Chordate Characteristics PDF

The animals most familiar to us belong to phylum Chordata (Chorda - cord), and humans share the characteristic that gives this group its name, the notochord. All members of the phylum possess this structure, although it can be restricted to early development.Its primary function is to support and st...

The animals most familiar to us belong to phylum Chordata (Chorda - cord), and humans share the characteristic that gives this group its name, the notochord. All members of the phylum possess this structure, although it can be restricted to early development.Its primary function is to support and stiffen the body, providing support for muscles. The structural plan of chordates share many features with invertebrate groups, such as bilateral symmetry, anterior-posterior axis, coelom (tube within a tube arrangement), metamerism, and cephalisation. However, there is debate about the exact phylogeny relationships between groups. Two possible lines have been proposed; the earlier speculation, which focused on arthropod-annelid mollusc group (Protostomia branch) of the invertebrates, has currently fallen out of favour. It is now believed that only members of the echinoderm-hemichordate group (Deuterosome branch) could be a sister group to the chordates. Reasons for this theory include the possession of common characteristics such as, radial embryonic cleavage, an anus derived from the first embryonic opening (blastopore), and a mouth derived from an opening of secondary origin. As a whole, there is more fundamental unity of plan throughout all the organs and systems of this phylum than there is in many other phyla. Ecologically, the chordates are amongst the most adaptable of all organic forms, and are able to utilise almost every habitat. They illustrate the basic evolutionary processes of the origin of new structures, adaptive strategies, and adaptive radiation. Chordate characteristics The five distinctive characteristics that set chordates apart from all other phyla are notochord, dorsal tubular nerve cord, pharyngeal pouches or slits endostyle and post anal tail. These characteristics are always found at some embryonic stage, although they may be altered or may disappear in later stages of the life cycle. All, but pharyngeal pouches/slits are unique to chordates. Hemichordates also possess these structures. The notochord is a flexible, rod-shaped structure extending the length of the body; it is the first part of the endoskeleton to appear in the embryo of all chordates. Notochord is an axis for muscle attachment, and, as it can bend without shortening, it permits adulatory movements of the body. It is composed of cells derived from the mesoderm. In lower vertebrates, it persists throughout life as the main axial support of the body, while in higher vertebrates it is replaced by the vertebral column. The notochord is found on the ventral surface of the neural tube. The dorsal tubular nerve cord is unique to the chordates. In most invertebrate phyla that have nerve cords, it is ventral to the alimentary canal and is solid. In chordates, the single cord is dorsal to other alimentary canal, and is a tube. The anterior end becomes enlarged to form a brain in most chordates. The hollow cord is produced in the embryo by in folding of ectodermic cells on the dorsal body surface above the notochord. In vertebrates, the nerve cord passes thorough the protective neural larches of the vertebrae. A bony or cartilaginous cranium surrounds the anterior brain. Pharyngeal slits are openings that lead from the pharyngeal cavity to the outside. They are formed by invagination of the endodermic lining of the pharynx (pharyngeal pouches). The perforated pharynx evolved as a filter-feeding device. Rows of beating cilia cause currents of water to flow through the mouth, through the pharyngeal slits and out of the body through a hole in the body wall called the atriopore. Small particles in the water are trapped by the cilia in different parts of the mouth chamber and separated into materials that the organism can eat. In primitive chordates, the pharyngeal slits are used to strain water and filter out food particles; in fish, they are modified for respiration. In tetrapod (four footed) vertebrates, they give rise to several different structures, including the Eustachian tube, middle ear cavity, tonsils and parathyroid gland. The endostyle, or thyroid gland, was not recognised as a chordate feature until recently. However, it (or its derivative, the thyroid gland) is found in all chordates, but in no other phyla. The end style, which is found in the pharyngeal floor, secretes mucus, which traps small food particles brought into the pharyngeal cavity. Some cells in the end style secrete iodinated proteins. These cells are homologous with the iodine-based hormones secreted by the thyroid gland of vertebrates. The post-anal tail, together with somatic musculature and the stiffening notochord provide motility for Laval tunicates and amphioxus. As a structure added to the body behind the end of the digestive tract, the tail has clearly evolved specifically for locomotion, efficiency is increased with the introduction of fins. The tail is vestigial in humans (the coccyx, a series