Nervous system and behavior Cephalopods are widely regarded as the most intelligent of the
invertebrates and have well-developed senses and large
brains (larger than those of
gastropods). The
nervous system of cephalopods is the most complex of the invertebrates and their brain-to-body-mass ratio falls between that of
endothermic and
ectothermic vertebrates. The brain is protected in a
cartilaginous cranium. The giant
nerve fibers of the cephalopod
mantle have been widely used for many years as experimental material in
neurophysiology; their large diameter (due to lack of
myelination) makes them relatively easy to study compared with other animals. Many cephalopods are social creatures; when isolated from their own kind, some species have been observed
shoaling with fish. Some cephalopods are able to fly through the air for distances of up to . While cephalopods are not particularly aerodynamic, they achieve these impressive ranges by jet-propulsion; water continues to be expelled from the funnel while the organism is in the air. The animals spread their fins and tentacles to form wings and actively control lift force with body posture. One species,
Todarodes pacificus, has been observed spreading tentacles in a flat fan shape with a mucus film between the individual tentacles, while another,
Sepioteuthis sepioidea, has been observed putting the tentacles in a circular arrangement.
Senses Cephalopods have advanced vision, can detect gravity with
statocysts, and have a variety of chemical sense organs. Consequently, cephalopod vision is acute: training experiments have shown that the
common octopus can distinguish the brightness, size, shape, and horizontal or vertical orientation of objects. The morphological construction gives cephalopod eyes the same performance as shark eyes; however, their construction differs, as cephalopods lack a cornea and have an everted retina. Unlike many other cephalopods,
nautiluses do not have good vision; their eye structure is highly developed, but lacks a solid
lens. They have a simple "
pinhole" eye through which water can pass. Instead of vision, the animal is thought to use
olfaction as the primary sense for
foraging, as well as locating or identifying potential mates. All octopuses are considered to be
color blind. Coleoid cephalopods (octopus, squid, cuttlefish) have a single photoreceptor type and lack the ability to determine color by comparing detected photon intensity across multiple spectral channels. When
camouflaging themselves, they use their chromatophores to change brightness and pattern according to the background they see, but their ability to match the specific color of a background may come from cells such as
iridophores and
leucophores that reflect light from the environment. They also produce visual pigments throughout their body and may sense light levels directly from their body. Evidence of
color vision has been found in the
sparkling enope squid (
Watasenia scintillans). It achieves color vision with three
photoreceptors, which are based on the same
opsin, but use distinct
retinal molecules as chromophores: A1 (retinal), A3 (3-dehydroretinal), and A4 (4-hydroxyretinal). The A1-photoreceptor is most sensitive to green-blue (484 nm), the A2-photoreceptor to blue-green (500 nm), and the A4-photoreceptor to blue (470 nm) light. In 2015, a novel mechanism for spectral discrimination in cephalopods was described. This relies on the exploitation of
chromatic aberration (wavelength-dependence of focal length). Numerical modeling shows that chromatic aberration can yield useful chromatic information through the dependence of image acuity on accommodation. The unusual off-axis slit and annular pupil shapes in cephalopods enhance this ability by acting as prisms which are scattering white light in all directions.
Photoreception In 2015, molecular evidence was published indicating that cephalopod chromatophores are photosensitive; reverse transcription polymerase chain reactions (RT-PCR) revealed
transcripts encoding
rhodopsin and
retinochrome within the retinas and skin of the
longfin inshore squid (
Doryteuthis pealeii), and the
common cuttlefish (
Sepia officinalis) and broadclub cuttlefish (
Sepia latimanus). The authors claim this is the first evidence that cephalopod dermal tissues may possess the required combination of molecules to respond to light.
Hearing Some octopuses and squids have been shown to detect sound using their
statocysts, but, in general, cephalopods are deaf.
Use of light (
Sepia latimanus) can change from camouflage tans and browns (top) to yellow with dark highlights (bottom) in less than a second. Most cephalopods possess an assemblage of skin components that interact with light. These may include
iridophores, leucophores,
chromatophores and (in some species)
photophores. Chromatophores are colored pigment cells that expand and contract in accordance to produce color and pattern which they can use in a startling array of fashions.
