Evolution of Fishes PDF

Osteichthyes (bony fishes) In the early to mid Silurian period, a group of fishes with bony endoskeletons gave rise to over 90% of all living fishes and all living tetrapods (four limbed animals). It was originally believed that these were the only fish with bony skeletons, but it is now known that...

Osteichthyes (bony fishes) In the early to mid Silurian period, a group of fishes with bony endoskeletons gave rise to over 90% of all living fishes and all living tetrapods (four limbed animals). It was originally believed that these were the only fish with bony skeletons, but it is now known that other groups such as the ostracoderms, placoderms and acanthodians also had bone. Bony fishes and tetrapods are united by the presence of endochondral bone (bone which replaces cartilage during development), the presence of lungs or a swim bladder derived from the gut, as well as dental and cranial characteristics. The term Osteichthyes is a convenient term, but does not represent a natural group, and is therefore not usually recognised as a valid taxon. Fossils of the earliest bony fishes show a bony operculum (gill cover) in common with acanthians, showing a likely common ancestor. By the middle of the Devonian, fishes had radiated into two main groups, the ray finned fishes (class Actinopterygii) including modern bony fishes, and the Sarcopterygii, (lobe finned fishes) which are represented today by only eight fish-like vertebrates, the lungfishes and coelacanths, which is a sister group to the land vertebrates. Several key adaptations contributed to their success. The bony operculum and associated muscles allow for more efficient respiration, and the progressively more specialised jaw musculature and skeletal elements have produced many effective feeding adaptations. Class Actinopterygii Ray finned fish number almost 23,600 species. Earliest examples (paleoniscids) were small, with large eyes, a heteroceral tail and thick interlocking scales. These fishes had a single dorsal fin and numerous bony rays derived from scales stacked end to end, and were very distinct from the lobe finned fishes with which they shared the Devonian waters. The paleoniscids were first seen as the ostracoderms. Placoderms and acanthodians were declining, which suggests morphological specialisations evolving in the actinopterygii gave them superiority over the other forms of the period. From these earliest forms, two major groups emerged. There were those with the most primitive characteristics (chondrosteans) represented today only by sturgeons and paddlefishes. These fishes show many similarities with their paleoniscid ancestors, including the tail and scale forms. Unfortunately, they are considered superior 'sport fish' and larger specimens are prized. The second major group that emerged from the palaeoniscid was the neopterygians, which appeared in the late Permian, radiating extensively throughout the Mesozoic. One branch resulted in the emergence of the modern bony fishes, the teleosts. There are two surviving genera of the early neopterygians, the bowfins, and gars. Both may surface to gulp air, filling their vasacularised swim bladder with air, which supplements the oxygen obtained via the gills. Teleost diversity is extraordinary, with the 23,600 (approximately) described species representing 96% of all living fish species and about half of all vertebrate species. It has also been estimated that between 5000 and 10,000 (or more) species have yet to be described. Almost all of the new species discovered yearly (averaging 200 species) are from tropical South America, or deep oceanic habitats. Teleosts are incredibly diverse, ranging from 10mm adult gobies to 17m oarfish They inhabit waters from 5200m above sea level (Tibetan streams) to 8000m ocean depths, in temperatures from -2 C in the Antarctic, to 44 C in hot springs, in complete darkness in caves, from three times the salinity of normal sea water to fresh water and swamps without oxygen, or, remarkably, making terrestrial journeys (mudskippers). This huge range of forms is due to several morphological trends. Light thin cycloid and ctenoid scales replaced the heavy dermal armour of primitive ray finned fishes; some (eels and catfishes) completely lack scales. The increased mobility and speed resulting from the loss of heavy armour improved predator avoidance and feeding efficiency. Most fish are protected by scales (overlapping bony plates covering all or part of the body). Scales can be classified into four types: placoid, cycloid, ctenoid, and ganoid. Ctenoid scales have small points on the surface and are rough to the touch. These, and the smooth, rounded cycloid scales are most common among modern fish