BONY FISHES (OSTEICHTHYES: CLASS SARCOPTERYGII AND CLASS ACTINOPTERYGII) MODULE 8 MODULE 8 BONY FISHES (Osteichthyes: Class Sarcopterygii and Class Actinopterygii) Unit 1 General Characteristics and Classification Sub-Unit 1: General Characteristics 1. The skeleton is more or less bony, notochord may persist in part and the tail is usually homocercal. 2. Skin has mucous glands and is embedded with dermal scales which are usually of three types: ganoid, cycloid or ctenoid scales. Some are without scales. 3. They possess both median and paired fins, with fin rays of cartilage or bone. 4. Mouth is usually terminal with or without teeth; jaws are present; olfactory sacs are paired and may or may not open into the mouth. 5. Respiration is by gills supported by bony arches and covered by a common operculum. 6. Swim bladder often present with or without duct connected to the pharynx. 7. The circulatory system consists of a two-chambered heart; arterial and venous systems, and characteristically four pairs of aortic arches; blood contains nucleated red cells. 8. Nervous system consists of a brain with small olfactory lobes and cerebrum; large optic lobes and cerebellum. There are ten pairs of cranial nerves and three pairs of semi- circular canals. 9. Sexes are separate although there is sex reversal in some. Gonads are paired. Fertilization is usually external. Sub-unit 2: Classification The Osteichthyes is divided into two classes: 1. Class Sarcopterygii (lobe finned fishes) 2. Class Actinopterygii (ray-finned fishes) a. Subclass Cladistia (bichirs) b. Subclass Chondrostei (Gr. chondros, cartilage + osteon, bone) e.g. sturgeons and paddlefishes AOE 1 BONY FISHES (OSTEICHTHYES: CLASS SARCOPTERYGII AND CLASS ACTINOPTERYGII) MODULE 8 c. Subclass Neopterygii (Gr. holos, entire + osteon, bone) e.g. bowfin, gars and teleosts. Unit 2 Class Sarcopterygii Members of the Sarcopterygii (G. sarkos, flesh + pteryx, fin or wing) includes the extinct rhipidistians, coelocanths and the lungfishes, which are collectively referred to as the lobe-finned fishes. The rhipidistians have a fusiform shape, two dorsal fins and a heterocercal tail. The paired fins bore sharp resemblance to tetrapod limb. They are believed to have been ancestors of ancient amphibians. The coelocanths have only one living order with two species: the West Indian Ocean coelocanth, Latimeria chalumnae (named after Majorie Courtenay- Latimer, South African Museum Director) and the Indonesian coelocanth, Latimeria menadoensis. The last coelacanth were believed to have become extinct 70 million years ago. However, in 1938, people fishing in deep water off the coast of South Africa brought up fishes that were identified as coelocanths. Since then, numerous other specimens have been caught in deep water around the Comoro Islands off Madagascar, where it was learned, native Comoran fishermen occasionally caught them with hand lines at great depths. Latimeria is large, up to 80kg, and has heavy scales. The tail is diphycercal but possesses a small lobe between the upper and lower caudal lobes, producing a three-pronged structure. Coelocanths show some degenerative features, such as more cartilaginous parts and a swim Figure 1: Latimeria chalumnae bladder that was either calcified or else persisted as a mere vestige. They also lack internal nostrils. Ancient coelocanths lived in freshwater lakes and rivers; therefore, the ancestors of Latimeria must have moved from freshwater habitats to the deep sea. AOE 2 BONY FISHES (OSTEICHTHYES: CLASS SARCOPTERYGII AND CLASS ACTINOPTERYGII) MODULE 8 The lung fishes and are represented today as three genera. All live in regions where seasonal droughts are common. The least specialized is Neoceratodus, the living Australian lungfish (from freshwaters of Queensland, Australia), which may which Figure 3: Neoceratodus may attain a length of 1.5m. This lungfish is able to survive in stagnant, oxygen-poor water by coming out and gulping air into its single lung, but it cannot live out of water. The other two genera are found in freshwater rivers and lakes of tropical Africa, Protopterus, and tropical South America, Lepidosiren. They have lost the use of gills for gas exchange and can survive when rivers and lakes are dry Figure 2: Lepidosiren by burrowing into the mud. Protopterus lives in African streams and rivers that run completely dry during the dry season (the West African lungfish is called Protopterus annectens). The fish burrows down at the approach of the dry season and secretes a copious slime that is mixed with mud to form a hard cocoon in which it aestivates until the rains return. They then emerge from their burrows to feed Figure 4: Protopterus and reproduce. Unit 3 Class Actinopterygii They are referred to as ray-finned fishes because their fins lack muscular lobes. They usually possess swim bladders, gas-filled sacs along the dorsal wall of the body cavity that regulate buoyancy. A modern classification system divides the Actinopterygii into