Jellyfish or sea jellies are the informal common names given to the medusa-phase of certain gelatinous members of the subphylum Medusozoa, a major part of the phylum Cnidaria. Jellyfish are free-swimming marine animals with umbrella-shaped bells and trailing tentacles, although a few are not mobile, being anchored to the seabed by stalks; the bell can pulsate to provide propulsion and efficient locomotion. The tentacles are armed with stinging cells and may be used to capture prey and defend against predators. Jellyfish have a complex life cycle. Jellyfish are found all from surface waters to the deep sea. Scyphozoans are marine, but some hydrozoans with a similar appearance live in freshwater. Large colorful, jellyfish are common in coastal zones worldwide; the medusae of most species are fast growing, mature within a few months and die soon after breeding, but the polyp stage, attached to the seabed, may be much more long-lived. Jellyfish have been in existence for at least 500 million years, 700 million years or more, making them the oldest multi-organ animal group.
Jellyfish are eaten by humans in certain cultures, being considered a delicacy in some Asian countries, where species in the Rhizostomae order are pressed and salted to remove excess water. They are used in research, where the green fluorescent protein, used by some species to cause bioluminescence, has been adapted as a fluorescent marker for genes inserted into other cells or organisms; the stinging cells used by jellyfish to subdue their prey can injure humans. Many thousands of swimmers are stung every year, with effects ranging from mild discomfort to serious injury or death; when conditions are favourable, jellyfish can form vast swarms. These can be responsible for damage to fishing gear by filling fishing nets, sometimes clog the cooling systems of power and desalination plants which draw their water from the sea; the name jellyfish, in use since 1796, has traditionally been applied to medusae and all similar animals including the comb jellies. The term jellies or sea jellies is more recent, having been introduced by public aquaria in an effort to avoid use of the word "fish" with its connotations of an animal with a backbone, though shellfish and starfish are not vertebrates either.
In scientific literature, "jelly" and "jellyfish" have been used interchangeably. Many sources refer to only scyphozoans as "true jellyfish"; the term jellyfish broadly corresponds to medusae, that is, a life-cycle stage in the Medusozoa. The American evolutionary biologist Paulyn Cartwright gives the following general definition: Typically, medusozoan cnidarians have a pelagic, predatory jellyfish stage in their life cycle; the Merriam-Webster dictionary defines jellyfish as follows: A free-swimming marine coelenterate, the sexually reproducing form of a hydrozoan or scyphozoan and has a nearly transparent saucer-shaped body and extensible marginal tentacles studded with stinging cells. Given that jellyfish is a common name, its mapping to biological groups is inexact; some authorities have called the comb jellies and certain salps jellyfish, though other authorities state that neither of these are jellyfish, which they consider should be limited to certain groups within the medusozoa. The non-medusozoan clades called jellyfish by some but not all authorities are indicated with "???" on the following cladogram of the animal kingdom: Jellyfish are not a clade, as they include most of the Medusozoa, barring some of the Hydrozoa.
The medusozoan groups included by authorities are indicated on the following phylogenetic tree by the presence of citations. Names of included jellyfish, in English; the subphylum Medusozoa includes all cnidarians with a medusa stage in their life cycle. The basic cycle is egg, planula larva, medusa, with the medusa being the sexual stage; the polyp stage is sometimes secondarily lost. The subphylum include the major taxa, Scyphozoa and Hydrozoa, excludes Anthozoa; this suggests. Medusozoans have tetramerous symmetry, with parts in multiples of four; the four major classes of medusozoan Cnidaria are: Scyphozoa are sometimes called true jellyfish, though they are no more jellyfish than the others listed here. They have tetra-radial symmetry. Most have tentacles around the outer margin of the bowl-shaped bell, long, oral arms around the mouth in the center of the subumbrella. Cubozoa have a box-shaped bell, their velarium assists them to swim more quickly. Box jellyfish may be related more to scyphozoan jellyfish than either are to the Hydrozoa.