species. Some, less highly developed fishes, have tough ganoid scales, while sharks and many rays have placoid scales, sharp and tooth-like. Changes in the fins of teleosts have increased manoeuvrability, speed, and have allowed fins to become specialised for other functions. The symmetrical shape of the homocercal tails of most teleosts (pointed, round, truncate, emarginated, forked and lunate) focus muscular effort on the tail resulting in increased speed. The dorsal fin shifted from providing a fixed keel to prevent rolling, to a flexible and highly specialised structure in higher teleosts. The changes in the morphology of fins were useful for camouflage, braking, streamlining, and social modification, even acting as a prey lure in anglerfish, venom delivery in scorpion fish, and the suctorial disc of shark suckers. The swim bladder changed from a primarily a respiratory function to one of buoyancy. Teleost taxonomy is complex; some of the orders of the more well-known forms are given below. Order Notacanthiformes, (halosaurs, spiny eels), Order Anguilliformes (true eels), Order Clupeiformes, (herrings, anchovies), Order Cypriniformes, (barbs, carp, danios, goldfishes, loaches, minnows, rasboras) Order Characiformes, (characins, pencilfishes, hatchetfishes, piranhas, tetras). Order Gymnotiformes, (electric eels, knifefishes), Order Siluriformes (catfishes), Order Salmoniformes, (salmon, trout), Order Esociformes (pike), Order Stomiiformes, ( bristlemouths , marine hatchetfishes), Order Aulopiformes (Bombay duck , lancetfishes), Order Myctophiformes,( lanternfishes), Order Lampriformes ( oarfish, opah ribbonfishes), Order Percopsiformes, ( cavefishes, trout-perches), Order Batrachoidiformes,( toadfishes), Order Lophiiformes, (anglerfishes) Order Gadiformes,( cods),Order Mugiliformes,(mullets), Order Beloniformes ( flyingfishes), Order Cetomimiformes (whalefishes), Order Cyprinodontiformes ( livebearers, killifishes),Order Zeiformes, (dories) Order Gasterosteiformes ( sticklebacks, pipefishes, seahorses, Order Tetraodontiformes ( filefishes, pufferfish), Order Pleuronectiformes, ( flatfishes), Order Scorpaeniformes (scorpionfishes, sculpins), and finally Order Perciformes 40% of all fish including anabantids, bass, cichlids, gobies, gouramis, mackerel, perches, scats, whiting, wrasses. Class Sarcopterygii (lobe finned fishes) Lobe finned fishes today are limited to eight species (six lungfish and two coelacanths); these are the survivors of an abundant Devonian group. All early sarcopterygians had lungs as well as gills and a heterocercal type tail. During the Palaeozoic era, the tail became symmetrical, with medial dorsal and ventral fins displaced posteriorly to form one continuous, flexible fin around the tail, also known as a diphycercal tail. The strong fleshy-paired lobed fins (pectoral and pelvic) may have been used much as legs are in terrestrial vertebrates, to move along the bottom. They had powerful jaws and skin covered in heavy scales. The Australian Lungfish (Neoceratodus) is the living form most like its ancestor. It can be up to 1.5m in length, and unlike the other lungfish, relies on gill respiration, and cannot survive for long out of water. The African lungfish, Protopterus, lives in streams and lakes that can dry out. When this occurs, the fish burrows down into the mud at the beginning of the dry season, and secretes copious slime, which mixes with the mud to form a cocoon, in which it lives until the rains come. Rhipidistians flourished in the late Palaeozoic era, before becoming extinct. They are a very important group, because they include the ancestors of the tetrapods. The Coelacanths also arose in the Devonian period, reaching their peak in the Mesozoic era; it was previously thought they became extinct at the end of the Mesozoic, 70 million years ago. In 1938, the remains of a fish that resembled these long dead animals, was found in a fisherman's net off South Africa. A population was found living at great depth off the Comoro islands near Madagascar, and they were thought to be the only living population. However, in 1998 a new species of coelacanth was discovered near Sulawesi, Indonesia, 10,000km from the Comoro islands. The 'modern' coelacanths are descendants of the Devonian freshwater species, the tail is diphyceral, but possesses a small lobe between the upper and lower caudal lobes, giving a three pronged structure. Coelacanths are a deep metallic blue, with irregular white or brassy flecks, which provide camouflage against the dark lava cave reefs they inhabit. Young are born fully formed after hatching internally from eggs 9 cm in diameter, the largest among bony fishes. Part 3: tetrapods, the move to land Adaptations for living on land is a