Bony Fishes Class - Osteichthyes Scientific ClassificationClass Osteichthyes Class Osteichthyes includes all bony fishes. Like all fishes, Osteichthyes are cold-blooded vertebrates that breathe through gills and use fins for swimming. Bony fishes share several distinguishing features: a skeleton of bone, scales, paired fins, one pair of gill openings, jaws, and paired nostrils. Scientific ClassificationClass Osteichthyes Osteichthyes includes the largest number of living species of all scientific classes of vertebrates, more than 28,000 species. Osteichthyes account for about 96% of all fish species. Fishes not included in the Osteichthyes are the Chondrichthyes (sharks and their relatives), the Myxini (hagfishes), and the Cephalaspidomorphi (lampreys). Subclasses Living Osteichthyes are divided into three subclasses: Dipnoi, Crossopterygii, and Actinopterygii. The subclass Dipnoi (lungfishes) is characterized by an upper jaw fused to the braincase, fused teeth, and the presence of an air-breathing organ that opens to the esophagus. A lungfish's caudal fin is continuous with its dorsal and anal fins. Its pelvic and pectoral fins are long and tubular. Subclasses- Crossopteryggii The subclass Crossopterygii (coelacanths) is characterized by a type of primitive scale called a cosmoid scale, two dorsal fins, and fleshy paired fins that contain skeletal elements. Scientists used to think that this entire subclass of fishes was extinct. Then in 1938, a living coelacanth (Latimeria chalumnae) was discovered off the coast of Southeast Africa. Several specimens have since been collected. Class Actinopterygii The subclass Actinopterygii includes all other living bony fishes. Actinopterygians are characterized by fins that are supported by bony elements called rays. Orders and Families All orders of bony fishes end in the suffix "iformes". While there is debate over how certain fishes should be classified, scientists recognize more than 500 different bony fish families. The names of bony fish families all end in the suffix "dae". Genera and Species More than 28,000 species of bony fishes have been documented. It's likely that many more, including some deep-sea species, have yet to be identified. Fossil Record Primitive fishes date back to the Cambrian period, about 550 million years ago. These jawless fishes lived relatively unchanged over the following 100 million years. The Devonian period, about 360 to 400 million years ago, is known as the "Age of Fishes", because of the abundance and diversity of fishes that appeared during this period. Fossil Record ◦ In the Devonian, fishes began to develop jaws and paired fins. All four living classes of fishes and the three subclasses of Osteichthyes were established by the mid-Devonian. ◦ Many species of fish that lived during the Devonian are now extinct. Fossil Records Bony fishes continued to evolve after the Devonian period. Most modern orders of bony fishes probably evolved during the Triassic period, about 200 million years ago. Today, the Actinoptergians are the dominant vertebrates in the oceans and in freshwater systems. The most recently evolved orders of bony fishes include the Pleuronectiformes (flatfishes) and Tetraodontiformes (triggerfishes, pufferfishes, and molas). Habitat & Distribution Bony fishes inhabit almost every body of water. They are found in tropical, temperate, and polar seas as well as virtually all fresh water environments. Some species of bony fishes live as deep as 11 km (6.8 mi.) in the deep sea. Other species inhabit lakes as high as 5 km (3.1 mi.) above sea level. About 58% of all species of bony fishes (more than 13,000 species) live in marine environments. Although only 0.01% of the earth's water is fresh water, freshwater fishes make up about 42% of fish species (more than 9,000 species). Habitat Bony fishes live in fresh water, sea water, and brackish (a combination of fresh water and salt water) environments. The salinity of sea water is about 35 ppt (parts per thousand). Some species can tolerate higher-salinity environments. Some species of gobies can tolerate salinity levels as high as 60 ppt. Habitat Fishes live in virtually all aquatic habitats. Different species of fish are adapted for different habitats: rocky shores, coral reefs, kelp forests, rivers and streams, lakes and ponds, under sea ice, the deep sea, and other environments of fresh, salt, and brackish water. Habitat Some fish are pelagic: they live in the open ocean. For example, tunas (several species in the family Scombridae, subfamily Thunninae) are pelagic fishes. Some species, such as the flatfishes (order Pleuronectiformes) are adapted for living along the bottom. Certain fishes, such as gobies (family Gobiidae) even burrow into the substrate or bury themselves in sand. Ocean sunfish (family Molidae) are most often spotted at the ocean's