Bioluminescence may also be used to entice prey, and some species use colorful displays to impress mates, startle predators, or even communicate with one another. Although color changes appear to rely primarily on vision input, there is evidence that skin cells, specifically
chromatophores, can detect light and adjust to light conditions independently of the eyes. The octopus changes skin color and texture during quiet and active sleep cycles. Cephalopods can use chromatophores like a muscle, which is why they can change their skin hue as rapidly as they do. Coloration is typically stronger in near-shore species than those living in the open ocean, whose functions tend to be restricted to
disruptive camouflage. Most octopuses mimic select structures in their field of view rather than becoming a composite color of their full background. Evidence of original coloration has been detected in cephalopod fossils dating as far back as the
Silurian; these orthoconic individuals bore concentric stripes, which are thought to have served as camouflage. Devonian cephalopods bear more complex color patterns, of unknown function.
Chromatophores Coleoids, a shell-less subclass of cephalopods (squid, cuttlefish, and octopuses), have complex pigment containing cells called chromatophores which are capable of producing rapidly changing color patterns. These cells store pigment within an elastic sac which produces the color seen from these cells. Coleoids can change the shape of this sac, called the cytoelastic sacculus, which then causes changes in the translucency and opacity of the cell. By rapidly changing multiple chromatophores of different colors, cephalopods are able to change the color of their skin at astonishing speeds, an adaptation that is especially notable in an organism that sees in black and white. Chromatophores are known to only contain three pigments, red, yellow, and brown, which cannot create the full color spectrum. However, cephalopods also have cells called iridophores, thin, layered protein cells that reflect light in ways that can produce colors chromatophores cannot. The mechanism of iridophore control is unknown, but chromatophores are under the control of neural pathways, allowing the cephalopod to coordinate elaborate displays. Together, chromatophores and iridophores are able to produce a large range of colors and pattern displays.
Adaptive value Cephalopods utilize chromatophores' color changing ability in order to camouflage themselves. Chromatophores allow coleoids to blend into many different environments, from coral reefs to the sandy sea floor. The color change of chromatophores works in concert with papillae, epithelial tissue which grows and deforms through hydrostatic motion to change skin texture. Chromatophores are able to perform two types of camouflage, mimicry and color matching. Mimicry is when an organism changes its appearance to appear like a different organism. The squid
Sepioteuthis sepioidea has been documented changing its appearance to appear as the non threatening herbivorous parrotfish to approach unaware prey. The octopus
Thaumoctopus mimicus is known to mimic a number of different venomous organisms it cohabitates with to deter predators. While background matching, a cephalopod changes its appearance to resemble its surroundings, hiding from its predators or concealing itself from prey. The ability to both mimic other organisms and match the appearance of their surroundings is notable given that cephalopods' vision is monochromatic. Cephalopods also use their fine control of body coloration and patterning to perform complex signaling displays for both conspecific and intraspecific communication. Coloration is used in concert with locomotion and texture to send signals to other organisms. Intraspecifically this can serve as a warning display to potential predators. For example, when the octopus
Callistoctopus macropus is threatened, it will turn a bright red brown color speckled with white dots as a high contrast display to startle predators. Conspecifically, color change is used for both mating displays and social communication. Cuttlefish have intricate mating displays from males to females. There is also male to male signaling that occurs during competition over mates, all of which are the product of chromatophore coloration displays.
Origin There are two hypotheses about the evolution of color change in cephalopods. One hypothesis is that the ability to change color may have evolved for social, sexual, and signaling functions. Another explanation is that it first evolved because of selective pressures encouraging predator avoidance and stealth hunting. For color change to have evolved as the result of social selection the environment of cephalopods' ancestors would have to fit a number of criteria. One, there would need to be some kind of mating ritual that involved signaling. Two, they would have to experience demonstrably high levels of sexual selection. And three, the ancestor would need to communicate using sexual signals that are visible to a conspecific receiver. For color change to have evolved as the result of natural selection different parameters would have to be met. For one, one would need some phenotypic diversity in body patterning among the population. The species would also need to cohabitate with predators which rely on vision for prey identification. These predators should have a high range of visual sensitivity, detecting not just motion or contrast but also colors. The habitats they occupy would also need to display a diversity of backgrounds. Experiments done in dwarf chameleons testing these hypotheses showed that chameleon taxa with greater capacity for color change had more visually conspicuous social signals but did not come from more visually diverse habitats, suggesting that color change ability likely evolved to facilitate social signaling, while camouflage is a useful byproduct. Because camouflage is used for multiple adaptive purposes in cephalopods, color change could have evolved for one use and the other developed later, or it evolved to regulate trade offs within both.