major theme for the rest of the vertebrate groups, these animals form a diverse group called tetrapods. Amphibians and amniotes (including reptiles, birds and mammals) represent two major branches of tetrapod evolution. The movement from water to land is perhaps the most dramatic evolutionary event known, as it involves the invasion of a physically hazardous environment. Life evolved in water, animals are mostly made of water, and all cellular activities occur in water. Tetrapods were not the first living organisms to solve these problems. Vascular plants, pulmonate snails and tracheae arthropods preceded vertebrates to the land, and winged insects were diversifying at approximately the same time vertebrates moved into terrestrial environments. The invasion of the land required modification of almost every body system. Aquatic and terrestrial vertebrates retain many basic structural land functional similarities, however, the transitional stages between the two forms is most clearly seen in the amphibians, which undergo the change in each individual life history. There are several important physical differences that animals must deal with in order to move from water to air. These include oxygen content, density temperature regulation, and habitat diversity. Air is at least 20% higher in oxygen than water, and diffuses much more rapidly in air than water. Consequently, terrestrial animals can obtain oxygen more easily than aquatic ones, once the necessary adaptations have occurred. As air is 1000 times less buoyant than water, and is 50 times less viscous, problems arise, as it provides less bodily support than water. Terrestrial animals needed to have stronger limbs and remodel their skeletons to deal with the increased effects of gravity. Air fluctuates in temperature to a much greater extent, experiencing harsh and unpredictable cycles of freezing and thawing with the often accompanying flood and drought. Terrestrial animals, thus, need to have behavioural land physiological adaptations to protect themselves from thermal extremes. The most important strategy, which solved this problem is homoeothermy (regulated constant body temperature) developed in birds and mammals. Despite these quite profound problems, the move to land had many benefits, from the increase in the number of available habitats, from forest (polar, temperate and tropical) grasslands, deserts, alpine, swamps, heathland, oceanic islands and Polar regions. Early evolution of terrestrial vertebrates The Devonian period, beginning 400 million years ago, was characterised by mild temperature and alternating drought and floods. During this era, some primarily aquatic vertebrates evolved two features that were vital for permitting subsequent terrestrial evolution - limbs and lungs. Devonian freshwater habitats were unstable, with periods of drying, flood and stagnant water. Only those fishes able to acquire oxygen from the air could survive these unpredictable conditions. Gills were unsuitable for these conditions, as in air the gill filaments collapse, dry, and lose the functioning capabilities. Virtually all freshwater fish that survived this period (including the lungfish and coelacanths) had a type of lung that developed as an outgrowth of the pharynx. Improving vascularity and surrounding it with a rich capillary network filled with arterial blood enhanced the efficiency of the air-filled cavity. Oxygenated blood returned directly to the heart by a pulmonary vein to form a complete pulmonary circuit. Thus, the double circulation characteristic of all tetrapods originated, a systematic circulation serving the body, and a pulmonary circulation supplying the lungs. Vertebrate limbs also evolved during the Devonian period. An examination of the bony elements of the paired lobed fins of sarcopterygians shows they broadly resemble the equivalent limbs of amphibians. The fins have long, ray-like dermal bones (lepidotrichia) that characterise the limbs as fins, but they also contain a well-developed endochondral or internal skeleton, such as appendicular bones (humerus and femur) that articulate with the pectoral and pelvic girdles, respectively. The diagram shows a comparison of the limb bones of a crossopterygian fish (right) and the limb bones of an early amphibian (left). r = radius u = ulna h = humerus Until recently, it was thought that early tetrapods had five 'fingers and toes', the pentadactyl arrangement found in most living tetrapods. However, there is fossil evidence to suggest that the very first tetrapods had many multiple digits, more like the crossopterygian fish, the five-digit arrangement stabilised soon afterwards. It is thought that, due to seasonal droughts, individuals that were able to flop or