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February 2002

Feature

 


What do we know about "Old Fourlegs"?

As a living fossil, the coelacanth has become a celebrity fish. Dr Phil Heemstra is an ichthyologist with the South African Institute for Aquatic Biodiversity. He is one of the fortunate few who has had first-hand experience of a living coelacanth. Here he shares information on some of its less - publicized secrets…such as Latimeria's head-stands. 

Dr Heemstra will be one of the key scientists involved in the Coelacanth Programme

 

Enter here for larger image depicting skeletons of a living coelacanth and that of a fossil.Coelacanths have not changed much over the past 380 million years. The skeleton of Macropoma lewesiensis, which is known from the upper Cretaceous, is virtually identical to that of the coelacanths caught off Sodwana Bay, Latimeria chalumnae, and differs little from the skeleton of most Devonian coelacanths. (Enter the image above to see skeletons of a living coelacanth and that of a fossil). Thanks to the efforts of JLB and Margaret Smith, Marjorie Courtenay-Latimer, French anatomists, and (since 1987) Hans Fricke and his co-workers, Jürgen Schauer and Karen Hissmann (who were the first to observe and film coelacanths in their natural habitat at Grand Comoro Island), we have learned much about the anatomy, biology and ecology of this fascinating fish.

Behaviour

Latimeria are nocturnal fish, and during the day, they are usually found in depths of 120-250 m, where they congregate in caves, with as many as 14 fish crowded together in a single cave. By resting in caves, they save energy and are also less vulnerable to large predators (deep-water sharks). Each coelacanth has its own distinctive pattern of white markings, and this allows recognition of individuals and tracking of their movements. Although several individuals occupy overlapping home ranges, no aggressive encounters between coelacanths have been observed. A single fish may frequent several caves within its home range, and three individuals were sighted within the same home range over a period of two years. After sunset, the fish leave their caves and drift slowly , 1-3 metres off the bottom, presumably looking for food. On these nightly hunting forays, Latimeria may travel as much as 8 km; and before dawn they shelter in the nearest cave.

Swimming

While searching for prey, or moving from one cave to another, Latimeria appear to move in slow motion, either drifting passively with the current and using the flexible pectoral and pelvic fins to adjust their position, or slowly swimming by a synchronous sculling movement of the second dorsal and anal fins. In slow forward swimming, the left pectoral and right pelvic fins move forward, while the right pectoral and left pelvic fins are pulled backward. This tandem movement of alternate paired fins resembles the movement of the fore- and hindlimbs of a tetrapod walking on land. Contrary to JLB's name "Old Fourlegs" and his idea that the coelacanth stalked its prey "by crawling quietly along gullies and channels", Latimeria do not use their lobed fins for walking on the bottom, and even when they are resting in caves their fins usually do not touch the walls or bottom of the cave. Like most slow-moving fishes, the coelacanth can make a sudden lunge or fast start by means of a quick sweep of its massive caudal fin.

Hunting for Food 

During their nightly foraging swims, Latimeria were often seen to perform head-stands, with the body in a vertical position, the head near the bottom and the tail fin curved perpendicular to the body. They then held this position for two or three minutes at a time. This curious behaviour may be used when Latimeria are scanning the bottom for prey. The large, sensory organ in the snout (known as the "rostral organ") is thought to function as an electoreceptive organ, in the same manner as the ampullae of Lorenzini, which assist sharks to find buried prey animals. Latimeria feed mainly on small fishes that occur in their deep demersal habitat. In addition to a swell shark and a skate, various bony fishes (a synaphobranchid eel, a deep-water cardinalfish, etc.) and a cuttlefish have also been found in their stomachs. Judging from their sluggish lifestyle and the relatively small surface area of the gills, Latimeria have one of the lowest metabolic rates of all fishes; consequently, their food (energy) requirements are relatively low for such a big fish. This energy-saving lifestyle is an advantage in their specialised habitat on the relatively barren volcanic slopes of the Comoros where prey is scarce.

Reproduction

Despite the lack of an obvious copulatory organ, Latimeria are "ovoviviparous", which means that the eggs are fertilised internally, and the pups (fetuses) are retained within the mother until they have grown large enough (36-38 cm) to fend for themselves. The enormous eggs (9 cm in diameter and over 325 g in weight) are the largest eggs known for fishes, and the huge yolk supplies all of the nutrients necessary for the growth of the embryo. Contrary to previous speculation, recent information confirms that Latimeria pups do not practise "embryonic cannibalism" or feed on eggs while in the uterus. In August 1991 a 179 cm, 98 kg, pregnant coelacanth was caught by a trawler off Pebane in northern Mozambique. This specimen was given to the Natural History Museum in Maputo, where it was dissected by the Director, Dr Augusto Cabral, who found that it contained 26 near-term pups, 31-36 cm in length. In view of the large size and advanced development of the pups from the Mozambique female, the size at birth for Latimeria is probably about 35-38 cm, and this agrees with the age estimate of 6 months assigned to a 43 cm juvenile that was caught on hook and line. The gestation period for Latimeria has been estimated at about 13 months.