Hydrozoa medusae have tetra-radial symmetry, nearly always have a velum attached just inside the bell margin, do not have oral arms, but a much smaller central stalk-like structure, the manubrium, with terminal mouth opening, are distinguished by the absence of cells in the mesoglea. Hydrozoa show great diversity of lifestyle. Stau
Countershading, or Thayer's law, is a method of camouflage in which an animal's coloration is darker on the upper side and lighter on the underside of the body. This pattern is found in many species of mammals, birds and insects, both predators and prey, has occurred since at least the Cretaceous period; when light falls from above on a uniformly coloured three-dimensional object such as a sphere, it makes the upper side appear lighter and the underside darker, grading from one to the other. This pattern of light and shade makes the object appear solid, therefore easier to detect; the classical form of countershading, discovered in 1909 by the artist Abbott Handerson Thayer, works by counterbalancing the effects of self-shadowing, again with grading from dark to light. In theory this could be useful for military camouflage, but in practice it has been applied, despite the best efforts of Thayer and in the Second World War, of the zoologist Hugh Cott; the precise function of various patterns of animal coloration that have been called countershading has been debated by zoologists such as Hannah Rowland, with the suggestion that there may be multiple functions including flattening and background matching when viewed from the side.
A related mechanism, counter-illumination, adds the creation of light by bioluminescence or lamps to match the actual brightness of a background. Counter-illumination camouflage is common in marine organisms such as squid, it has been studied up to the prototype stage for military use in ships and aircraft, but it too has or never been used in warfare. The reverse of countershading, with the belly pigmented darker than the back, enhances contrast and so makes animals more conspicuous, it is found in animals, such as skunks. The pattern is used both in startle or deimatic displays and as a signal to warn off experienced predators. However, animals that habitually live upside-down but lack strong defences, such as the Nile catfish and the Luna moth caterpillar, have upside-down countershading for camouflage; the English zoologist Edward Bagnall Poulton, author of The Colours of Animals discovered the countershading of various insects, including the pupa or chrysalis of the purple emperor butterfly, Apatura iris, the caterpillar larvae of the brimstone moth, Opisthograptis luteolata and of the peppered moth, Biston betularia.
However he did he suggest that the effect occurred widely. The New Hampshire artist Abbott Handerson Thayer was one of the first to study and write about countershading. In his 1909 book Concealing-Coloration in the Animal Kingdom, he described and illustrated countershading with photographs and paintings, but wrongly claimed that all animals are countershaded. For this reason countershading is sometimes called Thayer's law. Thayer wrote: Animals are painted by Nature darkest on those parts which tend to be most lighted by the sky's light, vice versa.... The fact that a vast majority of creatures of the whole animal kingdom wear this gradation, developed to an exquisitely minute degree, are famous for being hard to see in their homes, speaks for itself. Thayer observed and painted a number of examples, including the Luna moth caterpillar Actias luna, both in its habitual upside-down feeding position, where its countershading makes it appear flat, artificially inverted from that position, where sunlight and its inverted countershading combine to make it appear shaded and therefore solid.
Thayer obtained a patent in 1902 to paint warships, both submarines and surface ships, using countershading, but failed to convince the US Navy to adopt his ideas. Hugh Bamford Cott in his 1940 book Adaptive Coloration in Animals described many instances of countershading, following Thayer in general approach but criticising Thayer's excessive claim that all animals are camouflaged with countershading. Cott called this "Thayer straining the theory to a fantastic extreme". Both Thayer and Cott included in their books photographs of a non-countershaded white cockerel against a white background, to make the point that in Thayer's words "a monochrome object can not be'obliterated', no matter what its background" or in Cott's words "Colour resemblance alone is not sufficient to afford concealment". Cott explained that Contrary to what might have been expected by any one lacking in artistic perception, the bird appears conspicuous, the back looking lighter, the breast darker, than the background, although in actual fact, back and breast are all pure white."
