CEPHALOPODA
CEPHALOPODA
Origins: 452 million years ago (late Ordovician period, Katian stage)
Extinction: Still extant
Cephalopods form the mollusk class known as Cephalopoda (coming from the Greek kephalópodes, meaning "head-feet") and are represented by extant creatures such as nautili, squid and octopi. The study of cephalopods is known as teuthology (the study of mollusks in general is called malacology).
There are around 800 living species of cephalopods, though more keep getting discovered. They are found in all oceans of planet Earth. They are incapable of moving into freshwater due to multiple biochemical constraints, and over their long evolutionary history, they never ever ventured to freshwater courses.
Cephalopods are known to be quite intelligent, some captive ones have been observed getting out of their aquarium, travel out of it, move to other aquarium to feed on the local residents, and sneakily return to their home aquarium before anyone notices. Their brain is protected by a cartilaginous cranium. Some cephalopods are capable of gliding in the air for distances of up to 50 meters, after jumping from the water. Some of these cephalopods may spread their tentacles in a flat fan shape or in a circular arrangement to remain aerodynamic. Cephalopod eyes are very advanced, capable of sensing the plane of polarization of the light. Nautili are the least advanced, however, being a simple pinhole eye, through which water can pass through. Despite their advanced vision, most cephalopods are actually color blind, even though there is some level of spectral discrimination at play. They may often rely on chromatic aberration, a failure of the eye lens to focus all wavelengths in the same point, to detect variations in color. Molecular evidence shows that cephalopod chromatophores (color-changing cells) can be photosensitive and change their color in response to changes in light conditions. Many cephalopods have an assemblage of skin components that interact with light. Bioluminescence can sometimes be used to provide camouflage, shining lights downwards to mask their shadows. That bioluminescence is produced by bacterial endosymbionts. That bioluminescence can also be used to attract prey, mating, threat displays and even communication. As their chromatophores expand and retract, they can change their colors in a fraction of a second, and that can be used for signalling or for camouflage. Some soft-bodied cephalopods may mimic surrounding objects and animals, rather than just replicating the colors of the surrounding environment, to perfect their camouflage skills. Cephalopods also have iridophores, cells that can reflect light so to produce colors that chromatophores cannot on their own, so they may use both iridophores and chromatophores to produce unique colors. Thanks to that, cephalopods are capable of turning from a dull color to a very bright and aggressive one in little time, which can be used to scare away threats. There are different theories to explain why color changing capacities evolved in cephalopods, with one hypothesis defending the earliest function was for social and sexual purposes. Another hypothesis defends that its primary purpose was to evade predators and to perfect stealth hunting. The first hypothesis requires early cephalopods to have high visual acuity to detect the complex color-based signalling of other members of their species. Natural selection also requires the existence of pre-existing parameters for the evolution of this feature. One of them requires cephalopods to have coexisted with predators that relied a lot on sight to hunt them. Comparisons with the color-changing chameleons indicate that the greatest camouflaging capacities were tied with more conspicuous social signalling capacities, implying that color changing likely evolved from a social context and that camouflage would be the inevitable byproduct of that evolutionary pressure. As noted, cephalopod color changing capacities, despite impressive, are barely unique, and are found in a number of reptiles, amphibians, other fish, insects, spiders and other mollusks. With the exception of nautiloids, cephalopods can eject a cloud of ink when threatened, a blanket of a black substance that works as a smokescreen to confuse a predator and allowing the cephalopod to escape. Cephalopods are the only mollusks with a closed circulatory system. Coleoid cephalopods have two gill hearts that move blood through the capillaries of the gills. Then a single systemic heart pumps the oxygenated blood through the rest of the body. They have hemocyanin in their blood, unlike most vertebrates, which have hemoglobin, this one being more efficient in oxygen-rich acidic water, but less efficient than hemocyanin when in the context of cold and poorly oxygenated waters. Unlike hemoglobin as well, hemocyanin isn't attached to a red blood cell, and its instead a molecule that is floating freely in the blood stream. Cephalopods exchange gases by forcing sea water through their gills. Water enters the mantle and then the mantle contracts, forcing water through the gills. Fast moving cephalopods tend to have small gills, as water will be passed through them quickly, compensating for the small size. Cephalopods are