Unusual Anatomy of Latimeria

Latimeria differs from most other living fishes by a number of features, some of which are characteristic for coelacanths generally. The most obvious external feature that readily identifies a coelacanth are the fins. The lanceolate tail fin with the many-rayed upper and lower margins and central little supplementary fin projecting beyond the fin margin distinguishes Latimeria from all other living fishes. 

The fleshy, stalk-like pectoral and pelvic fins and similar fleshy second dorsal and anal fins are also unlike any other marine fishes. The paired fins of the Australian lungfish, Neoceratodus forsteri (another living fossil), are similar to those of Latimeria. The skeletal support for the fan-like first dorsal fin of Latimeria is a single bony plate, and the skeletons of the fleshy second dorsal and anal fins are identical to each other. In place of the bony vertebral column of most adult fishes, coelacanths have a large, thick tube of cartilage, called the 'notochord'. In the early development of most fishes, the notochord of the embryo or larva is gradually replaced by the bony (or calcificed) centra of the vertebral column. But in coelacanths, lungfishes and some primitive sharks, the transformation of notochord into a segmented bony (or calcified) vertebral column does not take place. In coelacanths, the hollow notochord is filled with oil and provides a strong, yet flexible support for the spinal cord. 

The cranium is divided transversely by the intracranial joint, which is better developed in coelacanths than in other lobefin fishes. The moveable front part of the cranium provides a larger gape when the mouth is opened, and this may be advantageous in feeding. Coelacanths have a large median 'rostral organ' in the snout, with three tubes leading to the exterior surface on each side of the snout. The nerve pathways that enervate the rostral organ indicate that it may function as an electroreceptive organ. In adult Latimeria, the brain is simple (not much convoluted), occupies less than 2% of the cranial cavity, and is confined to the rear part of the skull. In the late-term foetus, the brain fills the cranial cavity; it appears therefore, that soon after birth, the brain stops growing, while the head, body, fins (and everything else) continue to grow. This unusual configuration of a tiny brain situated at the rear of an enormous brain cavity is also seen in the sixgill stingray, Hexatrygon bickelli Heemstra and Smith, 1980. As suggested for the sixgill stingray, a small brain placed at the rear of the cranium may cause less electrical interference with the electroreceptive sensors in the snout. 

The jaws of coelacanths are unique. The lower jaw is attached by double (tandem) joints on each side of the head. The upper jaw is absent or reduced to a small median rostropremaxilla bone, and there is no maxillary bone. The toothed bones at the front of the palate serve as a functional upper jaw. The heart appears to be the most primitive of all adult vertebrates, with the auricle, ventricle and conus arteriosus arranged in straight line, rather than being doubled over one another. Unlike most bony fishes, which have a gas-filled swimbladder that provides buoyancy, the swimbladder of Latimeria is filled with fat. In addition to the fat-filled swimbladder (which also provides buoyancy, as fat is less dense than seawater), Latimeria also have a lot of oil in the liver, muscle tissue and in cells under the skin. In most fossil coelacanths, the swimbladder appears to be ossified (bony) and, consequently, these fishes were probably confined to shallow water. Latimeria have no branchiostegal rays supporting the opercular membranes.

Where Coelacanths Live

Enter here to see a map showing places where coelacanths have been caught or observed.In 1995 a coelacanth was caught in a gill net set in 150 m off the southwest coast of Madagascar. And in 1998, two coelacanths were caught at Sulawesi in the Indonesian Archipelago. Although it seems likely that the South African, Mozambique, Madagascar and Comoran specimens are all Latimeria chalumnae; a DNA comparison indicated that the recently discovered Indonesian coelacanths are a separate species

 
We still have much to Learn

Because of the difficulty of sampling and observing fishes in rough bottom habitats below depths of 50 m, the fish fauna of these areas is poorly known. This discovery of coelacanths at a depth of 104 m at Sodwana Bay, a popular dive site in South Africa, emphasises how little we know about life in the oceans and the need for further exploration and survey work to understand the fish diversity of southern Africa and the Western Indian Ocean.

 

 







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