Convergent evolution Color change is widespread in ectotherms including anoles, frogs, mollusks, many fish, insects, and spiders. The mechanism behind this color change can be either morphological or physiological. Morphological change is the result of a change in the density of pigment containing cells and tends to change over longer periods of time. Physiological change, the kind observed in cephalopod lineages, is typically the result of the movement of pigment within the chromatophore, changing where different pigments are localized within the cell. This physiological change typically occurs on much shorter timescales compared to morphological change. Cephalopods have a rare form of physiological color change which utilizes neural control of muscles to change the morphology of their chromatophores. This neural control of chromatophores has evolved convergently in both cephalopods and teleosts fishes.
Ink With the exception of the
Nautilidae and the species of
octopus belonging to the
suborder Cirrina, all known cephalopods have an ink sac, which can be used to expel a cloud of dark ink to confuse
predators. This sac is a muscular bag which originated as an extension of the hindgut. It lies beneath the gut and opens into the anus, into which its contents – almost pure
melanin – can be squirted; its proximity to the base of the funnel means the ink can be distributed by ejected water as the cephalopod uses its jet propulsion. formerly the pen-and-ink fish.
Circulatory system Cephalopods are the only molluscs with a closed circulatory system.
Coleoids have two gill
hearts (also known as
branchial hearts) that move blood through the capillaries of the
gills. A single systemic heart then pumps the oxygenated blood through the rest of the body. Like most molluscs, cephalopods use
hemocyanin, a copper-containing protein, rather than
hemoglobin, to transport oxygen. As a result, their blood is colorless when deoxygenated and turns blue when bonded to oxygen. In oxygen-rich environments and in acidic water, hemoglobin is more efficient, but in environments with little oxygen and in low temperatures, hemocyanin has the upper hand. The hemocyanin molecule is much larger than the hemoglobin molecule, allowing it to bond with 96 or molecules, instead of the hemoglobin's just four. But unlike hemoglobin, which are attached in millions on the surface of a single red blood cell, hemocyanin molecules float freely in the bloodstream.
Respiration Cephalopods exchange gases with the seawater by forcing water through their gills, which are attached to the roof of the organism. The gills, which are much more efficient than those of other mollusks, are attached to the ventral surface of the mantle cavity. There is a trade-off with gill size regarding lifestyle. To achieve fast speeds, gills need to be small – water will be passed through them quickly when energy is needed, compensating for their small size. However, organisms which spend most of their time moving slowly along the bottom do not naturally pass much water through their cavity for locomotion; thus they have larger gills, along with complex systems to ensure that water is constantly washing through their gills, even when the organism is stationary. The gills of cephalopods are supported by a skeleton of robust fibrous proteins; the lack of mucopolysaccharides distinguishes this matrix from cartilage. The gills are also thought to be involved in excretion, with NH4+ being swapped with K+ from the seawater. The relative efficiency of
jet propulsion decreases further as animal size increases;
paralarvae are far more efficient than juvenile and adult individuals. Since the
Paleozoic era, as competition with
fish produced an environment where efficient motion was crucial to survival, jet propulsion has taken a back role, with
fins and
tentacles used to maintain a steady velocity. and they can out-accelerate most fish. The jet is supplemented with fin motion; in the squid, the fins flap each time that a jet is released, amplifying the thrust; they are then extended between jets (presumably to avoid sinking). The velocity of the organism can be accurately predicted for a given mass and morphology of animal. Motion of the cephalopods is usually backward as water is forced out anteriorly through the hyponome, but direction can be controlled somewhat by pointing it in different directions. Some cephalopods accompany this expulsion of water with a gunshot-like popping noise, thought to function to frighten away potential predators. Cephalopods employ a similar method of propulsion despite their increasing size (as they grow) changing the dynamics of the water in which they find themselves. Thus their paralarvae do not extensively use their fins (which are less efficient at low
Reynolds numbers) and primarily use their jets to propel themselves upwards, whereas large adult cephalopods tend to swim less efficiently and with more reliance on their fins.
Nautilus is also capable of creating a jet by undulations of its funnel; this slower flow of water is more suited to the extraction of oxygen from the water. Jet thrust in cephalopods is controlled primarily by the maximum diameter of the funnel orifice (or, perhaps, the average diameter of the funnel) Changes in the size of the orifice are used most at intermediate velocities. Water refills the cavity by entering not only through the orifices, but also through the funnel. Some, such as
Nautilus, allow gas to diffuse into the gap between the mantle and the shell; others allow purer water to ooze from their kidneys, forcing out denser salt water from the body cavity; Females of two species,
Ocythoe tuberculata and
Haliphron atlanticus, have evolved a true
swim bladder.
