In this article we will discuss about:- 1. Geographical Distribution of Dipnoi 2. Habits and Habitat of Dipnoi 3. Peculiarities 4. Classifications 5. Affinities 6.Phylogeny and Significance.
Contents:
- Geographical Distribution of Dipnoi
- Habits and Habitat of Dipnoi
- Peculiarities of Dipnoi
- Classifications of Dipnoi
- Affinities of Dipnoi
- Phylogeny and Significance of Dipnoi
1. Geographical Distribution of Dipnoi:
The dipnoans, belonging to the order Dipnoi of the subclass Sarcopterygii of class Osteichthyes, are generally called “lung fishes”. The name Dipnoi (Gr., di = two, pnoe = breathing) means “double breathers” as they respire through gills as well as lungs. The dipnoans evolved during the middle Devonian and flourished well in the Permian and Triassic periods. They became rare after Triassic and are represented now by three specialised genera.
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Fossil dipnoans appeared in mid-Devonian probably as an offshot of the crossopterygian stem. They flourished moderately in Permian and Triassic and later became rare. At present, the Dipnoi are represented by three genera occurring in widely separated tropical and subtropical freshwater habitats.
There are only 3 living genera of lung fishes, one in each of the 3 continents of the Southern Hemisphere.
1. Neoceratodus (= Epiceratodus):
Single species Neoceratodus forsteri found in Australia in Burnett and Mary rivers of Queensland. A direct descendant of ancient lung fish. Attain a length of 1.5 metres and a weight of 45 kg.
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2. Protopterus:
4 or so species found in tropical Africa ranging from the rivers Senegal, White Nile and Zambesi and lake Tanganayika. Grow up to 1 or 2 metres and weight about 40 kg.
3. Lepidosiren:
Single species Lepidosiren paradoxa found in tropical South America in river Amazon and its tributaries.
2. Habits and Habitat of Dipnoi:
Lung fishes are large, bizarre fishes, inhabiting semipermanent freshwaters and swamps in Africa, South America and Australia. Protopterus lives in large lakes and rivers of tropical Africa. It can survive even when the rivers become completely dried up in summer. It can dig into the mud and aestivate (summer sleep) there for at least six months in a cocoon made out by clay and mucus. During the period of aestivation, nutrition is derived from the stored fat.
Lepidosiren has also the property of aestivation to escape death during summer. It lives in rivers which become shallow and stagnant in summer, but never dry up completely. It lives mostly at the bottom of the river. Neoceratodus thrives well by switching over to pulmonary respiration.
When the water is plenty and oxygen is easily available, Neoceratodus adopts gill-respiration. During summer when the water-level becomes low and toxic due to the decomposition of organic materials, Neoceratodus adopts well to pulmonary respiration.
All the surviving genera of the lung fishes are sluggish in nature and are bottom dwellers. They are carnivorous, although Lepidosiren sometimes feeds on plant materials. It lives mainly on snails. Protopterus is predaceous in nature. It feeds on worms, crustaceans, insects, frogs and many other small animals.
Protopterus is often found to attack its own species and bite off portions of the limbs, tail and other parts of the body. The lost parts of the body are regenerated by the these fishes. Neoceratodus is also a carnivorous fish which feeds on molluscs, crustaceans and other worms.
In Lepidosiren and Protopterus, parental care is present. The eggs are laid in muddy nests. The nest is simply a tunnel of about 30 cm deep. The male usually guard the nest till the development is complete. The parental care is highly specialised in Lepidosiren because its pelvic fins become specially modified during breeding season.
The pelvic fins become greatly enlarged and highly vascularised. The significance of such modification is either to release excess of oxygen to the surrounding water for respiration of the developing youngs or possibly to act as accessory respiratory structure to compensate aerial respiration while remaining around the nest.
3. Peculiarities of Dipnoi:
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1. External Features:
Lung fishes are slender fish-like or eel-like creatures, 1 to 2 metres in length. Body is covered by overlapping cycloid scales. Paired pectoral and pelvic fins, often designated as the limbs, are narrow lobe-like or filamentous, with a central axis of flesh and bone. Dorsal, anal and caudal fins are fused into a continuous, symmetrical, diphycercal tail, supported by partly calcified fin rays.
Snout is depressed bearing external nostrils enclosed within upper lip and two internal nostrils open into the mouth cavity. Mouth is subterminal or ventral. Gills are covered on either side by an operculum leaving a single slit-like gill-slit. Spiracles are absent. Eyes are small. The lateral line sensory system is well developed specially on head. The cloacal aperture lies at the root of the tail. Two abdominal pores usually open into the cloaca.
2. Internal Features:
Endoskeleton in dipnonans is mostly cartilaginous. It is composed of axial and appendicular skeleton.