of small vertebrae at the end of the spinal column, which is not visible externally). However, most other mammals have a movable tail that can perform locomotary, balance and communication functions, depending on species. Phylogeny and evolution Since the time of Darwin, zoologists have debated the question of chordate origins and relationships. However, the fact that the earliest protochordates were probably soft-bodied creatures, which were unlikely to have been preserved as fossils, has meant that much has needed to be inferred from living forms. Analysis of the early development of modern forms shows they tend to be more evolutionary conserved than the differentiated adult forms they become. Most early efforts to identify relationships between the chordate groups are now recognised as based on similarities related to analogy rather than homology. Analogous structures are those that perform similar functions (e.g. wings of birds and insects). Homologous structures share a common origin, but may look different, and perform very different functions, e.g. the vertebrate pentadactyl limb. Homologous structure reveals common ancestry, whilst analogous structures do not. The Deuterostomia (echinoderms, hemichordates, chordates) have their common origins in an ancient Precambrian ancestor. Several lines of anatomical developmental and molecular evidence suggest that, some time later, at the beginning of the Cambrian period, (570 million years ago) the first distinct chordates emerged from a line related to echinoderms and hemichordates. Some theorists propose that the Hemichordates are a sister group to the Chordates, citing pharyngeal slits as a shared derived characteristic, others suggest chordates arose from an extinct free-living echinoderm. While modern echinoderms have little in physical appearance to link them to chordates, an evolutionary relationship between the groups is obvious from the fossil record. One group of fossil echinoderms, the Calcichorda, has been suggested as a chordate ancestor. These small, non-symmetrical forms had a head, a structure that looked homologous to pharyngeal slits, a post anal tail, a notochord and muscle blocks. However, there are problems with this hypothesis, as the mineral of the skeleton differs (calcium phosphate in chordates and calcium carbonate in clavichordists) and this group was alive in the Ordovician, a more recent age than that proposed for the first chordates (fossil examples from early Cambrian). Despite the uncertainty over the ancestral chordate form, two living protochordate groups are descended from it. Subphylum Urochordata (Tunicata) The Urochordata, (tail-chordates), contains about 2000 species. They are commonly known as 'sea squirts.' They are found in all seas from near shore to great depths; most are sessile, although a few species are free swimming as adults. The older name of 'tunicate' refers to a usually tough, non-living tunic or test that surrounds the animal and contains cellulose. The body of an adult tunicate is highly specialised, being essentially a sack with two siphons through which water enters and exits. Water is filtered inside the sack-shaped body. However, all tunicates have a larva that is free-swimming and exhibits all chordate characteristics. This 'microscopic tadpole larva' will swim for some time, before (in many tunicates); it eventually attaches to a hard substrate, metamorphosing by losing its tail, its ability to move, and its nervous system apart from a single ganglion. A few tunicates are entirely pelagic (free swimming), known as salps, they typically have barrel-shaped bodies and may be extremely abundant in the open ocean. Urochordates have a sparse fossil record. Complete body fossils of tunicates are rare, but tunicates in some families generate microscopic spicules that may be preserved as microfossils. Examples of tunicate spicules have occasionally been described from the Jurassic period. Urochordata is divided into three classes; Ascidiacea, the most common and well known, they may be solitary, compound, sharing the same test, or colonial, Larvacea, which resemble the larval stages of Ascidiacea and the Thaliacea (pelagic salps). Paedomorphosis and chordate larval evolution The chordates have taken two paths in their early evolution, one path leading to the sedentary urochordates, the other to the more active, mobile cephalochordates and vertebrates discussed below. At the time of its discovery in 1869, the tadpole larva of tunicates was thought to be a descendant of an active free-swimming chordate ancestor. An Englishman, Walter Garstang, proposed in 1928 that the ancestral chordates were derived by retaining the larval active form of a tunicate into adulthood, instead of progressing to the normal sessile form. Evidence presented included the possession of notochord, hollow dorsal nerve cord, pharyngeal slits, endostyle and post anal tail by the larvae. Garstang proposed that a process he called paedomorphosis had occurred, meaning that, at some point, the larvae failed to metamorphose, but continued to develop gonads, reproducing in the larval stage. This was a departure from previous ideas suggesting evolution could occur in the larval stages of a life cycle. Paedomorphosis