pull themselves to a deeper neighbouring pool survived, and that those with fleshy fins had the greater ability to do this, circumstances, which directly support Darwin's theory of survival of the fittest. Unfortunately, fossils found recently have contradicted this very plausible explanation. This new evidence suggests that one of the earliest known tetrapods (Acanthostega) was a fully aquatic animal. A consensus among zoologists is now emerging that tetrapods evolved their limbs underwater, and subsequently moved onto land. The evidence points to lobe finned fishes as the closest relatives of tetrapods. In taxonomic terms they are sister groups. Both lobe finned fishes and early tetrapods such as Acanthostega and Ichthyostega share several characteristics of their skull, teeth and pectoral girdle. Ichthyostega represents an early offshoot of tetrapod phylogeny that possessed several adaptations (in addition to jointed limbs), which equipped it for life on land. These adaptations included stronger vertebrae and associated muscles to support the body in air, new muscles to elevate the head, strengthened shoulder and hip girdles, protective rib cage, modified ear structure for detecting airborne sounds, foreshortening of the skull, and lengthening of the snout which improved olfactory powers for detecting air borne odours. Yet, Ichthyostega still strongly resembled aquatic forms, as it retained a tail complete with fin rays, and had opercula (gill covers) bones. Radiation of the tetrapods The Devonian era was followed by the Carboniferous period, characterised by a warm, wet climate producing a swampy environment filled with mosses and ferns. Tetrapods radiated quickly into a variety of forms, feeding on the abundant insects and aquatic invertebrates. The exact relationships between the various groups of this period are still unclear, and have been revised many times. One group, the Lissamphibia, (which contains the modern amphibians) is distinguished by generally having only four digits on the forelimb. The lissamphibia diversified to produce the ancestors of the three major groups of modern amphibians, the Anura (frogs) Urodela or Caudata (salamanders) and Apoda or Gymnophiona (caecilians). Amphibians extended their adaptations to living in water during this period, including becoming flatter to move through shallow water. Early salamanders had weak limbs, and developed a tail useful for swimming. Even anurans, which are now largely terrestrial as adults, developed specialised hind limbs with webbing, which is better suited for swimming than walking on land. The adaptation of amphibians of being able to absorb oxygen via their skin in damp environments was very useful in the swampy carboniferous environment, but caused a significant desiccation problem for descendants in drier environments. Two additional groups of carboniferous and Permian tetrapods (lepospondyls and anthracosuars) are thought (by skull structure) to be closer to amniotes than the group containing the lissamphibia (known as the temnospondyls). These two groups are thought to be the animals whose descendants became the higher vertebrate groups. Modern amphibians The three living amphibian orders contain more than 4200 species. Most share terrestrial adaptations, including skeletal strengthening, a shift in specialised sense organs from the ancestral lateral line system to senses of smell and hearing. However, they still largely remain bound to an aquatic environment as they require water to reproduce, they lay eggs without casings (so at risk of desiccation), which hatch into gill bearing larvae. A metamorphosis then occurs in which gills are lost and the lungs, which are present throughout larval life, take over. As is common with animals, there are exceptions to this pattern with some salamanders (axolotls) retaining the larval form throughout life. Others live entirely on land, having no aquatic larval form. Some frogs have also developed a terrestrial habit by eliminating the larval stage. Completely terrestrial amphibians may lay eggs under rocks or logs, in flooded tree holes or in pockets on a parent's back. Despite these adaptations, even the exclusively terrestrial species still rely on moisture, as there skin is thin, requiring moisture to prevent desiccation. Amphibians also require moderately cool and constant temperatures, as they are exothermic (have the same body temperature as the environment). Order Gymnophiona (caecilians), consists of about 160 species of elongated, limbless, burrowing creatures. Their habitats are the tropical rainforests of South America, with a few species in Africa and South East Asia. They have vertebrae, long ribs, and a terminal anus; eyes are small, with most