Axial Skeleton:
Notochord is persistent as an unconstricted rod enclosed in a thick, fibrous sheath and extending up to pituitary region. The vertebrae are represented by paired neural or basidorsal and basiventral cartilages which form the neural arches in the trunk and both the neural and haemal arches in the tail region respectively. The vertebrae lack centra.
Distinct joint between the skull and vertebral column is lacking. Pleural ribs are present in the body wall like those of the bony fishes (teleosts). The skull is characterised by the loss of bones. Premaxillae, maxillae and nasal are absent.
The roof and the walls of cranium are formed by parietals and frontals, while the floor is formed by parasphenoid. The jaw suspension is autostylic because the mandible is attached with the skull by a palatoquadrate.
The lower jaw is composed of paired Meckel’s cartilages, tooth-bearing coronoid and supra-angulars. The hyoid and branchial arches are all cartilaginous. There are 4 to 6 cartilaginous branchial arches. The branchial arches bear gill-rakers.
Appendicular Skeleton:
The pectoral girdle consists of a stout cartilage with a pair of investing bones the cleithra and infraclavicle. The pelvic girdle is composed of a cartilaginous plate with a long epipubic and prepubic processes. The skeleton of the fin consists of a central axis of small cartilaginous rods with radials on both preaxial and postaxial sides. This is called an archipterygium.
Digestive System:
The teeth form characteristic tooth plates for crushing the mollusk an shells. The alimentary canal is a simple tube. The pharynx leads into an oesophagus. The lung-fishes lack distinct stomach. The posterior region of the oesophagus shows slight dilations which is regarded as a part of the stomach by many zoologists.
The cavities between the stomach and intestine are separated by a flap-like pyloric valve. The intestine is ciliated and contains a spiral valve running along the entire length of the intestine and makes about six and a half turns. The liver is a single massive gland. The gall-bladder is large and situated on the left margin of the liver.
Respiratory System:
Both gill and pulmonary respiration take place in the lung-fishes. Although the dipnoans possess the gills as well as lungs, they are mostly the lungs. The nostrils help in aerial respiration.
The external nostrils lie at the margin of the mouth and the internal nostrils open into the buccal cavity. A slit-like glottis is present in the floor of the oesophagus which opens into a short trachea. The trachea passes into the lungs. The glottis is provided with a fibro cartilaginous plate which resembles the epiglottis.
Air-bladder (swim-bladder) is modified into the “lung”, which is similar to that of higher vertebrates in structure and function. The internal cavity of lung is produced into numerous alveoli which lead into minute alveolar sacs.
Circulatory System:
The circulatory system is well developed. The heart is enclosed in a stiff pericardium and is situated somewhat posterior to the gills. The heart of lung fishes consists of four chambers the sinus venosus, auricle, ventricle and conus arteriosus. The sinus venosus is incompletely divided into two parts. It opens into the auricle by a broad sinuaricular opening.
The atrium is divided by a complete septum. Thus, the heart is incompletely divided into arterial and venous channels. Venous blood runs through the right side and oxygenated blood through the left side of the heart. The atrioventricular opening is closed or opened by means of a fibrous plug. Pulmonary vein bringing oxygenated blood from the lungs, passes through the sinus venosus and opens into the left auricle.
The deoxygenated blood from the rest of the body is poured into the right side of the sinus venosus passes to the right auricle. Ventricle is undivided and leads into the conus which is divided into two channels by means of valves.
The arrangement of the valves is such that the oxygenated blood flows through the first two arches into the carotid and cerebral circulation. Venous blood tends to go into the posterior branchial arches and is finally carried to the lungs.
The ventral aorta is small and gives off four pairs of afferent branchial arteries close to the conus. In the arches bearing gills, paired efferent branchial vessels are present and join to form the dorsal aorta. Pulmonary arteries arise from the paired dorsal aortae and carry blood to the lungs.
The conus arteriosus become spirally twisted and the cavity becomes complicated by the presence of valves. A spiral valve is present. A pulmonary artery is given off on either side from efferent branchial system. A pulmonary vein opens into left side of auricle. There are two precavals (ductus Cuvieri) and a large posterior vena cava (posterior cardinal vein) as in tetrapods.
Urinogenital Organs:
The excretory system is like that of Scoliodon (dog-fish). It consists of a pair of mesonephric kidneys which discharge into the cloaca through the ureters. In the female, a pair of ovaries are present close to the kidneys. Pair of oviducts open by funnel-like apertures into the abdominal cavity and join at their hinder ends before opening into the cloaca.