is known in other animals, e.g. Axolotl, a salamander that retains its juvenile gill bearing form throughout life. Modern evidence from molecular studies provides additional support for this theory, showing that sessile ascidians represent a derived body form, and that free- swimming lardaceous are perhaps the most similar to ancestral chordates. Subphylum Cephalochordata (Amphioxus) Cephalochordates have about 25 living species inhabiting shallow tropical and temperate oceans. Known as lancelets or as amphioxus (from the Greek for both \[ends\] pointed, (in reference to their shape), cephalochordates are small, (5cm 7cm long) eel-like animals that spend much of their time buried in sand. However, because of their remarkable morphology, they have proved crucial in understanding the morphology and evolution of the chordates. Cephalochordates have all the typical chordate features, although they lack features found in true vertebrates: the brain being very small and poorly developed, with poorly developed sense organs, and there are no true vertebrae. Water is taken in through the mouth, drawn in by the beating of cilia located on the wheel organ, a set of ridges lying inside the mouth. The oral cirri first filter the water (slender projections, which surround the opening of the mouth). It then passes through gill slits enclosed by folds of the body wall, which form a body cavity (the atrium). Food particles in the water are trapped by mucus, while water passes through the slits and out of the atrium through the atriopore, located towards the posterior end. The rest of the digestive system is fairly simple: a pouch or hepatic caecum secretes digestive enzymes; actual digestion takes place in a specialised part of the intestine known as the iliocolonic ring. Cephalochordates have a well-developed circulatory system and a simple excretory system composed of paired nephridia. Sexes are separate, with both sexes having multiple paired gonads. Eggs are fertilised externally, developing into free-swimming, fish-like larvae. Since cephalochordates have no hard parts, their fossil record is extremely sparse. However, fossil cephalochordates have been found predating the origin of the vertebrates. These fossils show that the chordate lineage appeared very early in the known history of the animal kingdom, and they strengthened the case for an origin of true vertebrates from a cephalochordate-like ancestor. Due to the lack of a defined head, it is thought that amphioxus resembles the ancestor that developed into the vertebrates, rather than the vertebrate ancestor itself. According to an article published in the New Scientist in 2008 amphioxus may be extremely common in shallow sandy environments: at Discovery Bay, Jamaica, up to 5000 individuals per square metre of sand have been reported. In some parts of the world, amphioxus is eaten by humans or by domestic animals; they are important food items in some parts of Asia, where they are commercially harvested. This example remains a popular one and still supports current research. Subphylum Vertebrata (Craniata) The third subphylum of chordates is a large and very diverse group, it shares the chordate features already mentioned with the other two subphyla but, in addition, members have a number of novel features. The alternative name of the group, 'Craniata', refers to the possession of a cranium, a bony or cartilaginous braincase; all (apart from some jawless fishes) also have vertebrae. The earliest vertebrates were larger than animals in the other subphyla, and considerably more active. Increased speed and mobility were benefits of modifications of the skeleton and muscles. The greater size and activity levels also required specialised structures for obtaining and digesting food, and other adaptations designed to maintain a high metabolic rate. Physiology Vertebrates have modifications to the respiratory, digestive, circulatory, excretory and reproductive systems needed to meet their increased metabolic requirements. The simply modified (for feeding) pharynx in early chordates is modified in the first larger predatory vertebrates into a muscular structure that actively pumps water. With the origin of highly visualised gills, the function changed to primarily being used for gaseous exchange. Changes to the digestive tract included a change from movement by ciliary action to muscular action, and the addition of accessory digestive glands, e.g. liver and pancreas, which were needed to utilise the higher volume of food needed. A ventral, chambered heart and erythrocytes (red blood cells) with haemoglobin improved transportation of nutrients, gases and other substances. Protochordates have no distinct kidneys, but the vertebrates have paired glomeruli kidneys that remove metabolic waste products, as well as regulating body fluids and salts (ions). Muscular-skeletal modifications Vertebrates possess an endoskeleton formed of cartilage or bone. Growing within the body, it has the advantage of allowing almost unlimited body size. The endoskeleton is thought to have initially been formed of cartilage, which later developed into bone. Cartilage, with its fast growth rate and flexibility forms the first skeleton of all embryonic vertebrates. The