nixon: A comparative study of protein composition in the chitin-protein complexes of the beak, pen, sucker disc, radula and oesophageal cuticle of cephalopods. Comparative Biochemistry and Physiology Part B: Comparative Biochemistry, Vol. 68, No. 4, 1981, pp. 535-546.
- Gabriele Kerber: Climate Change: Squid Instead of Sprats. In: Spektrum.de. July 1, 2020, accessed July 1, 2020 .