In the male, a pair of elongated testes is present close to the kidneys. The anterior part of the testis serves for spermatogenesis and the posterior part acts as vesicula seminalis. The two vasa deferentia join to open into the cloaca. Vestigial Mullerian ducts are also present. Vasa efferentia carry sperms through excretory part of kidney in male. Fertilisation is external. Development includes metamorphosis. Larvae may develop suckers and external gills.
Nervous System:
The brain of lung-fishes shows many characteristics resembling that of amphibians. A pair of well-developed cerebral hemispheres is present in the brain. Cerebellum is small and the optic lobes are fused to form a single oval body. A pineal body is present. The brain, thus, lacks the special features of elasmobranchs or teleosts.
4. Classifications of Dipnoi:
Order Dipnoi is divided into 2 Suborders:
Suborder 1. Monopneumona:
Lung Single:
Lateral jointed rays of archipterygium (paired fins) are well developed.
Examples: Living Australian Neoceratodus forsteri and extinct Ceratodus.
Suborder 2. Dipneumona:
Lung Double:
Lateral rays of archipterygium (paired fins) are vestigial or absent.
Examples: African Protopterus and Australian Lepidosiren paradoxa.
5. Affinities of Dipnoi:
With special features of their own, the Dipnoi combine characteristics in which they resemble different groups of fishes as well as Amphibia.
1. Affinities with Fishes (in general):
Lung-fishes are true fishes beyond doubt as they resemble them in general in the following features:
(i) Body spindle-shaped and streamlined.
(ii) Locomotory appendages fins.
(iii) Diphycercal tail fin.
(iv) Largely ossified, slender dermal fin rays,
(v) Body covered by overlapping cycloid scales.
(vi) Notochord persistent.
(vii) Vertebrae without centra.
(viii) Skull with little ossification and with many investing bones.
(ix) 4 to 6 pairs of branchial arches present,
(x) Aquatic respiration by gills.
(xi) Lateral line sense organs present.
2. Affinities with Elasmobranchii:
Lung-fishes resemble the cartilaginous fishes (elasmobranchs) in the following primitive characters:
(i) Spiral valve in the intestine.
(ii) Similar conus arteriosus.
(iii) No nephrostome in the kidney tubules.
(iv) Similar diencephalon.
(v) Similar female reproductive system.
(vi) Each gill-arch with 2 efferent arteries.
The above similarities are not sufficient enough to establish any relationship. The main differences from elasmobranchs are presence of diphycercal tail, opercula and lungs, absence of claspers and external fertilisation.
3. Affinities with Holocephali:
According to Jarvik (1964, 1967), dipnoans and holocephalians resemble remarkably with each other as follows:
(i) Teeth fused into dental plates on jaws.
(ii) Gills covered by operculum.
(iii) Operculo-gular membranes of both sides fused.
(iv) Jaw suspension autostylic.
(v) Lack of stomach. Intestine with a spiral valve.
(vi) Excurrent nostrils opening into the mouth cavity.
(vii) Similar kidneys, gonads and ducts.
(viii) Each gill-arch with 2 efferent arteries.
(ix) Similar cranial nerves.
Despite such similarities it is extremely difficult to establish any relationship between dipnoans and holocephalians. The main differences from holocephalians are presence of lungs, absence of claspers and external fertilisation.
4. Affinities with Actinopterygii:
Lung-fishes resemble subclass Actinopterygii in the following characters:
(i) Blunt snout.
(ii) Body covered with cycloid scales.
(iii) Powerful palatine and splenial teeth.
(iv) Presence of inferior jugular veins.
(v) Presence of swim-bladder (air-bladder).
(vi) Presence of operculum over gills.
However, the Actinopterygii belong to a separate evolutionary line. They have thin, broad fins modified for swimming, and the external nostrils never penetrate into the mouth. Most of them are small in size, have reduced snout, large eyes, single separate dorsal fin and a homocercal caudal fin.
5. Affinities with Crossopterygii:
The two orders Dipnoi and Crossopterygii were included under the subclass Sarcopterygii by Romer (1959). Instead of specialising for aquatic habitats (as did the Actinopterygii) they have adapted for semiaquatic existence.
They show many close similarities as follows:
(i) Powerful leg-like lobate fins.
(ii) Caudal fin diphycercal.
(iii) Presence of cosmine covering the cycloid scales.
(iv) Presence of blunt snout with ventral nostrils.
(v) Presence of powerful palatine and splenial bones.
(vi) Vertebral column reaching up to the end of caudal fin.
(vii) Paired inferior jugular veins are present.
(viii) Presence of air-bladder (swim-bladder) although modified into lungs with good pulmonary circulation.
(ix) Presence of operculum.
(x) Contractile conus arteriosus.
(xi) Larval forms in some cases with external gills as accessory respiratory organs.