endoskeleton of living hagfishes, lampreys, sharks (and related groups) is composed of cartilage, even in some 'bony' fish such as sturgeon, the skeleton is largely cartilaginous. Bone is thought to have conferred an adaptive advantage in several ways. The presence of bone in the skin of ostracoderms and other ancient fishes would have provided protection from predators. The structural strength of bone is superior to cartilage, making it ideal for muscle attachment in areas of high mechanical stress. One interesting idea about the development from cartilage to bone is the function of mineral regulation. Phosphorus and calcium are needed for many physiological processes, and are in high demand in animals with high metabolic rates; bone provides a reservoir of these important substances. It is surprising to note that many vertebrates also possess an exoskeleton, although it is highly modified in many forms. Some of the primate fishes were covered in bony, dermal armour, which is modified into scales in most advanced forms. Many of the bones encasing the brain of advanced vertebrates develop from tissue that originates from the embryonic dermis. The majority of vertebrates are further protected with keratinised structures derived from the epidermis, e.g. reptilian scales, hair, feathers, claws, nails and horn. Nervous and sensory systems When the ancestral vertebrates developed from filter feeders to active predation, new sensory, motor and integrative systems were essential for successful food capture. The anterior end of the nerve cord became enlarged as a tripartite brain (forebrain, midbrain and hindbrain), which is protected by a cartilaginous or bony cranium. Paired sense organs, designed for distance perception evolved, including eyes (with lenses and inverted retinas) pressure receptors (paired inner ears for equilibrium and audio reception) chemical reception (taste and olfactory), lateral line receptors for water vibrations, and chemoreceptors to detect the electrical currents of prospective prey. Development of the vertebrate's head and sense organs is largely the result of two unique vertebrate embryonic innovations, the neural crest and epidermal placodes. The neural crest is a group of ectodermal cells lying along the length of the embryonic neural tube, which gives rise to many different structures, including the cranium, pharyngeal skeleton, teeth dentine, some cranial nerves and ganglia, Schwann cells, and some endocrine glands. The epidermal placodes are plate- like ectodermal thickenings either side of the neural tube, which gives rise to the olfactory, epithelium, lens of the eye, inner ear, epithelium, some ganglia and cranial nerves, lateral line mechanoreceptors and electroreceptors, which induce the formation of taste buds. In summary, all vertebrates possess the five chordate features plus: endoskeleton (particularly the cranium and in most a vertebral column), muscular pharynx, muscular digestive tract, ventral hart with erythrocytes containing haemoglobin, paired glomeruli kidneys, tripartite brain, paired specialised sense organs, endocrine system of ductless glands and a well- developed coelom, divided into a pericardial cavity and a pleuroperitoneal cavity. Sexes are nearly always separate; gonads have ducts, which either discharge into a cloaca or an opening near the anus. Most have two pairs of appendages, supported by limb girdles and an appendicular skeleton. The earliest vertebrates The earliest known vertebrate fossils (until recently) were armoured jawless fishes called ostracoderms, from the late Cambrian and Ordovician deposits. However, in 1999 research discovered two fishlike vertebrates from the early Cambrian (about 530 million years ago). They possess some features of vertebrates such as a heart, cranium and fin rays, but there is a lack of evidence for mineralised tissues, which may explain the extreme rarity of fossils prior to the late Cambrian. The earliest ostracoderms (termed herterostracans) were armoured with bone in the dermis, but lacked paired fins, so important to later groups for stability. Slit mouths may have enabled them to filter feed (or maybe prey on small animals) whilst propelling them close to the bottom; movement would have probably been too imprecise to enable free, pelagic swimming. Without ever-evolving paired fins or jaws, these animals flourished for 150 million years, becoming extinct at the end of the Devonian period. Coexisting with the heterostracans, the osteostracans had the additional advantage of having paired pectoral fins, and are thought to be similar to modern lampreys. The anaspids were more streamlined, and existed in many different forms during the Silurian and Devonian periods. The jawless forms are called collectively agnathans, (superclass Agnatha) contrasting with all jawed vertebrates being gnathostomes (superclass Gnathostomata). The origin of jaws was one of the most important events in vertebrate evolution, as they allow predation on large and active forms of prey, not available to jawless vertebrates, and they permit manipulation of objects. Evidence shows that jaws arose thorough modifications of the first or second of the serially repeated, cartilaginous gill arches. The mandibular arch