Octopus vs. squid locomotion Two of the categories of cephalopods, octopus and squid, are vastly different in their movements despite being of the same class. Octopuses are generally not seen as active swimmers; they are often found scavenging the sea floor instead of swimming long distances through the water. Squid, on the other hand, can be found to travel vast distances, with some moving as much as 2,000 km in 2.5 months at an average pace of 0.9 body lengths per second. There is a major reason for the difference in movement type and efficiency: anatomy. Both octopuses and squids have mantles (referenced above) which function towards respiration and locomotion in the form of jetting. The composition of these mantles differs between the two families, however. In octopuses, the mantle is made up of three muscle types: longitudinal, radial, and circular. The longitudinal muscles run parallel to the length of the octopus and they are used in order to keep the mantle the same length throughout the jetting process. Given that they are muscles, it can be noted that this means the octopus must actively flex the longitudinal muscles during jetting in order to keep the mantle at a constant length. The radial muscles run perpendicular to the longitudinal muscles and are used to thicken and thin the wall of the mantle. Finally, the circular muscles are used as the main activators in jetting. They are muscle bands that surround the mantle and expand/contract the cavity. All three muscle types work in unison to produce a jet as a propulsion mechanism.
Shell Nautiluses are the only extant cephalopods with a true external shell. However, all molluscan shells are formed from the
ectoderm (outer layer of the embryo); in
cuttlefish (
Sepia spp.), for example, an invagination of the ectoderm forms during the embryonic period, resulting in a shell (
cuttlebone) that is internal in the adult. The same is true of the chitinous
gladius of squid
Cirrate octopods have
arch-shaped cartilaginous fin supports, which are sometimes referred to as a "shell vestige" or "gladius". The
Incirrina have either a pair of rod-shaped
stylets or no vestige of an internal shell, and some squid also lack a gladius. The shelled coleoids do not form a clade or even a paraphyletic group. Shells that are "lost" may be lost by resorption of the calcium carbonate component. Females of the octopus genus
Argonauta secrete a specialized paper-thin egg case in which they reside, and this is popularly regarded as a "shell", although it is not attached to the body of the animal and has a separate evolutionary origin. The largest group of shelled cephalopods, the
ammonites, are extinct, but their shells are very common as
fossils. Ammonites thrived during the Paleozoic and Mesozoic eras. Their distinctive, spiral-shaped shells are found in sedimentary rocks worldwide and subsequently in many human creations. The deposition of carbonate, leading to a mineralized shell, appears to be related to the acidity of the organic shell matrix (see
Mollusc shell); shell-forming cephalopods have an acidic matrix, whereas the gladius of squid has a basic matrix. The basic arrangement of the cephalopod outer wall is: an outer (spherulitic) prismatic layer, a laminar (nacreous) layer and an inner prismatic layer. The thickness of every layer depends on the taxa. In modern cephalopods, the Ca carbonate is aragonite. As for other mollusc shells or coral skeletons, the smallest visible units are irregular rounded granules.
Head appendages Cephalopods, as the name implies, have muscular appendages extending from their heads and surrounding their mouths. These are used in feeding, mobility, and even reproduction. In
coleoids they number eight or ten. Decapods such as cuttlefish and squid have five pairs. The longer two, termed "
tentacles", are actively involved in capturing prey; The tentacle consists of a thick central nerve cord (which must be thick to allow each sucker to be controlled independently) surrounded by circular and radial muscles. Because the volume of the tentacle remains constant, contracting the circular muscles decreases the radius and permits the rapid increase in length. Typically, a 70% lengthening is achieved by decreasing the width by 23%. Externally shelled
nautilids (
Nautilus and
Allonautilus) have on the order of 90 finger-like appendages, termed
tentacles, which lack suckers but are sticky instead, and are partly retractable.
Feeding ,
Architeuthis sp. All living cephalopods have a two-part
beak; They feed by capturing prey with their tentacles, drawing it into their mouth and taking bites from it. However, octopus arms use a family of cephalopod-specific chemotactile receptors (CRs) to be their "taste by touch" system.