species being blind as adults. They have special sensory tentacles on the snout, which they use to locate their prey of worms and other invertebrates underground. Fertilisation is internal, males have a protrusible copulatory organ, and eggs are laid in moist ground near water. Larvae may be aquatic or larval development is completed within the egg. In some species, eggs are guarded in folds of the body; viviparity is common, with embryos obtaining nourishment by eating the wall of the oviduct. Order Caudata (Salamanders) are the tailed amphibians, with approximately 360 species that can be found in almost all northern temperate regions, being particularly abundant in North America. They are also found in tropical central and northern South America. Salamander are typically small, most are under 15cm long, although an aquatic Japanese species can grow up to 1.5m long. Most salamanders have limbs at right angles to their body, with fore and hind limbs of equal size, although in some burrowing forms, limbs are rudimentary or absent. They are carnivorous both when larvae and adult, preying on worms, small arthropods and molluscs. Most are only attracted to moving prey. Like other amphibians they are exothermic, and have a low metabolic rate. Some species have an entirely aquatic life cycle, although most are metamorphic, having aquatic larvae and terrestrial adult forms that live in moist environments under logs and stones. Fertilisation is usually internal, normally after the female picks up a packet of sperm (spermatophore) that has been deposited on the substratum. The male deposits the spermatophore, then leads the following female over it, tilting her front end up by arching his tail so her vent contacts the spermatophore. Aquatic species lay eggs in clusters or strings in water. Eggs hatch to produce a 'tadpole' with external gills and a fin-like tail. Completely terrestrial species lay eggs under logs, or in moist earth. Many species guard eggs, and they have direct development, bypassing the larval stage. Order Anura (frogs and toads) This group is first known in the fossil record from the Jurassic period, 150 million years ago, and today the order Anura contains 5250 species in 33 families, of which the Leptodactylidae (1100 spp.), Hylidae (800 spp.) and Ranidae (750 spp.) are the richest in species. About 88% of amphibian species are frogs. The use of the common names 'frog' and 'toad' has no taxonomic justification. From a taxonomic perspective, all members of the order, Anura, are frogs, but only members of the family, Bufonidae, are considered 'true toads'. The use of the term 'frog' in common names usually refers to species that are aquatic or semi-aquatic with smooth and/or moist skins, and the term 'toad' generally refers to species that tend to be terrestrial with dry, warty skin. Frogs and toads are broadly classified into three suborders: Archaeobatrachia, which includes four families of primitive frogs; Mesobatrachia, which includes five families of evolutionary intermediate frogs; and Neobatrachia, by far the largest group, which contains the remaining 24 families of 'modern' frogs, including the most common species throughout the world. This classification is based on such morphological features as the number of vertebrae, the structure of the pectoral girdle, and the morphology of tadpoles. While this classification is largely accepted, relationships amongst families of frogs are still debated. Future studies of molecular genetics should soon provide further insights into the evolutionary relationships amongst frog families The morphology of frogs is unique among amphibians. Compared with the other two groups of amphibians, frogs are unusual, as they lack tails as adults and legs are more suited to jumping than walking. The physiology of frogs is similar to other amphibians, because oxygen can pass through their highly permeable skin; this makes frogs susceptible to many toxins in the environment, some of which can similarly dissolve in the layer of water and be passed into their bloodstream. This may be a reason for the decline in frog populations. Widespread population crashes and mass localised extinction have been noted since the 1980s all over the world. These declines are perceived as one of the most critical threats to global biodiversity, and several causes are believed to be involved, including habitat destruction/modification, over-exploitation, pollution, pesticide use, introduced species, climate change, increased ultraviolet-B radiation (UV-B) and disease. However, many of the causes of amphibian declines are poorly understood. Many characteristics are not shared by all of the approximately 5250 described frog species. However, some general