Countershading is observed in a wide range of animal groups, both terrestrial, such as deer, marine, such as sharks. It prey, it is used alongside other forms of camouflage including colour matching and disruptive coloration. Among predatory fish, the gray snapper, Lutianus griseus, is flattened by its countershading, while it hunts an "almost invisible" prey, the hardhead fish, Atherina laticeps which swims over greyish sands. Other countershaded marine animals include blue shark and dolphin, it tones the canvas on which are painted the Leopard's spots, the Tiger's stripes... It is the dress universally worn by rodents... It is the essential uniform adopt
In evolutionary biology, mimicry is an evolved resemblance between an organism and another object an organism of another species. Mimicry may evolve between individuals of the same species. Mimicry functions to protect a species from predators, making it an antipredator adaptation. Mimicry evolves if a receiver perceives the similarity between a mimic and a model and as a result changes its behaviour in a way that provides a selective advantage to the mimic; the resemblances that evolve in mimicry can be visual, chemical, tactile, or electric, or combinations of these sensory modalities. Mimicry may be to the advantage of both organisms that share a resemblance, in which case it is a form of mutualism; the evolutionary convergence between groups is driven by the selective action of a signal-receiver or dupe. Birds, for example, use whilst avoiding the noxious ones. Over time, palatable insects may evolve to resemble noxious ones, making them mimics and the noxious ones models. In the case of mutualism, sometimes both groups are referred to as "co-mimics".
It is thought that models must be more abundant than mimics, but this is not so. Mimicry may involve numerous species. Mimicry between prey species and their predators involves three or more species. In its broadest definition, mimicry can include non-living models; the specific terms masquerade and mimesis are sometimes used. For example, animals such as flower mantises, planthoppers and geometer moth caterpillars resemble twigs, leaves, bird droppings or flowers. Many animals bear eyespots, they may not resemble any specific organism's eyes, whether or not animals respond to them as eyes is unclear. Nonetheless, eyespots are the subject of a rich contemporary literature; the model is another species, except in automimicry, where members of the species mimic other members, or other parts of their own bodies, in inter-sexual mimicry, where members of one sex mimic members of the other. Mimicry can result in an evolutionary arms race if mimicry negatively affects the model, the model can evolve a different appearance from the mimic.p161 Mimicry should not be confused with other forms of convergent evolution that occurs when species come to resemble each other by adapting to similar lifestyles that have nothing to do with a common signal receiver.
Mimics may have different models for different life cycle stages, or they may be polymorphic, with different individuals imitating different models, such as in Heliconius butterflies. Models themselves may have more than one mimic, though frequency dependent selection favours mimicry where models outnumber mimics. Models tend to be closely related organisms, but mimicry of vastly different species is known. Most known mimics are insects, though many other examples including vertebrates are known. Plants and fungi may be mimics, though less research has been carried out in this area. Use of the word mimicry dates to 1637, it derives from the Greek term mimetikos, "imitative", in turn from mimetos, the verbal adjective of mimeisthai, "to imitate". Used to describe people, "mimetic" was used in zoology from 1851, "mimicry" from 1861. Many types of mimicry have been described. An overview of each follows, highlighting the similarities and differences between the various forms. Classification is based on function with respect to the mimic.
Some cases may belong to more than one class, e.g. automimicry and aggressive mimicry are not mutually exclusive, as one describes the species relationship between model and mimic, while the other describes the function for the mimic. The terminology used is not without debate and attempts to clarify have led to new terms being included; the term "masquerade" is sometimes used when the model is inanimate but it is differentiated from "crypsis" in its strict sense by the potential response of the signal receiver. In crypsis the receiver is assumed to not respond while a masquerader confuses the recognition system of the receiver that would otherwise seek the signaller. In the other forms of mimicry, the signal is not filtered out by the sensory system of the receiver; these are not mutually exclusive and in the evolution of wasp-like appearance, it has been argued that insects evolve to masquerade wasps since predatory wasps do not attack each other but this mimetic resemblance deters vertebrate predators.
Defensive or protective mimicry takes place when organisms are able to avoid harmful encounters by deceiving enemies into treating them as something else. The first three such cases discussed here entail mimicry of animals protected by warning coloration: Batesian mimicry, where a harmless mimic poses as harmful. Müllerian mimicry, where two or more harmful species mutually advertise themselves as harmful. Mertensian mimicry, where a deadly mimic resembles a less harmful but lesson-teaching model; the fourth case, Vavilovian mimicry, where weeds resemble crops, involves humans as the agent of selection. In Batesian mimicry the mimic shares signals similar to the model, but does not have the attribute that makes it unprofitable to predators. In other words, a Batesian mimic is a sheep in wolf's clothing, it is named after Henry Walter Bates, an English naturalist whose
Abbott Handerson Thayer
Abbott Handerson Thayer was an American artist and teacher. As a painter of portraits, figures and landscapes, he enjoyed a certain prominence during his lifetime, his paintings are represented in the major American art collections, he is best known for his'angel' paintings, some of which use his children as models. During the last third of his life, he worked together with his son, Gerald Handerson Thayer, on a book about protective coloration in nature, titled Concealing-Coloration in the Animal Kingdom. First published by Macmillan in 1909 reissued in 1918, it may have had an effect on military camouflage during World War I; however it was roundly mocked by Theodore Roosevelt and others for its assumption that all animal coloration is cryptic. Thayer influenced American art through his efforts as a teacher, training apprentices in his New Hampshire studio. Thayer was born in Boston to William Henry Ellen Handerson; the son of a country doctor, he spent his childhood in rural New Hampshire, near Keene, at the foot of Mount Monadnock.
