Dive into the World of Gnathostomata: Vertebrates & Evolutionary Adaptations

3. The Gnathostomata are Chordates, Craniate and Vertebrates • The class includes Chondrichthyes, Osteichthyes, Amphibians, reptiles, Aves & Mammals

4. Some of the earliest known vertebrates, collectively referred to as ostracoderms, consisted of several groups of small, armored, jawless fishes that lived on the bottom and strained their food from the water. • Thick bony plates protected their heads from predators

5. Large (average about 2 m), body fusiform, or dorsoventrally depressed, with a heterocercal caudal fin (diphycercal in chimaeras) ; paired pectoral and pelvic fins; pelvic fins in male modified as “claspers” • Mouth ventral; two olfactory sacs that do not open into the mouth cavity in elasmobranchs; nostrils open into mouth cavity in chimaeras; jaws present • Skin with placoid scales or naked • Endoskeleton entirely cartilaginous; notochord persistent but reduced; vertebrae complete and separate (vertebrae present but centra absent in chimaeras) • Digestive system with J-shaped stomach (stomach absent in chimaeras); intestine with spiral valve; often with large oil-filled liver for buoyancy. • Circulatory system of several pairs of aortic arches; single circulation; hepatic portal and renal portal systems; heart with sinus venosus, atrium, ventricle, and conus arteriosus

6. Large (average about 2 m), body fusiform, or dorsoventrally depressed, with a heterocercal caudal fin (diphycercal in chimaeras) ; paired pectoral and pelvic fins; pelvic fins in male modified as “claspers” • Mouth ventral; two olfactory sacs that do not open into the mouth cavity in elasmobranchs; nostrils open into mouth cavity in chimaeras; jaws present • Skin with placoid scales or naked • Endoskeleton entirely cartilaginous; notochord persistent but reduced; vertebrae complete and separate (vertebrae present but centra absent in chimaeras) • Digestive system with J-shaped stomach (stomach absent in chimaeras); intestine with spiral valve; often with large oil-filled liver for buoyancy. • Circulatory system of several pairs of aortic arches; single circulation; hepatic portal and renal portal systems; heart with sinus venosus, atrium, ventricle, and conus arteriosus

7. Several key adaptations contributed to the radiation of bony fishes. • They have an operculum over the gill composed of bony plates and attached to a series of muscles. • This feature increases respiratory efficiency because outward rotation of the operculum creates a negative pressure so that water would be drawn across the gills, as well as pushed across by the mouth pump. • A gas-filled derivative of the esophagus provides an additional means of gas exchange in hypoxic waters and an efficient means for achieving neutral buoyancy. • In fishes that use these pouches primarily for gas exchange, the pouches are called lungs, while in fishes that use these pouches primarily for buoyancy, the pouches are called swim bladders • Progressive specialization of jaw musculature and skeletal elements involved in feeding is another key feature of bony fish evolution

8. Lives in both marine and freshwater • Body spindle or stream-lined • Tail usually homocercal • Endoskeleton chiefly of bones • Cloaca lacking, anus present • Respiration by 4 pairs of gills covered by operculum • Air/ swim bladder often present • Ventrally positioned 2-chambered heart • Unlike most sharks, bony fi shes are oviparous. They lay an impressive number of eggs and fertilize them externally. The ocean sunfi sh, for example, lays more than 300 million eggs! • Of course, most of the eggs and young become food for other animals. • The probability of survival is increased by certain behavioural adaptations. For example, many species of fishes build nests for their eggs and protect them.

9. Superclass Gnathostomataa) (Gr. gnathos, jaw, + stoma, mouth): jawed fishes, tetrapods. With jaws and (usually) paired appendages. • Class Chondrichthyes sharks, skates, rays, chimaeras. • Cartilaginous skeleton; intestine with spiral valve; claspers present in males; no swim bladder. About 970 species. • Class Osteichthyes- Bony Fishes • Sub-Class Actinopterygii are the ray-finned fishes. Skeleton ossified; single gill opening covered by operculum; paired fins supported primarily by dermal rays; appendage musculature within body; swim bladder mainly a hydrostatic organ, if present; atrium and ventricle not divided. About 27,000 species. • Eg Tilapia zilli, Oreochromis niloticus, Sarotherodon galileus • Sub-Class Sarcopterygii (Gr. sarkos, flesh, + pteryx, fin, wing): lobe-finned fishes. Skeleton ossified, single gill opening covered by operculum; paired fins with sturdy internal skeleton and musculature within appendage; diphycercal tail; intestine with spiral valve; usually with lunglike swim bladder; atrium and ventricle at least partly divided. 8 species. Eg South American lungfish, Lepidosiren, African lungfish, Protopterus

