In this article we will discuss about Latimeria:- 1. History of Discovery of Latimeria 2. Fossils of Latimeria from India 3. External Structures 4. Skeletal Structures 5. Digestive System 6. Swim-Bladder 7. Respiratory System 8. Circulatory System 9. Nervous System 10. Urinogenital System 11. Zoological Importance.
Contents:
- History of Discovery of Latimeria
- Fossils of Latimeria from India
- External Structures of Latimeria
- Skeletal Structures of Latimeria
- Digestive System of Latimeria
- Swim-Bladder of Latimeria
- Respiratory System of Latimeria
- Circulatory System of Latimeria
- Nervous System of Latimeria
- Urinogenital System of Latimeria
- Zoological Importance of Latimeria
1. History of Discovery of Latimeria:
Latimeria is a peculiar bony fish belonging to the subclass Crossopterygii. It is the sole surviving representative of the coelacanths. The discovery of this ‘living fossil’ created a sensational history in the science of fishes. The first living specimen was caught at a depth of about 80 m near the eastern coast of South Africa in the year 1938.
The fish was steel-blue in colour measuring about 1.5 m in length. Miss C. Latimer, the then curator of the museum of East London (South Africa), suspected its peculiarities and preserved the skin of the fish with a special note on its principal characteristics. Professor J. L. B. Smith, a well-known Ichthyologist at Rhodes University of Grahams town identified the specimen as a coelacanth and gave the name Latimeria chalumnae after the name of Miss C. Latimer.
Extensive searches have been made to procure live specimens of this particular type of fish. The second specimen was collected from the Anjouan Island (Madagascar) in 1952. Smith studied the animal in detail and suggested a new generic name Malania.
But a thorough study on the other specimens by Millot and his associates revealed the biology of these fishes and in course of time many other coelacanths have been trawled around Comoro Islands (Madagascar) from the depths of 150-400m.
All these fishes are large in size weighing up to 80 kg. It is now unanimously accepted that the generic name of all these coelacanths, discovered so far, will be Latimeria and Malania as suggested by Smith would be a synonym. Latimeria has been extensively studied by Millot (1955) and Thomson (1969).
In 1998, another specimen Latimeria menadoensis, has been discovered in 100 to 150 m deep off the northwest corner of Sulawesi, a central Indonesian island situated in between Borneo, Philippines, and New Guinea. It was named after the city nearby where the 2 specimens were caught. The next species was caught 10,000 km away from the Comoro Island.
2. Fossils of Latimeria from India:
Palaeogeologists of Indian Statistical Institute, Calcutta have discovered some fossils of coelacanths in 1973 from the Lower Kota in Godavari Valley. The coelacanths were 70 cm. long and had jaws without teeth, perhaps they consumed the food by swallowing. Approximate age of these fossils has estimated about 1,65,00000 years. So, at that time these fishes lived in south-west region of India.
The place of occurrence also indicates the habitats in which they lived. The coelacanths favoured the lakes, rivers, etc. as a habitat, though most of present coelacanths have been captured near the bottom of about 300 meters depth in sea around Comoro Archipelago between Madagascar and Mozambique.
3. External Structures of Latimeria:
The body is covered with cosmoid scales. The scales are round and overlap in such a fashion that the body becomes three scales thick. The paired fins act as active swimming organs and are also used for movement at the bottom of the sea. The paired fins are of pedunculate type (Fig. 6.36 A,C).
Each such fin has a short scale-covered lobe and the fin-rays are arranged on the tip in a fan-shaped fashion. There are two dorsal fins. The anterior dorsal fin is supported by a bony plate while the posterior one is almost like that of paired fins.
The tail is diphycercal with a small median lobe. Like that of other teleosts, the fin-rays supporting the fins are highly mobile. Latimeria swims by rotating the pectoral fins and the paired fins are highly mobile.
4. Skeletal Structures of Latimeria:
The notochord persists as a massive un-constricted rod-like structure. The vertebral column is represented by a hollow un-ossified tube with the ossified neural and haemal spines. The fin spines are hollow, hence the name of the group, coelacanths or hollow spine.
The skull has a well-developed joint between a condyle of the posterior end of basisphenoid and a cavity at the anterior end of the occipital region of the skull (Fig. 6.36B). The joint allows movement of the anterior portion of the skull and helps to catch the prey.