may have first become enlarged to assist in gill ventilation. Later the anterior gill arches hinged and bent forward into the characteristic position of the vertebrate jaw. An additional feature of all gnathostomes is the presence of paired pectoral and pelvic fins or limbs, although pectoral fins seem to have appeared before pectoral appendages. These are likely to have originated as stabilisers to correct yaw, pitch and roll during active swimming. Early vertebrates such as the anaspids had intermediate 'paired flaps'. Amongst the first jawed vertebrates were the heavily armoured placoderms, which first appeared in the early Silurian period. They evolved into a large variety of forms. They were armoured with diamond shaped scales, or with plates of bone. All appeared to have become extinct by the end of the Devonian, and there are no known living descendents. Contemporaries of these animals were the acanthodians, another group of early-jawed fishes characterised by fins with large spines. It is thought they gave rise to modern day bony fishes, which dominate the world's waters today. Part 2: the fishes In common usage, the word 'fish' applies to many different aquatic organisms, from cnidarians (jellyfish) and echinoderms (starfish) to molluscs (shellfish) and crustaceans (crayfish). However, we will consider 'fish' to mean an aquatic vertebrate with gills, limbs, and usually a skin covered in dermal scales. The term 'fish' even in this context is not a taxonomic group but is used for convenience. Fish are of ancient ancestry, having descended form an unknown free-swimming protochordate ancestor. Superclass Agnatha Living jawless fishes number about 106 species divided into two classes, the Myxini (hagfish, 65 species) and Cephalaspidomorphi (lampreys, 41 species) and the extinct class of Ostracodermi. All these forms are either scavengers or parasites. Members of living classes lack (as well as jaws) internal ossification (including no or only rudimentary vertebrae), scales and paired fins, they have pore-like gill openings and an eel-like body form. In other respects, the groups are very distinct, with hagfishes being the more primitive animals; lampreys are closer in form to gnathostomes than hagfish. Hagfish have no paired appendages, stomachs, cerebellum and are almost completely blind. Being scavengers and predators, they are quickly attracted to food by keenly developed senses of smell and touch. A hagfish enters a dead or dying animal via an orifice or by digging inside. For extra leverage, it can tie itself in a knot, pressing the knot against the carcass. They are known for their copious production of slime, and, if disturbed, they exude a milky substance, which when mixed with seawater makes the animal so slippery it is impossible to grasp. Unlike any other vertebrate the body fluids of a hagfish are in osmotic equilibrium with seawater. As in most marine invertebrates, they also have three additional accessory hearts in addition to the main heart positioned behind the gills. Reproduction in the hagfish is poorly understood, although in some species females outnumber males 100 to 1. There is no larva stage. Lampreys have one or two pairs of dorsal fins, but no paired appendages. Their sucker like mouth is used either to maintain position in a fast flowing current, or to attach to larger animals to feed. All breed in fresh water, even those species which normally reside in marine habitats. Adults die soon after spawning, eggs hatch into small larvae, which resemble the protochordate Amphioxus. The larvae live as suspension feeders, growing for 3-7 years. After metamorphosis into the adult form, parasitic forms live for up to three years feeding on fish body fluids. Non-parasitic forms do not feed when adults, they metamorphose, then breed and die in the space of a few months. Superclass Gnathostomata (jawed fishes) Class Chonrichthyes (cartilaginous fishes) There are nearly 850 living species in this class. They are an ancient, compact and highly developed group. This group is smaller and less diverse than bony fishes, but their impressive array of well-developed sense organs, powerful jaws, swimming musculature and predatory habits have made them a very successful and long-lived group. Distinctive features of the class are a cartilaginous skeleton, although calcification may be extensive, bone is absent. This is a curious evolutionary feature, as the class is derived from ancestors who had well-developed bone. All but 28 species are marine species. With the exception of whales, sharks are the largest living vertebrates, some reaching 12m in length. All chondrichthyans have internal fertilisation, but maternal support of the embryo is highly variable. Many lay large, yolky eggs immediately after fertilisation (termed oviparous). Some oviparous sharks and rays deposit eggs in a horny capsule (mermaid's purse), which has tendrils that wrap around the first firm object it encounters. Embryos are nourished from the yolk for a long period (average six to nine months, one species up to two years), before hatching into tiny replicas of adults. Many sharks retain embryos within their reproductive tract for development. Some are ovoviviparous (young are retained, but are