10. Skeleton with bone of endochondral origin; caudal fin heterocercal in ancestral forms, usually homocercal in descendant forms ; skin with mucous glands and embedded dermal scales. • Paired and median fins present, supported by long dermal rays (lepidotrichia); muscles controlling fin movement within body • Jaws present; teeth usually present with enamaloid covering; olfactory sacs do not open into mouth; spiral valve present in ancestral forms. • Respiration primarily by gills supported by arches and covered with an operculum • Swim bladder often present with or without a duct connecting to esophagus, usually functioning in buoyancy • Circulation consisting of a heart with a sinus venosus, an undivided atrium, and an undivided ventricle; single circulation; typically four aortic arches; nucleated erythrocytes • Excretory system of paired opisthonephric kidneys; sexes usually separate; fertilization usually external; larval forms may differ greatly from adults • Nervous system of a brain with small cerebrum, optic lobes, and cerebellum; 10 pairs of cranial nerves; three pairs of semicircular canals

11. Skeleton with bone of endochondral origin; caudalfin diphycercal in living representatives, heterocercal in ancestral forms; skin with embedded dermal scales with a layer of dentinelike material, cosmine, in ancestral forms • Paired and median fi ns present; paired fi ns with a single basal skeletal element and short dermal rays; muscles that move paired fi ns located on appendage • Jaws present; teeth are covered with true enamel and typically are crushing plates restricted to palate; olfactory sacs paired, may or may not open into mouth; intestine with spiral valve • Gills supported by bony arches and covered with an operculum • Swim bladder vascularized and used for respiration and buoyancy (fat-filled in coelacanths) • Circulation consisting of heart with a sinus venosus, two atria, a partly divided ventricle, and a conus arteriosus; double circulation with pulmonary and systemic circuits; characteristically five aortic arches • Nervous system with a cerebrum, a cerebellum, and optic lobes; 10 pairs of cranial nerves; three pairs of semicircular canals • Sexes separate; fertilization external or internal

12. Members of class Chondrichthyes, the cartilaginous fishes, evolved as successful marine forms in the Devonian period. Class Chondrichthyes, which is considered monophyletic, includes the sharks, rays, and skates. • Most species are ocean dwellers, but a few have invaded fresh water. With the exception of whales, the sharks are the largest living vertebrates. • Some whale sharks (Rhincodon) exceed 15 m (49 ft) in length, making them the largest fish. • All chondrichthyes have paired jaws and two pairs of fins. The skin contains placoid scales. • Each scale is a toothlike structure consisting of an outer layer of enamel and an inner layer of dentine. • The lining of the mouth contains larger, but essentially similar, scales that serve as teeth. The teeth of other vertebrates are homologous with these scales. • Shark teeth are embedded in the flesh and not attached to the jawbones; new teeth develop continuously in rows behind the functional teeth and migrate forward to replace any that are lost.

13. Anaspid fishes (agnathans) showed first evidence of paired fins around 420 mya. Likely bony keels; with/without flimsy webs/spines. Always in anterior (pectoral) position. • Anaspids had primitive fins toward the front of the body. • Ostracoderms too had paired fins around 400 mybp, and were also jawless. • Their paired fins were in the forward position in the anterior, pectoral region. • Paired fins likely derived from outgrowths in lateral plate mesoderm.

14. Recall that generalized vertebrate embryo: it has an ectoderm, mesoderm, and endoderm. • The mesoderm is the middle germ cell layer which differentiates to form important tissues and structures, which led to paired fins. • These important tissues include bone, cartilage, and glands. The position of the fin buds is regulated by the Hox genes. What is unique about the Hox genes in vertebrates and why? Vertebrates have many more Hox genes than invertebrates, which is important because Hox genes control development of structures on the body.