surface. Habitat Some lungfishes "hibernate" throughout a summer drought season, buried under the mud of a dried-up pond. Several fish species live in freshwater habitats in the darkness of caves. Habitat Depending on the species, bony fishes can live at various temperatures. Some live at extreme temperatures. Some desert pupfish (Cyprinodon macularius) live in California hot springs that reach temperatures greater than 45°C (113°F). At the opposite extreme, some species of bony fishes can survive freezing temperatures of the Arctic and Antarctic. Certain glycoprotein molecules present in the blood of these speciallyadapted fishes lower the freezing point of the blood. The arctic cod (Boreogadus saida) can survive temperatures as low as -2°C (28°F). Habitat In general, fishes rely on oxygen dissolved in water for respiration. Some species of bony fishes require large amounts of dissolved oxygen. The brown trout (Salmo trutta) requires up to 11 mg of dissolved oxygen per liter (11 ppm, or parts per million). Misgurnus fossillis, a type of loach, can survive in water with an oxygen concentration as low as 0.5 mg per liter (0.5 ppm). Habitat Mudskippers (family Periophthalmidae) can carry a small amount of water in their gill cavities. They commonly spend time on land, returning to mud holes when their water supply begins to evaporate. African lungfishes (subclass Dipnoi) gulp air into a "lung" for respiration. In fact, these fishes must have access to the water's surface or they will drown. Size Thousands of species of bony fishes are less than a few centimeters long as adults. Among the smallest is the endangered dwarf pygmy goby (Pandaka pygmaea). Adult males reach just 15 mm (0.6 in.), and adult females reach only about 9 mm (0.4 in.). Size Some species can reach tremendous sizes - much larger than a human. The longest bony fish is the oarfish (Regalecus glesne), which can reach 11 m (36 ft.). Among the heaviest of the bony fishes is the common ocean sunfish (Mola mola), which lives throughout warm and temperate seas worldwide. A large sunfish can reach 3.3 m (10.8 ft.) and 2,300 kg (5,071 lb.). Size Many sturgeons (family Acipenseridae) grow very large. The largest is the beluga sturgeon (Huso huso), which inhabits the Caspian, Black, and Adriatic Seas and can reach 5 m (16.4 ft.) and 2,000 kg (4,409 lb.). Black marlin (Makaira indica) reach 4.7 m (15.4 ft.) and 750 kg (1,653 lb.). The European wels catfish (Silurus glanis) reaches 5 m (16.4 ft.) and about 300 kg (661 lb.). Body Shape Bony fishes show great variety in body shape, but the "typical" fish body shape is roughly cylindrical and tapering at both ends. This characteristic fusiform shape is quite energy efficient for swimming. Compared to other body shapes, this body shape creates less drag (the opposing force an object generates as it travels through water). Body Shape Various species of fishes deviate from the fusiform body shape in three ways: compression, depression, and elongation A laterally compressed (flattened, side-toside) body shape is common in bony fishes that live in dense cover or within coral reefs. Butterflyfishes (family Chaetodontidae) are an example of bony fishes with a laterally compressed body shape. Body shape A depressed (flattened, top-to-bottom) body shape is common in bottomdwelling fishes. Goosefishes (family Lophidae) and batfishes (family Ogcocephalidae) are examples of bony fishes with a depressed body shape. The body shape of an eel (for example, the morays, family Muraenidae) is an extreme example of anelongated shape. Coloration Most fish species have pigmentation. Pigment is mostly contained in cells called chromatophores. Most fishes can contract and expand their chromatophores to change colors. Reflective cells called iridocytes can change color rapidly. Because the different wavelengths of light are absorbed at various depths, fishes may appear a different color underwater than at the surface. Some fish, such as the ghost glass catfish (Kryptopterus bicirrhis), lack pigmentation. coloration Some fish bioluminate (emit light).Certain pigments (called luciferins) emit light when oxidized. Some fish produce light in luminescent organs or in cells called photophores. In some fish, it is light-producing bacteria that live in or on the fish that actually produce the light. Depending on species, bioluminescence may attract mates, deter or confuse predators, attract prey, or act as "headlights" to help a fish see in the dark. Fins All fishes have fins. Bony fish families show various degrees of fin fusion and reduction. Fins help stabilize or propel a fish in the water. Except in the lungfishes and the coelacanth, fins lack bones. In Actinopterygians, fins are supported by structures called rays. fins The spiny fin rays of some species are associated with venom glands. Fishes in the family Scorpaenidae