one of the few invertebrates that are capable of very fast motions in the water column, in ways comparable to vertebrates, at least. Cephalopods usually have to move backwards, as water is propelled through their siphon, which is located near the tentacles, but the siphon can change directions to coordinate changes in motion. The first cephalopods, such as nautiloids, would have created a jet by undulating their body into their shells, as the slower flow of water is more suited to extract oxygen from the water. If nautiloids don't move, they can only extract 20% of the oxygen in the water. Nautiloids, at least the modern ones, are much slower than coleoids, in terms of jet propulsion. In comparison, squid can eject up to over 90% of the water within their mantle cavity in a single jet thrust, producing massive acceleration. Although most cephalopods float, they achieve that in different ways. Squids have it harder because they're negatively buoyant, so to maintain the same depth, they have to expend more energy to stay in the position they want. Some cephalopods can master their underwater movement to even mimick the swimming styles of surrounding animals. The mantle of modern soft-bodied cephalopods, namely squids and octopi, differ from each other. Octopi have more flexible mantles, so they need to keep flexing their longitudinal muscles to keep the mantle in the same shape as they swim. Squids instead have a tunic that keep the mantle in a more constant shape. This permits squids to have more efficiency in jet propulsion swimming. Thanks to that, they don't need to expend energy on musculature to keep the mantle in a fixed hydrodynamic shape. Collagen fibres is the material that permits the mantle to contract so actively. Although the first cephalopods had a shell, most modern ones lack it, often being reduced to a simple chitinous gladius found in the mantle of squid and octopi. Female argonaut octopi actually secrete a specialized egg case that resembles a shell, but that is not part of the animal. The basic arrangement of the cephalopod outer wall involves an outer (spherulitic) prismatic layer, a laminar (nacreous) layer and an inner prismatic layer. The muscular appendages that extend from the head of cephalopods, sometimes called arms or tentacles, have multiple uses including for feeding, mobility and even reproduction. The largest of cephalopod tentacles can reach up to 8 meters long. They may end in a broad sucker-coated club. Tentacles is the term refered to only a pair of, typically longer, appendages, with the remaining, typically four pairs, of appendages, are called arms, and those are used to holding and manipulate prey. The tentacle has a thick central nerve cord. It is surrounded by circular and radial muscles, permitting them to retract and increase their length effectively without losing volume. The digestive gland of cephalopods is rather short. Cells in the digestive gland directly release pigmented excretory chemicals into the lumen of the gut, which are then bound with mucus passed through the anus as long dark strings, ejected with the aid of exhaled water from the funnel. Some cephalopods have specialized chemotactile receptors in their arms to allow them to taste what they touch. Their radula has teeth that are variable in terms of structure, depending on the species. Most cephalopods have a pair of large nephridia used for their excretory system, with waste being released into the mantle cavity through a pore. Because protein is a major constituent of their diet, they produce large amounts of ammonium ions as waste. The gills are the main organs responsible for expelling excess ammonium. Most cephalopods don't provide parental care, except for a few that stick around the development of their egg clutch to increase their chance of survival. Developing cephalopod embryos are greatly affected by changes in temperature, salinity and oxygen saturation, so that influences a lot on their survival probabilities. Food availability also influences the time of spawning. After mating, coleoid cephalopods die. Sexual maturation is attained when the gonads of the cephalopod become enlarged. Another indication of sexual maturity comes from females, that start developing brachial photophores to attract mates. Cephalopods are not broadcast spawners. The males have to fertilize the female internally, contracting the mantle several times to release the sperm. Some cephalopods swell their eggs with perivitelline fluid (PVF), which prevents premature hatching. Because most cephalopods won't stick around to protect their eggs, some may cover them with ink to camouflage the embryos from predators. Often in the mating season, males may threat display to each other, as a form of competition for a female. In many cephalopods, females tend to be larger than males, in the most extreme cases, the female may be tens of thousands of times larger than the adult male. Cephalopod eggs can range from 1 to 30 millimetres in diameter. The shells, whether external or internal, develop from the ectoderm, in the case of internally shelled ones, the ectoderm forms an invagination where the shell forms inside. Juvenile cephalopods learn quickly how to hunt by experimenting in interaction with prey.