15. The Tilapias belong to the family Cichlidae. • Three genera are well-known namely Oreochromis, Tilapia and Sarotherodon, of which Nile Tilapia belongs to genus Oreochromis. • This species is naturally distributed in Nigeria water bodies, the Nile River as well as most part of African river and lakes. • Its rising popularity is due to their hardiness, resistance to disease, ease to breeding, reasonable growth rate, good taste, and tolerance to a wide range of environmental conditions including temperature and salinity.

16. Tilapia are mainly freshwater fish inhabiting shallow streams, ponds, rivers and lakes and less commonly found living in brackish water. • Other than their temperature sensitivity, tilapia exist in or can adapt to a very wide range of conditions. • Tilapia are also known to be a mouth breeding species. • Mouth breeding means they carry the fertilized eggs and young fish in their mouths for several days after the yolk sac is absorbed • Historically, they have been of major importance in artisan fishing in Africa and the Middle East, and they are of increasing importance in aquaculture and aquaponics. 

17. This means they are efficient feeders that can capture and process a wide variety of food items. • Their mouths are protrusible, usually bordered with wide and often swollen lips. • The jaws have conical teeth. • Typically tilapia have a long dorsal fin, and a lateral line which often breaks towards the end of the dorsal fin, and starts again two or three rows of scales below. • Some Nile tilapia can grow as long as two feet.

18. Most Tilapias species of the tribe Tilapinii now being used in aquaculture were grouped initially into one genus, Tilapia. • The species within this genus were later classified according to differences in their mode of reproduction. • Species which evolved as substrate spawner but guard their eggs were retained in the genus tilapia while those which orally rear their clutches were grouped into a new species Sarotherodon. • Classification of the three genera Tilapia, Sarotherodon and Oreochromis was based largely on the differences on their reproduction, feeding habits and biogeography.

19. Body is divided into head, trunk and tail • Most are streamlined body that swim actively and catch their prey. • Tilapia typically have laterally compressed, deep bodies. Like other cichlids, their lower pharyngeal bones are fused into a single tooth-bearing structure. • A complex set of muscles allows the upper and lower pharyngeal bones to be used as a second set of jaws for processing food, allowing a division of labour between the "true jaws" (mandibles) and the "pharyngeal jaws".

20. Head consist of lateral eyes, mouth, operculum that protects the gills • Lateral line serves as sense organs are found on the body

25. Gas exchange takes place through their five to seven pairs of gills. • A current of water enters the mouth and passes over the gills and out the gill slits, constantly providing the fish with a fresh supply of dissolved oxygen. • Tilapa can actively swim depend on their motion to enhance gas exchange. • They can spend time on the in water using muscles of the jaw and pharynx to pump water over their gills.

26. Tilapia ingest a wide variety of natural food organisms, including plankton, some aquatic macrophytes, planktonic and benthic. • For their sustenance, newly hatched fry depend on their yolk sacs until consumed. • Then, they eat the smallest phytoplankton present in the pond. As the fry become bigger, they eat larger organisms and supplemental feeds such as rice-bran, fishmeal and others. • Tilapia feed on a variety of phytoplankton as their primary food items. They are cannibalistic and will feed on their fry if food is not abundant. The tilapia has a short esophagus leading to a small sac-like stomach with an exceptionally long intestine (4x the body long). • The O. niloticus has firm pharyngeal teeth set on a triangular blade. Its role is to prepare food for digestion, shredding the coarser materials and breaking some of the cell walls before passing it on to the stomach

27. The digestive tract consists of the mouth cavity; a long pharynx leading to the stomach; a short, straight intestine;. • The liver and pancreas discharge digestive juices into the intestine.

28. Fins: to maintain its position, move, steer and stop : the fins are supported by spines that are rigid and may be quite sharp thus playing a defensive role • Paired fins – pectoral (chest) fin with spines; – pelvic (hip) fins located anteriorly – help to stabilise fish and assist with steering, stopping and hovering. • Unpaired fins – long spiny dorsal fin – anal fin with spines • reduce rolling motion during swimming and help in turning movements • primarily help fish to not roll over onto their sides – caudal fin: main fin for propulsion to move the fish forward. • homocercal, not deeply forked. – • Thin, forked tail fins are for continuous, fast swimming.– Less differentiated tail fins are for less active fishes that may use a short burst of speed to escape a predator or catch a prey.