In that rural setting, he became a hunter and a trapper. Thayer studied Audubon's Birds of America, experimented with taxidermy, made his first artworks: watercolor paintings of animals. At the age of fifteen he was sent to the Chauncy Hall School in Boston, where he met Henry D. Morse, an amateur artist who painted animals. With guidance from Morse, Abbott developed and improved his painting skills, focusing on depictions of birds and other wildlife, soon began painting animal portraits on commission, he taught his sister, Ellen Thayer Fisher, techniques that he was learning. At age 18, he relocated to Brooklyn, New York, to study painting at the Brooklyn Art School and the National Academy of Design. Studying under Lemuel Wilmarth, he met many emerging and progressive artists during this period in New York, including his future wife, Kate Bloede and his close friend, Daniel Chester French. He showed work at the newly formed Society of American Artists, continued refining his skills as an animal and landscape painter.
In 1875, after having married Kate Bloede, he moved to Paris, where he studied for four years at the École des Beaux-Arts, with Henri Lehmann and Jean-Léon Gérôme, where his closest friend became the American artist George de Forest Brush. Returning to New York, he established his own portrait studio, became active in the Society of American Painters, began to take in apprentices. Life became all but unbearable for Thayer and his wife during the early 1880s, when two of their small children died unexpectedly, just one year apart. Devastated, they spent the next several years relocating from place to place. Although he was not yet secure financially, Thayer's growing reputation resulted in more portrait commissions than he could accept. Among his sitters were George Washington Cable, Mark Twain, Henry James, he made numerous portraits of the three remaining Thayer children, Mary and Gladys, used them as models for symbolic compositions such as Angel and Virgin Enthroned. After her father died, Thayer's wife lapsed into an irreversible melancholia, which led to her confinement in an asylum, the decline of her health, her eventual death on May 3, 1891, from a lung infection.
Soon after, Thayer married their long-time friend, Emmeline "Emma" Buckingham Beach, whose father Moses Yale Beach owned The New York Sun. He and his second wife spent their remaining years in rural New Hampshire and working productively. In 1901, they settled permanently in New Hampshire, where Thayer had grown up. Eccentric and opinionated, Thayer grew more so as he aged, his family's manner of living reflected his strong beliefs: the Thayers slept outdoors year-round in order to enjoy the benefits of fresh air, the three children were never enrolled in a school; the younger two and Gladys, shared their father's enthusiasms, became painters. In 1898, Thayer visited St Ives, Cornwall and, with an introductory letter from C. Hart Merrian, the Chief of the US Biological Survey in Washington, D. C. applied to the lord of the Manor of St Ives and Treloyhan, Henry Arthur Mornington Wellesley, the 3rd Earl Cowley, for permission to collect specimens of birds from the cliffs at St Ives. During this latter part of his life, among Thayer's neighbors was George de Forest Brush, with whom he collaborated on camouflage.
