The following poionts highlight the top thirteen types of fish. The types are: 1. Cetorhinus (Family Lamnidae) 2. Pristis (Family Pristidae) 3. Sphyrna (Family Sphyrnidae) 4. Polypterus (Bichir) (Family Polypteridae) 5. Acipenser (Family Acipenseridae) 6. Amia (Family Amiidae) 7. Lepisosteus (Family Lepidosteidae) 8. Exocoetus (Family Exocoetidae) 9. Echeneis (Family Echeneidae) 10. Lophius (Family Lophiidae) and Others.
Type # 1. Cetorhinus (Family Lamnidae):
Cetorhinus is a peculiar shark under the order Lamniformes. It is commonly known as basking shark (Fig. 6.47A). It is not predaceous in habit. It collects its food by straining small planktonic forms by special comb-like structures produced by the gill-rakers.
By this feature, it resembles the whale bone of Cetaceans. Numerous small eggs are produced which develop within a ‘uterus’. Although the development is intrauterine, no placenta is formed.
Type # 2. Pristis (Family Pristidae):
Pristis (or the saw fish) is a specialised representative of the order Rajiiformes. The body is elongated and becomes slightly broadened laterally. The rostrum is drawn out into a blade armed with two lateral rows of knife-like denticles (Fig. 6.47B).
The length of the ‘saw’ equals the length of one-third part of the body. The head strikes from side to side among the shoals of fishes and may also give slashing blows to its enemies. The saw fishes may attack the soft parts of the whale and tear off pieces of flesh by their ‘saws’. There are a few species under this genus which inhabit fresh water.
The saw fishes are abundant in India, China and the Gulf of Mexico. The Saw fishes may reach a length of about 3-6 m. The Saw fishes are viviparous. The most common Saw fishes of the Indian coast are: Pristis cuspidatus and Pristis microdon.
Type # 3. Sphyrna (Family Sphyrnidae):
Sphyrna (or the Hammer-headed shark) is characterised by having a laterally flattened head (Fig. 6.47C). The head is prolonged laterally into two large lobes. These lobes are provided with cartilaginous support from the nasal part of the skull. The head assumes the shape of a hammer.
The eyes are situated at the tip of the lateral projections of the head. The hammer-headed sharks are very ferocious in nature. These sharks are used in extracting liver oil. They may attain the length of about 4.5 m. The common Hammer-headed shark of Indian coast is Sphyrna zygaena.
Type # 4. Polypterus (Bichir) (Family Polypteridae):
Polypterus (see Fig. 6.48A) has a slender body covered by thick ganoid rhombic scales. The dorsal fin is divided into eight or more dorsal fin-lets (Sails) each of which is preceded by a spine. The tail is arrow-shaped and diphycercal.
The swim-bladder is lung-like and is capable of breathing in air. The larvae bear slender external gills. The conus arteriosus contains three longitudinal rows of valves, each row has got nine valves.
One pair of gular plates is present between the rami of the lower jaw. Polypterus bichir of tropical Africa attains a length of about 1 m. The other related genus, Calamoichthys (= Erpetoichthys) of the rivers of Nigeria and Congo has an elongated body devoid of pelvic fins.
Type # 5. Acipenser (Family Acipenseridae):
Acipenser (see Fig. 6.48B) is a large sized fish living in the seas and rivers of Northern hemisphere. They feed on worms, molluscs, small fishes and aquatic plants. It has an elongated shark-like body. The snout is long. The mouth is small, toothless and ventrally located.
Four barbels are present in front of the mouth. The body bears a few rows of bony scutes (modified ganoid scales). The tail fin is heterocercal. The stomach lacks the blind diverticulum. Presence of un-constricted notochord and loss of ossification in the whole skeleton are the most notable features.
The common sturgeons are: Acipenser sturio (common sturgeon), A. transmontanus (white pacific sturgeon), A. fulvescens (rock sturgeon), Scaphirhynchus platorhynchus (fresh-water shovel-nosed sturgeon). Polyodon (spoonbill sturgeon of Mississippi), Psephurus (spoonbill sturgeon of China). Atipenser possesses spiracle, but Scaphirhynchus is without spiracle. Polyodon lacks bony scutes and barbels.
Type # 6. Amia (Family Amiidae):
Amia calva (Fig. 6.48C) is the solitary species of the family Amiidae and lives in the fresh-water rivers and lakes of North America. It is a predatory fish and lives on small fishes. It has a fusiform body covered by overlapping cycloid scales. The dorsal fin is long and continuous. The tail fin is homocercal. A dorsal bilobed cellular swim-bladder is present which helps in respiration.
The spiral valve is present in the intestine. Presence of a small valvular conus arteriosus and one large jugal plate between the rami of the lower jaw are some of the primitive features. The vertebrae are solid and possess amphicoelous centra. The pyloric caeca are absent. The spiracles are also absent. Two comb-like structures are situated on the throat.
The male individuals are smaller than the females. The males are sharply distinguishable from the female by having a shiny black spot on the basal part of the caudal fin. Before spawning the male prepares a saucer- shaped crude nest with aquatic plants. After the eggs being laid by the female in the nest, the male guards the nest until the hatching of the eggs.
Type # 7. Lepisosteus (Family Lepidosteidae):
Lepidosteus is a common fresh-water garpike. (Fig. 6.48D) of North and Central America. It has an elongated body covered over by thick ganoid rhombic scales. The scales of the trunk are arranged in oblique rows. The snout is much elongated. The vertebral column is bony. Lepisosteus possesses an admixture of primitive and advanced characters.
