Small order of freshwater bony fishes.
Dipnoi (Gr. di-two, pnoe-breathing) is a small order of fresh water bony fishes.
They respire by gills and lungs.
Dipnoi evolved during Devonian period.
They are characterized by short jaws, crushing plate like teeth, internal nares, reduced exo- and endo- skeleton, and diphycercal tail.
The air bladder i.e., so called ‘lungs’ are one or two. They are functional with related changes in the circulatory system and in the heart.
Distribution of Dipnoi
Modern lung fishes show discontinuous distribution.
The three surviving genera of lung fishes are Neoceratodus (=Epiceratodus), Protopterus and Lepidosiren. All are inhabitants of river.
Neoceratodus is the only living genus of the family Ceratodontidae, the other being extinct Ceratodus. It is found only in the Burnett and Mary rivers of Queens-land in Australia, so commonly called as ‘Burnett Salmon’ or Australian lungfish.
Protopterus lives in large lakes and rivers of tropical Africa. It is commonly called as ‘Nile lungfish’ or African lung fish.
Lepidosiren is found in river Amazon and Paraguay basin in South America. It is commonly called as ‘Amazon lungfish’ or South American lungfish.
Primitive characters of Dipnoi
Unconstricted notochord. Presence of cloaca. Spiral valves in intestine. Valves in the conus. Diphycercal tail. Ventral inferior nostril. Persistent notochord without any constriction. Cartilaginous autostylic skull.
Specialised characters of dipnoi:
Internal nares, possibly help in breathing through the nose.
Respiration by lungs (modified air bladder) in addition to gill-respiration.
Auricle is partly divided into two and nearly three-chambered heart.
One of the paired auricles receives oxygenated blood through a special pulmonary arch from the lungs.
Conus arteriosus spirally twisted and contractile in nature.
Separation of pulmonary and systemic circulation.
Large paired cerebral hemispheres.
Well-developed Mullerian duct.
Presence of characteristic tooth plates, used for crushing of shelled invertebrates.
Bones absent in the jaw.
General Organization of Dipnoi
External Structures of Dipnoi:
The three extant dipnoans have elongated piscean body covered by overlapping cycloid scales.
The dorsal, anal and tail fins are continuous.
The pectoral and pelvic fins are usually designated as the ‘limbs’.
These are extremely elongated, filamentous structures and are devoid of finrays.
The tail is diphycercal (Protocercal or isocercal) in the living genera
The operculum and a slit-like branchial opening are present on either side.
The external nostrils are enclosed within the upper lip and two internal nostrils open into the mouth cavity.
The lateral line sensory system is well-developed.
The cloacal aperture lies at the root of the tail.
Two abdominal pores usually open into the cloaca.
Affinities of Dipnoi