include the stonefish (Synanceja spp.), the lionfish (Pterois spp.), and the scorpionfish (Scorpaena spp.) - some of the most venomous fishes in the world. Glands in the dorsal, anal, and pelvic spines produce venom that is intensely painful and occasionally fatal to humans. fins Fishes have two kinds of fins: paired fins (pectoral and pelvic) and median fins (dorsal, caudal, and anal) Typically, the paired pectoral fins help a fish turn. In some fishes, pectoral fins are adapted for other functions. Paired pelvic fins add stability, and some fishes use them for slowing. In the clingfishes (family Gobiesocidae), the pelvic fins are adapted as a sucking appendage, which helps a fish hold on to stationary objects on the ocean bottom. fins The dorsal fin may be a single fin or separated into several fins. In most bony fishes, the dorsal fin is used for sudden direction changes and acts as a "keel", keeping the fish stable in the water. In some fishes, the dorsal fin is adapted for other functions. The caudal fin, or tail, is responsible for propulsion in most bony fishes. Caudal fins come in many shapes. Many continuously swimming fishes have forked caudal fins. Fishes with lunate caudal fins, such as tunas, tend to be fast swimmers that can maintain rapid speed for long durations. fins The anal fin adds stability. In some fishes, the anal fin is adapted for other functions. Some species of bony fishes have reduced or absent fins. For example, morays (family Muraenidae) lack pectoral fins and pelvic fins. Several species lack an anal fin. Head Eye size and position vary depending on the habitat and behavior of the species. In most species, the gills are protected by a flexible plate called an operculum. Most bony fishes have a single pair of gill openings. Some bony fishes such as eels (family Anguillidae) have a pair of gill holes or pores that aren't covered by an operculum. The nostrils of most bony fishes have no connection with the mouth or gills. In some bony fishes (such as eels), the nostrils' incurrent and excurrent openings are widely separated. Mouth shape and size are good indications of bony fish's feeding habits. Scales Most species of bony fishes are covered with and protected by a layer of plates called scales. There are four different kinds of bony fish scales: cosmoid, ganoid, cycloid, and ctenoid. scales True cosmoid scales are found only on extinct Crossopterygians. The inner layer of a cosmoid scale is compact bone. On top of this bone layer lays a spongy layer and then a layer of cosmine (a type of dentin). The upper surface is enamel. Gars (family Lepisosteidae), bichirs, and reedfishes (family Polypteridae) have ganoid scales. They are similar to cosmoid scales, but a layer of ganoin (a hard, enamel-like substance) lies over the cosmine layer and under the enamel. Ganoid scales are diamond-shaped, shiny, and hard. scales Most bony fishes have cycloid or ctenoid scales. Both cycloid and ctenoid scales consist of an outer layer of calcium and an inner layer of connective tissue. Cycloid scales overlap from head to tail, an arrangement that helps reduce drag as a fish swims. Cycloid scales are circular and smooth. They are most common on fishes with soft fin rays. Ctenoid scales have a characteristic toothed edge. They are most common on fishes with spiny fin rays. As a fish grows, cycloid and ctenoid scales add concentric layers. Body Spines Body spines are modified scales. Protective spines are common in slowswimming fishes and others that need to protect themselves without moving. Some fishes actively engage spines. Mucus A fish secretes a layer of mucus that covers its entire body. Mucus helps protect a fish from infection. In some bony fishes, mucus may serve additional functions. ◦ Some species of parrotfishes (family Scaridae) envelop their bodies in mucous bubbles at night while they rest. This mucous barrier may "hide" the parrotfish from nocturnal predators that rely on their sense of smell to locate prey. ◦ Young discus (Symphysodon discus) feed on the parent fish's mucus. Skeletal System The skeleton of bony fishes is made of bone and cartilage. The vertebral column, cranium, jaw, ribs, and intramuscular bones make up a bony fish's skeleton. The skeleton of a bony fish gives structure, provides protection, assists in leverage, and (along with the spleen and the kidney) is a site of red blood cell production. Muscular System The muscles of the tail and trunk consist of a series of muscle blocks called myotomes. The myotomes usually resemble a sideways letter "W". A connective tissue called myosepta separates the myotomes. A horizontal septum separates the myotomes into dorsal (top) myotomes and ventral (bottom) myotomes. Nervous System The nervous system of fishes is poorly developed compared to that of other vertebrates. A bony fish's brain is divided into