29. Tilapia has a complex brain and a spinal cord that is protected by vertebrae. • The well-developed sense organs very effectively locate prey in the water. • They may detect other animals electrically before sensing them by sight or smell. • Electroreceptors on the Tilapia’s head sense weak electric currents generated by the muscular activity of animals. • The lateral line organ, found in all fishes, is a groove along each side of the body with many tiny openings to the outside. • Sensory cells in the lateral line organ are sensitive to waves and other motion in the water, alerting the shark to the presence of predator or prey. • Other than their temperature sensitivity, tilapia exist in or can adapt to a very wide range of conditions.

30. Consist of 2 kidneys just beneath the vertebral column in the abdominal cavity • Nitrogenous cavities is carried posteriorly from each kidney in a slender, tubular ureter • Both ureters unite into a common urinary duct with a thin-walled urinary bladder which is not homologous with that of tetrapods

31. In all Oreochromis species the male excavates a nest in the pond bottom (generally in water shallower than 3 feet) and mates with several females. • After a short mating ritual the female spawns in the nest (about two to four eggs per gram of brood female), the male fertilizes the eggs, and she then holds and incubates the eggs in her mouth (buccal cavity) until they hatch. • Fry remain in the female’s mouth through yolk sac absorption and often seek refuge in her mouth for several days after they begin to feed. • Sexual maturity in tilapia is a function of age, size and environmental conditions.

32. In all Oreochromis species the male excavates a nest in the pond bottom (generally in water shallower than 3 feet) and mates with several females. • After a short mating ritual the female spawns in the nest (about two to four eggs per gram of brood female), the male fertilizes the eggs, and she then holds and incubates the eggs in her mouth (buccal cavity) until they hatch. • Fry remain in the female’s mouth through yolk sac absorption and often seek refuge in her mouth for several days after they begin to feed. • Sexual maturity in tilapia is a function of age, size and environmental conditions.

33. Genus Tilapia (Substrate Spawners) • Both parents guard, protect, aerate the brood, and help move clutch to different nest sites. Fry at first feeding are 4-6 mm and show feeble swimming ability. Fry survival relatively low. (Eg: Tilapia zillii) • Genus Sarotherodon (Paternal/biparental) • Both parents stay close to each other. Eggs and fry brooded in oral cavity up until they are ready for released. Fry are between 7-9 mm at first feeding, well developed fins for swimming. Fry survival high. (Eg: Sarotherodon melanotheron) • Genus Oreochromis (Maternal) • Female solely involved in broad care. After spawning, female leaves nest to rear her clutch in safety. Fry brooded up until free swimming. There is an external period of care during which fry seek shelter in buccal cavity for safety. First feeders have well-developed fins for swimming. Fry survival high. (Eg: Oreochromis niloticus, Oreochromis massambicus, Oreochromis aureus, Oreochromis spilurus)

34. The Nile Tilapia is a mouth-brooder. Mouth brooding is an advanced reproductive tactic, a form of intensive care whereby the seed can be protected from the outside world until their development is more advanced. • The male establishes a territory and builds a round nest in the pond bottom. (Usually the diameter of a nest is 30 – 60 cm. The size of the nest is correlated to the size of the male.) The female enters the nest and lay the eggs. • The eggs are fertilized by male. The female then collects and incubates the eggs in her mouth. The eggs are yellow in colour. Eggs hatch in about five to seven days. After hatching the fry remain in the mouth of the female for another 4-7 days. The fry begin to swim freely in schools.

35. Growth of tilapia is dependent on stocking rates, food supply and water quality. • Males grow faster by 10-20 % than females. • The growth of fish will be drastically reduced if fingerling production is not controlled. • The growth of tilapia is directly related to the amount of food available in the pond.

36. Sex identification of tilapia is relatively simple. The male has two openings just in front of anal fin. • The large opening is the anus and the smaller opening at the tip is the urogenital pore. • The female has three openings: the anus, the genital pore, and the urinary pore. • The genital papilla is usually smaller in the female. • Tilapia can be sexed when it has attained the weight of 15 grams. • Application of ink or dyes to the papillae may increase the accuracy of sexing and may allow sexing of smaller fish. • By rubbing ink along the papillae of the tilapia, sexes can be readily distinguished.