5. Digestive System of Latimeria:
The jaws and palate are provided with numerous minute teeth. The buccal cavity is spacious and leads into a powerful muscular oesophagus. The oesophagus leads into a large bag-like stomach. The intestine is spiral and continues as the rectum and cloacal pouch.
The intestine contains a complicated spiral valve. A median nodular organ (homologous with the rectal gland of selachians) is present which pours its secretion into the cloaca. The cloacal pouch bears a urinogenital papilla. The liver is a bilobed structure with an enlarged gall-bladder. The pancreas is well-formed.
6. Swim-Bladder of Latimeria:
The swim-bladder is variable in size and shape in different individuals. It arises as a tube 3-8 cm in length from the ventral side of the oesophagus and continues backwards to occupy the dorsal side of the abdominal cavity. The cavity of the swim-bladder is greatly reduced and contains about 95% fatty tissue. The flesh of Latimeria is also excessively oily in nature. The swim-bladder is neither a hydrostatic nor a respiratory organ.
7. Respiratory System of Latimeria:
Respiration takes place by gills. There is a deep spiracular pouch in Latimeria which lacks mandibular pseudo branch and external opening. The gills are composed of a small hyoidean hemi branch on the posterior surface of the hyoid arch and four holobranchs. The fifth branchial arch is greatly reduced and lacks gill.
8. Circulatory System of Latimeria:
The heart is a very simple structure. It is composed of sinus venosus and auricle located behind the ventricle and conus arteriosus which bears four rows of endocardial thickenings on its inner wall. These thickenings represent the pseudo valves or reduced valves. Presence of such valves speaks about the primitive condition. The red blood cells are large in size resembling that of dipnoans and amphibians.
9. Nervous System of Latimeria:
The brain is small and occupies less than one-hundredth part of the cranial cavity. The rest of the space is filled up with fatty substance. There is a large corpus striatum in the forebrain and the roof is very thin.
Sense Organs of Latimeria:
Besides the eyes, internal ears and lateral line sense organs, there are two peculiar sense organs in the rostral region. (Fig. 6.36D). They are designated as rostral sense organs. Each is composed of a large sac-like median organ of unknown function and communicates with the exterior by three rostral tubes on each side of the snout.
The rostral sense organ is not connected with the olfactory organ and is filled up with a gelatinous substance. It bears similarity with the ampullae of Lorenzini. The rostral sense organ is innervated by the superficial ophthalmic nerve. The other sensory organ is olfactory in function. It is a paired structure and communicates with the exterior by nasal tubes.
10. Urinogenital System of Latimeria:
The kidneys are fused to form a median structure and remain attached to the ventral wall of the abdominal cavity. The ventral position of kidney is a peculiar occurrence and not found in other vertebrates. There are two ureters, each of which dilates into an expanded bladder. The ureters open into the urinogenital papilla. The gonads are usually paired and the gonoducts open between the bases of the pelvic fins.
11. Zoological Importance of Latimeria:
The crossopterygians constitute a peculiar collection of fishes which throw much light on the problems of the origin of land vertebrates. The coelacanths, a side line of the crossopterygians, draw special interest because these fishes survived for much longer a period than any other vertebrate forms.
Originating from the common crossopterygian ancestor possibly in the late Devonian, they lived almost without any significant change for such a long span of time. The first coelacanth fossil, Coelacanthus, was discovered in Carboniferous stratum and that of Undina in Mesozoic strata. Coelacanthus was a fresh-water form and Undina was marine.
Many fossil forms of the Mesozoic era have been discovered. No coelacanth was recorded beyond Mesozoic era. It was then naturally thought that the group has become extinct about 50,000,000 years ago. But the discovery of a living example of the group, Latimeria in the year 1938 added new dimension to our knowledge.
Latimeria persists since the Jurassic time or earlier with very little change. The anatomical organisation of Latimeria is very peculiar and presents the development of structures parallel to those of the Teleostei rather than the dipnoans with which it possibly shares its ancestry.
What causes Latimeria to remain practically unchanged for such a long period of time is not understood. Several features are found to be paedomorphic in Latimeria, which may be responsible for such a long survival.