The primitive features are:
(a) The presence of a long valvular conus arteriosus,
(b) Absence of a bulbus arteriosus,
(c) The walls of swim-bladder are cellular,
(d) Semi-hetero- cercal tail fin and
(e) Thick ganoid scales.
The specialised features are:
(a) The reduction of maxillae and their functional replacement by many tooth-bearing infraorbital structures,
(b) Opisthocoelous vertebrae composing the vertebral column and
(c) The presence of numerous cheek plates. Loss of central jugal plate, absence of spiracles and presence of pyloric caeca are the individual characteristics. The garpikes are voracious eaters and prey on smaller fishes. Lepisosteus tristaechus attains a length of about 3 metres.
Type # 8. Exocoetus (Family Exocoetidae):
Exocoetus is a common flying fish (see Fig. 6.49A) of tropical seas. The fishes usually live in groups. The body is covered by scales. The pelvic fins are devoid of spines and each bears six rays. The dorsal fin is situated above the anal fin.
The pectoral fins become greatly elongated into parachute-like organs. Both the jaws are short. Exocoetus volitans can leave water by a powerful tail movement and float rapidly through air in a straight line by the activity of the expanded pectoral fins.
The related genera of Exocoetus are: Cypselurus (flying fish), Belone (garfish or gar- pike or swordfish with no fin-let and the jaws are elongated into a beak), Hemirhamphus (half-beak with the lower jaw extended beyond the upper jaw), Scombersesox (skipper or saury with a beak and fin-lets behind the anal and dorsal fins).
Type # 9. Echeneis (Family Echeneidae):
The sucker fishes or remoras (Fig. 6.49B, C) belong to the family Echeneidae under the order Echeneiformes. They are peculiar bony fishes found in all seas. Echeneis is the typical genus of the group. The related genera are: Phtheirichthyes, Remora, Remilega, Rhombochirus. The size of the fishes reaches up to 53 cm.
The anterior dorsal fin is transformed into an oval adhesive vacuum disc. This disc is flat and is surrounded by membranous margin. It is transversely furrowed and situated on the upper surface of the head (Fig. 6.49C).
The sucker fishes can attach themselves to the body of other fishes, turtles, whales or any, other floating objects by producing numerous suction chambers and are thus carried from one place to the other. They leave the host periodically to capture small fishes.
The adhering power of the adhesive disc of remoras is quite high and the sucker of a large individual can resist a vertical pull of about 18 kg. The pelvic fins are provided with external spines and are thoracic in position. Other characteristic features are the absence of pseudo-branch, swim-bladder and fin-lets.
Type # 10. Lophius (Family Lophiidae):
This particular fish is also commonly called by the names, frog-fish, monk-fish, sea- devil and by many other names. The head and anterior portion of the body are greatly enlarged (Fig. 6.49D). The scales are absent. The dorsal fin is ‘spinous’ and is provided with a few isolated flexible rays.
Of the rays, the first one is extremely elongated, situated on the head and bears a dilated bulb at the tip. Lophius piscatorius has a broad head with a huge mouth. The jaws are beset with numerous teeth. It attains a length of about 1.2 m. Many phosphorescent organs are present.
The foremost long fin-ray bears a terminal bulb to attract small prey. The terminal bulb is capable of rapid regeneration. The outline of the body is partially obliterated by stumpy dermal appendages which mimic the leaves of aquatic weeds. The pectoral fins are broad and are used to ‘walk’ on the ground in search of prey.
Type # 11. Syngnathus (Family Syngnathidae):
This genus is found in all the warm seas and is called greater pipefish (Fig. 6.50A). The body of the fish is slender with very small fins. The scales are transformed into ring-like plates covering the body. The gills are lophobranchs and the gill-openings are minute. The dorsal fin is soft and the pelvic fins are absent. Like that of Hippocampus, the snout is tubular with suctorial toothless mouth.
The male possess brood pouch where the eggs are stored (Fig. 6.50B). The tail is non-prehensile with small caudal fin. Other genera of pipe-fishes are: Siphonostoma, Doryichthys, Nerophis, Proto-campus, Ichthyocampus, Stigmatophora, etc.
Type # 12. Phycodurus (Family Syngnathidae):
Phycodurus eques is the sea dragon of Australian sea-shores. It resembles Hippocampus very closely and exhibits excellent mimetic resemblance with a piece of sea-weed (Fig. 6.50C). The outline of the body is broken up by the development of numerous membranous leaf-like processes. The snout is tubular and the mouth is toothless. The gills are lophobranchs. The tail is prehensile and lacks caudal fin.
Type # 13. Hippocampus (Family Syngnathidae):
This peculiar marine fish is called the ‘seahorse’, because its head bears a superficial resemblance with the head of horse (Fig. 6.50D). The toothless mouth is suctorial and the snout is tubular. The head is large and is present at right angle to its body. The skin is covered by bony plates. The gills are composed of many small-rounded lobes attached to the gill-arches (Fig. 6.50E).
Such gills are called lophobranchs. The operculum is a large plate-like structure. The musculature is feebly developed. It is a bad swimmer and is usually carried passively by currents. It can swim vertically with the help of the dorsal fin.
The caudal fin is absent and the tail is prehensile. The male individual is characterised by having brood pouch at base of the tail where the eggs are housed until hatching. The other seahorses are: Castrotikeus, Solenognathus, Phyllopteryx, etc..