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The Class Osteichthyes • (Greek, Osteon = bone, ichthyes = fish). • The bony fish are the most diverse class of vertebrates. They comprise more than 95% of all fish and 50% of all species of vertebrates. • consists of the bony fishes and is the largest class of vertebrates, • with over 24,000 species.
Bony fish skeleton is made of bone, which is an important characteristic. operculum–a flap on each side of the head that covers the gills. The movements of these flaps allow the fish to breathe without moving.
Bony fishes range in size from the very small Philippine Goby Mugilogobius parvus ,(topmost) which is only about 10 mm long, to the ocean sunfish Mola mola, which can grow up to 4 m in length and 1500kg in weight (bottom).
THE BONY FISH • As well as many vertebrae for structural support and calcium storage, the bony fishes have special adaptations to control their buoyancy. A special organ called the swim bladder acts as a gas-filled chamber to control their depth in the water.
Another prominent feature of the bony fishes is the operculum. This constitutes the lateral fleshy grooves on the fish that cover the chambers housing the gills. The operculum allows the fish to breathe (even if it is not swimming) by moving water currents over the gills. Other features include paired fins, dermal scales in the skin (in most species) and numerous vertebrae.
Major Sub-groups in the Phylum There are two subclasses of Osteichthyes: • ACTINOPTERYGII- ray-finned fish • SARCOPTERYGII- lobe-finned fish
ACTINOPTERYGII • The ray-finned fishes • are very diverse and successful, especially the teleosts, and can be found in nearly all aquatic environments. • are oviviparous and lay many thousands of tiny eggs at one time, which are then fertilized externally.
ACTINOPTERYGII- infraclass Chondrostei
Most chondrostean species lived during the late Palaeozoic. There are only two chondrostean lineages that survive today, the paddlefish of Canada and China and the sturgeons of Europe, Asia and Canada. • Both lineages have secondarily lost a number of their actinopterygian characters: • scales lost on most of the body, • cartilaginous skeleton, • shark-like, heterocercal tail, • rostrum extending past the mouth, which forms the paddle of the paddlefish.
The holosteans arose in the Permian. • They succeeded the Chondrosteans as the dominant fishes in the middle of the Mesozoic. Even fewer holosteans than chondrosteans survive today. These include the bowfin Amia of eastern Canada, and seven species of garpikes (Lepisosteus and Atractosteus) from North and Central America. • share with other primitive fishes a mixture of characteristics of teleosts and sharks
Holosteans have more advanced jaws than the chondrosteans. The posterior end of the maxilla is free, which allows it to swing forward, stretching the fold of skin. Instead of snapping the mouth shut, the fish produces a powerful sucton. Jaws are shorter and better supported and the scales tend to lose their shiny ganoid covering. From the Jurassic onwards, the holosteans dominated the world oceans, but, they became rare in the Cretaceous and are now largely extinct.
The numbers of teleost species far exceed those of any other fish group, as 96% of living fishes are teleosts. They are the most successful fish group since their radiation in the Cretaceous. Teleosts have been found to survive in freezing Antarctic waters, in hot springs (up to 44 oC), in alkaline lakes, in acid streams, in the deep sea and in shallow rivers. • By the end of the Cretaceous, teleosts had become the dominant fishes in both the oceans and in freshwater habitats. Teleosts are characterized by a fully movable maxilla and premaxilla (which form the biting surface of the upper jaw); the movable upper jaw makes it possible for teleosts to protrude their jaws when opening the mouth. Teleosts are also distinguished by having fully homocercal (symmetrical) tails. • Teleosts include eels, catfish, tarpon, tuna, halibut, flounder, trout, cod, herring, salmon, and many other fishes.
SARCOPTERYGII • The lobe-finned bony fishes. The sarcopteryians are a very ancient subclasses of fishes with three orders: RHIPIDISTIA, ACTINISTIA, and DIPNOI. All sarcopterygians are viviparous, i.e. they give birth to one or two highly developed young at one time.
The rhipidistians are a mixed group of lobe-finned fishes that form outgroups ('ancestors') to the two main living groups, the Actinistia and the Dipnoi. Rhipistians also include ancestors of the tetrapods. These fish groups - such as the porolepiform Holoptychius and the osteolepiform Eusthenopteron, were restricted in time mainly to the Devonian, with a few survivors into the Carboniferous. The tristichopterid 'rhipidistian' Eusthenopteron
Rhipidistian characters that ally them with tetrapods are seen in the arrangement of the bones in the skull and in their lobe fins - many details of the skeleton of the pectoral and pelvic fins match fairly closely the bones in the forelimb and hindlimb of tetrapods.
The coelacanths arose in the Devonian. They are characterized by their three-lobed tail: the tail is symmetrical, with a curved upper and lower section, and a longer, projecting, rather lobed middle portion. Famously, the coelacanths were thought to have gone extinct in the Cretaceous, when the laast fossils are found. But then in 1938, a living (or recently dead) specimen of a 2-metre-long coelacanth was fished up in the Indian Ocean, off East Africa. Since 1938, as many as 200 specimens of the living coelacanth, Latimeria, have been found, some of them now off Indonesia.
The lungfishes also arose in the Devonian, when forms such as Dipterus were important predators. Since the Devonian, lungfishes evolved specialized tooth plates, and other characters, so they had most of their modern features by the Permian. Today there are three species of lungfishes, living in Australia, souther Africa, and South America. Lungfishes do indeed have lungs, as do all the other sacropterygians. Indeed, lungs were probably present in most basal osteichthyans, and they have simply been lost subsequently in the actinopterygians.
BIOLOGY • All bony fish possess gills. For the majority this is their sole or main means of Respiration. Lungfish and other osteichthyan species, are capable of respiration through lungs or vascularized swim bladders. Other species can respire through their skin, intestines, and/or stomach • ectothermic – cold-blooded • They can be any type of heterotroph: omnivore, carnivore, herbivore, or detrivore.
Some bony fish are hermaphrodites, and a number of species exhibit parthenogenesis • Fertilization is usually external, but can be internal. Development is usually oviparous (egg-laying) but can be ovoviviparous, or viviparous.
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.
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. • 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 Devonian sarcopterygians included lungfishes and coelacanths, both groups still surviving. But modern lungfishes and coelacanths are a modest remnant of once-diverse groups, and they are often called 'living fossils'. The other Devonian sarcipterygians are often grouped loosely as the 'rhipidistians', a paraphyletic ('incomplete') group that includes forms such as the osteoelpiforms, tristichopterids and panderichthyids, that lie on the line of ancestry to the tetrapods.
Early actinopterygians, such as the Devonian Cheirolepis, had heavy bony scales over the whole body, and large bony plates over the head region. The jaws operated as a simple hinge, with modest movements of other skull bones to widen the gape. Late actinopterygians had more complex jaw apparatuses that allowed them to project the jaws forward and to use suction in feeding. This perhaps was one of the keys to the success of the teleosts, which arose in the Triassic or Jurassic, but radiated dramatically in the Cretaceous and Tertiary. • Fossil species from all sub-groups -the Chondrosteans, the Holosteans, the Teleosteans, Rhipidistia, Actinistia, and Dipnoi- have been identified.
OSTEICHTHYES Modern Forms
Lionfish Blackeye goby
DID YOU KNOW???? • The largest bony fish ever was Leedsichthys • The ocean sunfish is the largest bony fish in the world • while the longest is oarfish • The smallest fish is Philippine goby