three sections: the forebrain, the midbrain, and the hindbrain. The forebrain is responsible for the bony fish's ability to smell. Bony fishes that have an especially good sense of smell, such as eels, have an enlarged forebrain. Nervous system The midbrain processes vision, learning, and motor responses. Blind bony fishes, such as blind cavefishes in the family Amblyopsidae, have a reduced midbrain. The hindbrain (medulla oblongata and cerebellum) coordinates movement, muscle tone, and balance. Fast-swimming bony fishes usually have an enlarged hindbrain. Cardiovasular System A bony fish's heart has two chambers: an atrium and a ventricle. The venous side of the heart is preceded by an enlarged chamber called the sinus venosus. The arterial side of the heart is followed by a thickened muscular cavity called the bulbus arteriosus. Digestive System The esophagus in bony fishes is short and expandable so that large objects can be swallowed. The esophagus walls are layered with muscle. Most species of bony fishes have a stomach. Usually the stomach is a bent muscular tube in a "U" or "V" shape. Gastric glands release substances that break down food to prepare it for digestion. At the end of the stomach, many bony fishes have blind sacs called pyloric caeca. The pyloric caeca are an adaptation for increasing the gut area; they digest food. Swim Bladder Many species of bony fishes have a gas-filled bladder called a swim bladder. Apparently the swim bladder originally developed in fish as an organ of respiration, as evidenced by the "lung" of the lungfishes. In modern bony fishes that possess a swim bladder, the organ serves principally in maintaining neutral buoyancy. In some fishes the swim bladder has adapted to function as a sound amplifier. Acoustic Senses The ears of a bony fish function in equilibrium, detecting acceleration, and hearing.There are no external openings to the ears. Sound waves travel through soft tissue to the ears. (A fish's soft body tissue has about the same acoustic density as water). In some bony fish species, the swim bladder is associated with adaptations for enhanced sound reception at higher frequencies. In some, the swim bladder lies against the ear and acts as an amplifier to enhance sound detection. In other species, such as goldfish (Carassius auratus), a series of small bones connects the swim bladder to the ear. Eyesight Bony fishes have a basic vertebrate eye, with various structural adaptations. A bony fish's eye includes rods and cones. Bony fishes, especially those that live in shallow-water habitats, probably have color vision. Certain visual cells are specialized to particular wavelengths and intensities. The eyesight in some species of bony fishes may be well developed. Goldfish (Carassius auratus) have excellent visual acuity up to 4.8 m (16 ft.) away. Taste Bony fishes have taste buds in their mouths. Some species have taste buds along the head and ventral side of the body. Taste perception hasn't been extensively studied in bony fishes. Some species can detect some sensations, such as salty, sweet, bitter, and acid stimuli. Taste may be responsible for the final acceptance or rejection of prey items. Smell Olfactory cells in the nasal sac detect tiny amounts of chemicals in solution. In general, the sense of smell is well developed in fishes. The nasal areas and extent of the sense of smell vary among species. Electroreception Some bony fishes in the families Electrophoridae, Gymnotidae, and Mormyridae produce a lowvoltage electric current that sets up a field around the fish. Tiny skin organs on the fish detect disruptions in the electric field that are caused by prey or inanimate objects. Electric organs are made up of cells called electrocytes that have evolved from muscle cells. Electrocytes typically are thin and stacked on top of one another. Electroreception is an adaptation for detecting prey and for navigation in murky water. Fertilization and Embryonic Development Some species release unfertilized eggs and sperm.Young develop from eggs that are fertilized in the water. Some species have internal fertilization; these species mate. For species with internal fertilization, there is great variation in the development stage at which offspring are released: fertilized eggs, larvae, juvenile fish, or even sexually mature adults. development Oviparous bony fishes release eggs, and the developing embryo is nourished by a yolk sac. In ovoviviparous fishes, one parent (usually female) retains the fertilized eggs in her body, and the developing embryo is nourished by a yolk sac formed prior to fertilization. There is no nutrient connection between the parent and the developing embryos. development In viviparous fishes, the female retains the fertilized eggs in her ovary or uterus, and the developing embryo is nourished by connection with the mother.