nourished by a yolk sack until born). Others are true viviparous species, with the embryo receiving nourishment from the maternal bloodstream through a placenta, in the form of nutritive secretions 'uterine milk' produced by the mother. Tiger shark embryos receive additional nutrition by eating eggs and siblings. Prolonged retention of embryos in elasmobranches was an important innovation, contributing to their success; however, there is no parental care of young. Subclass Elasmobranchii This is a subclass within the Chondrichthyes (cartilaginous fishes) that includes the sharks (Euselachii) and the skates and rays (Batoidei). The other subclass within Chondrichthyes, according to traditional classifications, is the Holocephali (chimaeras, or ratfish). It is probable that both groups arose independently, during the Silurian or Early Devonian, from a group of extinct armoured fishes, the Placodermi. The elasmobranches are distinguished by separate gill openings, and sensory electro receptors (ampullae of Lorenzini) in the head region. Elasmobranchii have the following characteristics: o o o A variably calcified cartilaginous endoskeleton Placoid scales (tooth-like dermal structure, which reduces water turbulence when swimming) Urea-retention mechanism (in order to maintain a similar osmotic pressure with the environment, which prevents water loss into the concentrated marine solution, they retain nitrogenous compounds in their extra cellular fluid, causing the blood salt concentration to be higher than the surrounding water) o o Clasper organs in the male for internal fertilisation (the medial part of the pelvic fin in males is specialised for internal fertilisation) The absence of an air (swim) bladder. Order Carchardiniformes (ground sharks) are often found in coastal waters, and range from tiger and bull sharks to the more unusual forms such as hammerheads. There are over 270 species, which are characterised by the presence of a nictitating membrane over the eye, two dorsal fins, an anal fin, and five-gill slits. Order Lamniformes contain several large pelagic forms dangerous to humans including Great Whites and mako sharks. The order also contains the extinct Megalodon shark, thought to have grown as large as 16m (52ft) long, and weighed 48 tons. It is thought to have preyed on whales. Characteristics of the order include: two dorsal fins, an anal fin, five gill slits, eyes without nictitating membrane, and mouth extending behind the eyes. Order Squaliformes contains dogfish and their relatives; they have two dorsal fins, no anal fin, no nictitating membrane and five gill slits. Order Rajiformes (also called Batiodea) is the group of rays (sawfish rays, electric rays, stingrays, eagle rays and devil rays). Batoids are flat-bodied, and like other elasmobranches, are species of cartilaginous marine fish. Like all sharks, they have slot-like body openings (gill slits) that lead from the gills to the outside environment. Most members of this group are adapted for bottom dwelling. Gill slits lie under the pectoral fins on the underside, whereas in other orders, they are on the sides of the head. Most batoids have a flat, disk-like body, with the exception of the guitarfish and sawfish, while most sharks have a streamlined body. Many species of batoid have developed their pectoral fins into broad flat wing-like appendages. The eyes and spiracles (where water is taken in by the gills to prevent clogging with the substratum) are located on top of the head. This group has developed some novel defence mechanisms. Stingrays have a slender, whip-like tail armed with one or more saw-edged spines with venom glands at the base. Electric rays are sluggish fish with large electrical organs on each side of the head. Each organ is composed of vertical stacks of disc-like cells, connected in parallel, so that when all cells discharge simultaneously, a high amperage current is produced that flows into the surrounding water, causing a useful deterrent to predators, and stunning prey. Interestingly, electric rays were used by the ancient Egyptians for a form of electrotherapy in the treatment of ailments such as arthritis and gout. Subclass Holocephali (Chimaeras) Members of this small subclass are known as ratfish, rabbit fish, spook fish and ghost fish, and are remnants of a line that diverged from shark lineage at least 360 million years ago. Fossil examples, dating from the Devonian, were prolific in the Cretaceous and early Tertiary and have now declined to the 31 species known today. Chimaeras live in temperate ocean floors and grow up to two metres long. Like other members of the class, Chondrichthyes, chimaeras have a skeleton constructed of cartilage. Their skin is smooth and lacks scales, and their colour ranges from black to brownish grey. Anatomically, chimaeras have several features in common with the elasmobranches, but also have several unique characteristics. Instead of a toothed mouth, jaws bear large flat plates. The upper jaw is fused to the cranium, a most usual fish feature. They eat seaweed, molluscs, echinoderms, crustaceans and fish, an unusually mixed diet for a specialised grinding dentition. For defence, most chimaeras have a venomous spine located in front of the dorsal fin.