It is difficult to categorize Thayer as an artist. He was described in first-person accounts as eccentric and mercurial, there is a parallel contradictory mixture of academic tradition and improvisation in his artistic methods. For example, he is known as a painter of "ideal figures", in which he portrayed women as embodiments of virtue, adorned in flowing white tunics and equipped with feathered angel’s wings. At the same time, he did this using methods that were unorthodox, such as purposely mixing dirt into the paint, or using a broom instead of a brush to lessen the sense of rigidity in a newly finished, still-wet painting. Thayer was surrounded by women, be they his family, models or students. Biographer Ross Anderson believed that in his mind "feminine virtue and aesthetic grandeur were inextricably linked"; when he began to add wings to his figures in the late 1880s, he was making more obvious the transcendent quali
Sea anemones are a group of marine, predatory animals of the order Actiniaria. They are named after the anemone, a terrestrial flowering plant, because of the colourful appearance of many. Sea anemones are classified in the phylum class Anthozoa, subclass Hexacorallia; as cnidarians, sea anemones are related to corals, tube-dwelling anemones, Hydra. Unlike jellyfish, sea anemones do not have a medusa stage in their life cycle. A typical sea anemone is a single polyp attached to a hard surface by its base, but some species live in soft sediment and a few float near the surface of the water; the polyp has a columnar trunk topped by an oral disc with a ring of a central mouth. The tentacles can be expanded to catch passing prey, they are armed with cnidocytes. In many species, additional nourishment comes from a symbiotic relationship with single-celled dinoflagellates, zooxanthellae or with green algae, that live within the cells; some species of sea anemone live in association with hermit crabs, small fish or other animals to their mutual benefit.
Sea anemones breed by liberating sperm and eggs through the mouth into the sea. The resulting fertilized eggs develop into planula larvae which, after being planktonic for a while, settle on the seabed and develop directly into juvenile polyps. Sea anemones breed asexually, by breaking in half or into smaller pieces which regenerate into polyps. Sea anemones are sometimes kept in reef aquariums. A typical sea anemone is a sessile polyp attached at the base to the surface beneath it by an adhesive foot, called a basal or pedal disc, with a column-shaped body topped by an oral disc. Most are from 1 to 5 cm in diameter and 1.5 to 10 cm in length, but they are inflatable and vary in dimensions. Some are large; some species burrow in soft sediment and lack a basal disc, having instead a bulbous lower end, the physa, which anchors them in place. The column or trunk is more or less cylindrical and may be plain and smooth or may bear specialist structures. In some species the part below the oral disc is constricted and is known as the capitulum.
When the animal contracts, the oral disc and capitulum fold inside the pharynx and are held in place by a strong sphincter muscle part way up the column. There may be a fold in the body wall, known as a parapet, at this point, this parapet covers and protects the anemone when it is retracted; the oral disc has a central mouth slit-shaped, surrounded by one or more whorls of tentacles. The ends of the slit lead to grooves in the wall of the pharynx known as siphonoglyphs; the tentacles are tapered and tipped by a pore, but in some species they are branched, club-tipped, or reduced to low knobs. The tentacles are armed with many cnidocytes, cells that are both defensive and used to capture prey. Cnidocytes contain stinging nematocysts, capsule-like organelles capable of everting giving the phylum Cnidaria its name; each nematocyst contains a small venom vesicle filled with actinotoxins, an inner filament, an external sensory hair. A touch to the hair mechanically triggers a cell explosion, which launches a harpoon-like structure that attaches to the organism that triggered it, injects a dose of venom in the flesh of the aggressor or prey.
At the base of the tentacles in some species lie acrorhagi, elongated inflatable tentacle-like organs armed with cnidocytes, that can flail around and fend off other encroaching anemones. The venom is a mix of toxins, including neurotoxins, that paralyzes the prey so the anemone can move it to the mouth for digestion inside the gastrovascular cavity. Actinotoxins are toxic to prey species of fish and crustaceans. However, small banded fish in various colours, are not affected by their host anemone's sting and shelter themselves from predators among its tentacles. Several other species have similar adaptions and are unaffected. Most sea anemones are harmless to humans, but a few toxic species have caused severe injuries and are lethal. Sea anemones have. Waste and undigested matter is excreted through this opening; the mouth is slit-like in shape, bears a groove at one or both ends. The groove, termed a siphonoglyph, is ciliated, helps to move food particles inwards and circulate water through the gastrovascular cavity.