characteristics distinguish them from other amphibians. Frogs are usually well suited to jumping, with long hind legs and elongated ankle bones. They have a short vertebral column, with no more than ten free vertebrae, followed by a fused tailbone (urostyle or coccyx), typically resulting in a tailless phenotype. Frogs range in size from 10mm (Brachycephalus didactylus of Brazil and Eleutherodactylus iberia of Cuba) to 300mm (goliath frog, Conraua goliath, of Cameroon). The skin hangs loosely on the body, because of the lack of loose connective tissue. Skin texture varies from smooth to warty or folded. Frogs have three eyelid membranes: one is transparent to protect the eyes underwater, and two vary from translucent to opaque. Frogs have a tympanum on each side of the head, which is involved in hearing and, in some species, is covered by skin. Most frogs have teeth (maxillary teeth), consisting of a ridge of very small cone teeth around the upper edge of the jaw. They may also have vomerine teeth on the roof of their mouth. There are no teeth on the lower jaw, and food is usually swallowed whole. True toads do not possess teeth. Their aquatic reproductive habits and water permeable skin prevent them from being too far from moist environments, and the exothermic temperature regulation prevents them from occupying polar and sub polar environments. Camouflage is a common defensive mechanism, although many frogs are also nocturnal to reduce predation and dessciation. Some species can alter their skin colour, although it is usually restricted to shades of one or two colours. For example, White\'s tree frog varies in shades of green and brown. Some species change colour between night and day, and light and moisture stimulate the pigmented skin cells and cause them to expand or contract. Many frogs contain mild toxins making them distasteful to potential predators. All toads have large poison glands (parotid glands), behind the eyes on the top of the head. The chemical makeup of toxins in frogs varies from irritants to hallucinogens, convulsants, nerve poisons, and vasoconstrictors. Many predators of frogs have adapted to tolerate high levels of these poisons. Poisonous frogs tend to advertise their toxicity with bright colours, an adaptive strategy known as aposematism. When mature, Anurans usually assemble at a water source (pond or stream) to breed. Many return to the bodies of water where they were born, often resulting in annual migrations involving thousands of frogs. Once at the breeding ground, male frogs call to attract a mate, the call is species specific, attracting only females of that species. The male and female frogs then undergo amplexus. This involves the male mounting the female and gripping her tightly. Fertilisation is external. The female releases her eggs, which the male frog covers with a sperm solution. The eggs then swell and develop a protective coating. The eggs are typically brown or black, with a clear, gelatine-like covering. Most temperate species of frogs reproduce between late autumn and early spring. Water temperatures at this time of year are relatively low (4-10 Celsius) so this helps the developing tadpoles, because dissolved oxygen concentrations in the water are highest at cold temperatures. More importantly, reproducing early in the season ensures that appropriate food is available to the developing frogs at the right time. It is estimated that up to 20% of amphibian species may care for their young in one way or another, and there is a great diversity in parental behaviours. Some species of poison dart frog lay eggs on the forest floor and protect them, guarding the eggs from predation and keeping them moist with their own urine. After hatching, a parent (the sex depends upon the species) will move them, on its back, to a water-holding plant. The parent then feeds them by laying unfertilised eggs in the 'pool' until the young metamorphose. Other frogs carry the eggs and tadpoles on their hind legs or back (Alytes spp.) or protect their offspring inside their own bodies (Assa darlingtoni). It has pouches along its side in which the tadpoles reside until metamorphosis. Female gastric-brooding frogs (Rheobatrachus) swallow their tadpoles, which then develop in the stomach. However, despite all the incredible adaptations of the animals described in this module, they are all still tied to water, at least to reproduce. Eggs and adult skin have little or no protection against very hot, very cold or dry conditions, which has greatly restricted them to moist warm/temperate environments. The evolution of species, which have overcome these problems, will be dealt with in the next module, particularly the evolution of the amniote egg, reptiles and birds.