Cephalopods first evolved as shelled animals, with their ancestors developing a siphuncle that would have permitted them to fill it with gases and therefore making them buoyant, allowing them to keep their shells upright and crawl in the ocean floor more easily. The first cephalopods were probably already apex predators. Stem-cephalopods became incredibly diverse in the Ordovician, and since then they became abundant in the Paleozoic and Mesozoic seas. In their early days, the cephalopod lineage was abundant in shelled forms, from straight-shelled to coiled-shelled ones. The Devonian period saw a major early diversification period for the neocephalopods (including ammonites and coleoids) which probably was triggered by the paralel diversification of fish. The evolutionary origin of the cephalopod tentacles has been a matter of debate, and it was initially believed that they may have been homologous to the head tentacles of gastropods, though now they seem to be more agreed to be derived from the mollusk foot structure. Cephalopod genomes (and specially those of coleoids) are very strange and appear to show a lot of repetitions, leading some to believe cephalopods went through entire genome duplications. However subsequent analysis show that the genome of cephalopods contained patterns found in other marine invertebrates, indicating these genomic additions aren't that unique. In their genome, substantial replications of two gene families are observed. Most significantly, those gene families were previously only known to exhibit replicative behaviour in vertebrates. One gene family in question are the protocadherins, attributed to neuron development, whose patterns of gene expansion appear to have evolved independently in cephalopods and vertebrates. Analysis to the cephalopod genome also showed a significant presence of transposable elements as well as transposon expression that activated genomic diversity between neurons, when these were applied on fruit flies. Many cephalopod genes are dedicated to the nervous system, which seems to explain how they get their extraordinary intelligence. Modern cephalopod taxonomy is complicated to ascertain, though nautili are considered the most basal ones, with vampire squid lying outside of the natural squid group, though some analysis result in unwanted polytomies, feeding the uncertainty on this topic. There are three widely agreed subclasses of cephalopods. These are Nautiloidea (nautili and extinct kin), Ammonoidea (ammonites) and Coleoidea (octopi, squid, vampire squid and extinct kin). Other classifications split nautiloids into more groups, as very basal forms that resemble nautilids may actually retain a plesiomorphic shape and those shouldn't reflect on their actual phylogenetic placement as relatives of nautili. The coleoids, however, are very consistently regarded as monophyletic.
Cephalopods have been a great figure of inspiration for humans, serving as the base for impressive mythological creatures, such as the mighty Kraken. More marginally, cephalopods have also been associated with eroticism, specially in eastern Asia.
main source: Wikipedia
PHYLOGENY
- Bivalvia
- Cephalopoda
- Cenoceras intermedium
- Simoniteuthis michaelyi
- Teudopsis subcostata
- Clarkeiteuthis conocauda
- Passaloteuthis paxillosa
- Acrocoelites sp.
- Youngibelus tubularis
- Phylloceras heterophyllum
- Eleganticeras sp.
- Harpoceras subplanatum
- Lytoceras fimbriatum
- Catacoeloceras crassum
- Dactylioceras commune
NAME: Cenoceras intermedium
SIZE: 10-45 centimeters of shell diameter
DESCRIBED BY: Sowerby, 1816
CLOSEST LIVING RELATIVES: Nautilus
DEPICTED IN: The Life of a Temnodontosaurus episode 9
Cenoceras intermedium (the genus name stands for "recent horn") is a member of the Nautilidae family, which includes the modern day nautili. It had a characteristic spiral shell that was covered in gas to stay buoyant in the water column.