The mouth opens into a flattened pharynx. This consists of an in-folding of the body wall, is therefore lined by the animal's epidermis; the pharynx runs for about one third the length of the body before opening into the gastrovascular cavity that occupies the remainder of the body. The gastrovascular cavity itself is divided into a number of chambers by mesenteries radiating inwards from the body wall; some of the mesenteries form complete partitions with a free edge at the base of the pharynx, where t
Tuatara are reptiles endemic to New Zealand. Although resembling most lizards, they are part of the order Rhynchocephalia, their name derives from the Māori language, means "peaks on the back". The single species of tuatara is the only surviving member of its order, which flourished around 200 million years ago, their most recent common ancestor with any other extant group is with the squamates. For this reason, tuatara are of interest in the study of the evolution of lizards and snakes, for the reconstruction of the appearance and habits of the earliest diapsids, a group of amniote tetrapods that includes dinosaurs and crocodilians. Tuatara are greenish brown and grey, measure up to 80 cm from head to tail-tip and weigh up to 1.3 kg with a spiny crest along the back pronounced in males. They have two rows of teeth in the upper jaw overlapping one row on the lower jaw, unique among living species, they are unusual in having a pronounced photoreceptive eye, the third eye, thought to be involved in setting circadian and seasonal cycles.
They are able to hear, although no external ear is present, have unique features in their skeleton, some of them evolutionarily retained from fish. Tuatara are sometimes referred to as "living fossils", which has generated significant scientific debate. While mapping its genome, researchers have discovered that the species has between five and six billion base pairs of DNA sequence, nearly twice that of humans; the tuatara Sphenodon punctatus has been protected by law since 1895. A second species, S. guntheri, was recognised in 1989, but since 2009 it has been reclassified as a subspecies. Tuatara, like many of New Zealand's native animals, are threatened by habitat loss and introduced predators, such as the Polynesian rat. Tuatara were extinct on the mainland, with the remaining populations confined to 32 offshore islands until the first North Island release into the fenced and monitored Karori Sanctuary in 2005. During routine maintenance work at Karori Sanctuary in late 2008, a tuatara nest was uncovered, with a hatchling found the following autumn.
This is thought to be the first case of tuatara breeding in the wild on the New Zealand North Island in over 200 years. Tuatara, along with other, now-extinct members of the order Sphenodontia, belong to the superorder Lepidosauria, the only surviving taxon within Lepidosauromorpha. Squamates and tuatara both show caudal autotomy, have transverse cloacal slits; the origin of the tuatara lies close to the split between the Lepidosauromorpha and the Archosauromorpha. Though tuatara resemble lizards, the similarity is superficial, because the family has several characteristics unique among reptiles; the typical lizard shape is common for the early amniotes. Tuatara were classified as lizards in 1831 when the British Museum received a skull; the genus remained misclassified until 1867, when Albert Günther of the British Museum noted features similar to birds and crocodiles. He proposed the order Rhynchocephalia for its fossil relatives. Many disparately related species were subsequently added to the Rhynchocephalia, resulting in what taxonomists call a "wastebasket taxon".
Williston proposed the Sphenodontia to include only tuatara and their closest fossil relatives in 1925. Sphenodon is derived from the Greek for "wedge" and "tooth". Tuatara have been referred to as living fossils, which means that they retain many basal characteristics from around the time of the squamate–rhynchocephalian split. Morphometric analyses of variation in jaw morphology among tuatara and extinct Rhynchocephalian relatives have been argued to demonstrate morphological conservatism and support for the classification of tuatara as a'living fossil', but the reliability of these results has been criticised and debated. Taxonomic research on Sphenodontia indicates that the group has undergone a variety of changes throughout the Mesozoic, the rate of molecular evolution for tuatara has been estimated to be among the fastest of any animal yet examined. Many of the niches occupied by lizards today were held by sphenodontians. There was a successful group of aquatic sphenodontians known as pleurosaurs, which differed markedly from living tuatara.
Tuatara show cold-weather adaptations. For instance, Palaeopleurosaurus appears to have had a much shorter lifespan compared to the modern tuatara. A species of sphenodontine is known from the Miocene Saint Bathans Fauna. Whether it is referable to Sphenodon proper is not clear, but is assumed to be related to tuataras. While there is considered to be only one living species of tuatara, two species were identified: Sphenodon punctatus, or northern tuatara, the much rarer Sphenodon guntheri, or Brothers Island tuatara, confined to North Brother Island in Cook Strait; the specific name punctatus is Latin for "spotted", guntheri refers to German-born British herpetologist Albert Günther. A 2009 paper re-examined the genetic bases used to distinguish the two supposed species of tuatara, concluded they only represent geographic variants, only one species should be recognized; the northern tuatara was classified as Sphenodon punctatus punctatus and the Brothers Island tuatara as Sphenodon punctatus guntheri.