Radula '' eating a crab The cephalopod radula consists of multiple symmetrical rows of up to nine teeth – thirteen in fossil classes. The organ is reduced or even vestigial in certain octopus species and is absent in
Spirula. They are usually preserved within the cephalopod's body chamber, commonly in conjunction with the mandibles; but this need not always be the case; many radulae are preserved in a range of settings in the Mason Creek. Radulae are usually difficult to detect, even when they are preserved in fossils, as the rock must weather and crack in exactly the right fashion to expose them; for instance, radulae have only been found in nine of the 43 ammonite genera, and they are rarer still in non-ammonoid forms: only three pre-Mesozoic species possess one.
Nautilus, unusually, possesses four nephridia, none of which are connected to the pericardial cavities. The incorporation of
ammonia is important for shell formation in terrestrial molluscs and other non-molluscan lineages. Because
protein (i.e., flesh) is a major constituent of the cephalopod diet, large amounts of
ammonium ions are produced as waste. The main organs involved with the release of this excess ammonium are the gills. The rate of release is lowest in the shelled cephalopods
Nautilus and
Sepia as a result of their using
nitrogen to fill their shells with gas to increase buoyancy. Stearns (1992) suggested that in order to produce the largest possible number of viable offspring, spawning events depend on the ecological environmental factors of the organism. The majority of cephalopods do not provide parental care to their offspring, except, for example, octopus, which helps this organism increase the survival rate of their offspring. The development of a cephalopod embryo can be greatly affected by temperature, oxygen saturation, pollution, light intensity, and salinity. Mating would be a poor indicator of sexual maturation in females; they can receive sperm when not fully reproductively mature and store them until they are ready to fertilize the eggs. Most cephalopod males develop a hectocotylus, an arm tip which is capable of transferring their spermatozoa into the female mantle cavity. Though not all species use a hectocotylus; for example, the adult nautilus releases a
spadix. Some male squids, mainly deep-water species, have instead evolved a penis longer than their own body length, the longest penis in any free-living animals. It is assumed these males simply attach a
spermatophore anywhere on a female's body. An indication of sexual maturity of females is the development of brachial photophores to attract mates.
Fertilization Cephalopods are not
broadcast spawners. During the process of fertilization, the females use sperm provided by the male via
external fertilization.
Internal fertilization is seen only in octopuses. Cephalopods often mate several times, which influences males to mate longer with females that have previously, nearly tripling the number of contractions of the mantle. Others, like the
Japanese flying squid, will spawn neutrally buoyant egg masses which will float at the interface between water layers of slightly different densities, or the female will swim around while carrying the eggs with her. Most species are
semelparous (only reproduce once before dying), the only known exceptions are the
vampire squid, the
lesser Pacific striped octopus and the
nautilus, which are
iteroparous. In some species of cephalopods, egg clutches are anchored to substrates by a mucilaginous adhesive substance. These eggs are swelled with perivitelline fluid (PVF), a hypertonic fluid that prevents premature hatching. Fertilized egg clusters are neutrally buoyant depending on the depth that they were laid, but can also be found in substrates such as sand, a matrix of corals, or seaweed. During mate competition males also participate in a technique called flushing. This technique is used by the second male attempting to mate with a female. Flushing removes spermatophores in the buccal cavity that was placed there by the first mate by forcing water into the cavity.
Mate choice Mate choice is seen in cuttlefish species, where females prefer some males over others, though characteristics of the preferred males are unknown.
Sexual dimorphism In a variety of marine organisms, it is seen that females are larger in size compared to the males in some closely related species. In some lineages, such as the
blanket octopus, males become structurally smaller and smaller resembling a term, "dwarfism" dwarf males usually occurs at low densities. The blanket octopus male is an example of sexual-evolutionary dwarfism; females grow 10,000 to 40,000 times larger than the males and the sex ratio between males and females can be distinguished right after hatching of the eggs. The funnel of cephalopods develops on the top of their head, whereas the mouth develops on the opposite surface. The early embryological stages are reminiscent of ancestral
gastropods and extant
Monoplacophora. The process from spawning to hatching follows a similar trajectory in all species, the main variable being the amount of yolk available to the young and when it is absorbed by the embryo. In contrast, hatchling nautili are not referred to by a specific technical term, as they resemble miniatures of the adults. Neonate cephalopods quickly learn how to hunt, using encounters with prey to refine their strategies. == Evolution ==