Besides, the fossil crossopterygians (rhipidistians such as Devonian Osteolepis) and fossil dipnoans (such as Devonian Dipterus) show even closer affinity in:
(i) Similar body shapes and sizes (20 to 70 cm).
(ii) Separate 2 dorsal, 1 anal and 1 heterocercal caudal fin supported by dermal bony rays.
(iii) Paired fins somewhat lobate with a fleshy scaly central axis. Pectoral fins placed high.
(iv) Presence of internal nostrils.
(v) Cycloid scales modified from cosmoid type.
(vi) Similar skull bones.
(vii) No vertebral centra.
(viii) Similar opercular and gular bones.
(ix) Comparable lower jaw.
(x) Similar lateral line sensory system.
(xi) Presence of swim-bladder (air-bladder) functioning as the lung.
(xii) Intestine contains spiral valves.
(xiii) Conus arteriosus is contractile.
(xiv) External gills are present in the larvae.
This led to the belief that the dipnoans are degenerate descendants of the crossopterygians, which the early dipnoans closely resembled. But Jarvik (1968) and others throw doubt upon this belief. According to them, certain structures are more specialised in dipnoans than the crossopterygians.
Their basic differences are in structural organisation of food crushing apparatus, fin skeleton, vertebral column, visceral skeleton, neural endocranium, snout, division of heart, atrium, blood supply of swim-bladder, etc.
Affinities with Amphibia:
Lung-fishes (dipnoans) resemble the amphibians in several features, such as:
(i) Semiaquatic and marshy habitat.
(ii) Skin glands multicellular.
(iii) Presence of dermal scales in Gymnophiona.
(iv) Internal nostrils piercing roof of mouth cavity.
(v) Presence of vomerine teeth.
(vi) Lungs capable of pulmonary respiration. A pulmonary artery and a pulmonary vein are present.
(vii) Spiracles lacking.
(viii) Auricle and sinus venosus partially divided into right and left halves.
(ix) Conus arteriosus spirally twisted and divided into two by a longitudinal partition.
(x) Pericardium is thin-walled.
(xi) Ventral aorta short.
(xii) Presence of anterior abdominal vein, posterior vena cava.
(xiii) Jaw suspension autostylic.
(xiv) Brain similar in structure of cerebrum and cerebellum.
(xv) Sperms carried through excretory part of mesonephric kidney.
(xvi) Similar structure and development.
(xvii) Larval stages possess external gills and sucker.
The close similarities led early workers to conclude that lung-fishes (dipnoans) gave rise to Amphibians, a view no longer held now-a-days. According to Dolo, these similarities probably were due to convergent evolution and have possibly from adaptive modifications on account of similar habits and habitat. On the other hand, lung-fishes have many specialised features by which they differ from amphibians.
These are as follows:
(i) Lobate fins instead of limbs for locomotion.
(ii) Fin skeleton not like that of primitive tetrapods.
(iii) Skull mainly cartilaginous with little ossification.
(iv) Presence of characteristic tooth-plates instead of teeth.
(v) Maxillae and premaxillae absent.
(vi) Some anterior vertebrae fused with skull.
(vii) Lungs located dorsal to gut.
(viii) Urinary bladder develops from dorsal wall of cloaca in Dipnoi but from ventral wall in Amphibia.
6. Phylogeny and Significance of Dipnoi:
The origin, evolution and affinities of Dipnoi are still an unsolved problem due to diverse opinions. They combine characteristics in which they resemble almost all the other groups of fishes as well as Amphibia. Fossil primitive Dipnoi (e.g., Dipterus), shows greater similarity with fossil crossopterygians (e.g., Osteolepis), than do their living members.
During the course of their evolution, the modem Dipnoi has undergone several changes or specialisations which are as follows:
(i) Anterior dorsal fin was reduced and eventually lost.
(ii) Remaining median fins elongated and fused so that originally heterocercal tail became symmetrically diphycercal.
(iii) Reduction in number of dermal plates or bones of skull and operculum.
(iv) Thick bony cosmoid scales modified into thin cycloid scales.
(v) Extensive sheet of cosmine covering head and body was lost.
(vi) Fusion of conical teeth to form characteristic crushing tooth-plates with ridges to feed effectively upon shell fish.
(vii) Air-bladder became functional lungs not unlike those of higher vertebrates.
Evidence indicates that Dipnoi are degenerate descendants of Crossopterygii which early dipnoans resembled. Romer (1945) thought that Dipnoi and rhipidistian Crossopterygii had a common ancestor.
On the other hand close similarity between Dipnoi and Amphibia led early workers to conclude that dipnoans gave rise to amphibians, a view no longer held now. However, it is universally accepted that Amphibia have originated either directly from rhipidistian Crossopterygii or from some common ancestor.