three subclasses; these are the Cladistia (bichirs), Chondrostei, (sturgeons and paddlefishes); and the Neopterygii (gars, bowfin and teleosts). Members of the subclass Cladistia (bichirs) have long, slender bodies that are covered by rhombic ganoid scales; examples include bichirs of Nigerian freshwaters such as Polypterus senegalus and P. bichir. Polypterus has a lung-like swim bladder. The sturgeons and AOE 3 BONY FISHES (OSTEICHTHYES: CLASS SARCOPTERYGII AND CLASS ACTINOPTERYGII) MODULE 8 paddlefishes have cartilaginous skeletons. Chondrosteans have a tail with a large upper lobe. Sturgeons are large (up to 1,000kg), and bony plates cover the anterior portion of the body, heavy scales cover the tail. The sturgeon mouth is small, and jaws are weak. They feed on invertebrates using their snout and they are valued for their caviar (eggs). Paddlefishes are large freshwater chondrosteans. They have large, paddle-like rostrum that is innervated with sensory organs believed to detect weak electrical fields. They swim through the water with their large mouths open, Figure 5: (a) Bichir (b) Sturgeon filtering crustaceans and small fishes. (c) Paddlefish The Neopterygii comprise the bowfin, gars and teleosts. The bowfin, Amia (Greek name of perch- like fish) of shallow weedy waters of the Great Lakes and Mississippi Valley; the gars, Lepisosteus (garpike), of eastern North America; and the Figure 6: Bowfin teleosts or modern fishes. The teleosts have evolved into a variety of body forms today. They have a symmetrical caudal fin and a swim bladder (for hydrostatic functions) that Figure 7: Garpike has lost its connection to the digestive tract. Figure 8: Teleosts (a) Tilapia (b) Catfish (c) Sardines (d) African pike (e) Mormyrid (f) Flatfish AOE 4 BONY FISHES (OSTEICHTHYES: CLASS SARCOPTERYGII AND CLASS ACTINOPTERYGII) MODULE 8 Unit 4 Success of Bony Fishes Bony fishes have adapted themselves to nearly every available aquatic habitat. The remarkable success of bony fishes has resulted from a series of significant adaptations that have enabled them to dominate life in water. These include the swim bladder, lateral line system and gill cover. 1. Swim bladder: possession of swim bladder keeps bony fishes buoyant despite the fact that bones are heavier than cartilaginous skeleton. The swim bladder is a gas-filled sac that allows them to regulate their buoyant density and so remain suspended in water effortlessly at any depth. In most of today’s bony fishes, the swim bladder is an independent organ that is filled and drained of gases, mostly nitrogen and oxygen internally. It turns out that the gases are released from their blood. The gas flow is regulated by lactic acid, the acidity of which drives nitrogen out of the blood. The lower pH also alters the shape of the haemoglobin so that it is less able to bind oxygen. 2. Lateral line system: bony fishes possess a fully developed lateral line system, which consist of a series of sensory organs that project into a canal beneath the surface of the skin. Movement of water past the fish forces water through the canal. The sensory organs are deflected by the slightest movement of water over them. Nerve impulses from these sensory organs permit the fish to assess its rate of movement through water, sensing the movement as pressure waves against its lateral line. The lateral line enables a fish to detect motionless objects at a distance by the movement of water reflected off the object. In a very real sense, this is the fish equivalent of hearing. 3. Gill cover: most bony fishes have a hard plate, called the operculum, covering the gills on each side of the head. Flexing the operculum permits bony fishes to pump water over gills. When the mouth is open, the operculum is closed thereby sealing off the exit. This increases the volume of the mouth cavity so that the water is drawn into the mouth. When the mouth is closed, the operculum opens thereby decreasing the volume of the mouth cavity, forcing water past the gills to the outside. AOE 5 BONY FISHES (OSTEICHTHYES: CLASS SARCOPTERYGII AND CLASS ACTINOPTERYGII) MODULE 8 Unit 5 Biology of Bony Fish Teleosts have internal skeleton almost completely ossified. They have developed thin cycloid scales from the thick, ganoid type of earlier fishes. There is also the absence of spiracles. Locomotion The streamlined shape of a fish and the mucoid secretions that lubricates its body surface reduce friction between the fish and the water. The buoyant properties of water also contribute to the efficiency of a fish swimming through the water. The propulsive mechanism of a fish is its trunk and tail musculature. The musculature is composed of zig-zag muscle bands (myotomes). The slimy surface of a fish reduces water friction by at least 66% as compared with the same surface from which the slime has been removed. Nutrition and the Digestive System Most fishes are carnivores that prey on various animal foods from zooplankton and insect larvae to large vertebrates.