Number of families 1
Evolution and systematics
The living coelacanths are often celebrated as the most unusual case and important example of animal evolution. The first fossil coelacanths were recognized in rocks between 380 and 75 million years old. More than 100 years ago Woodward published the first review on these fishes. Fossils younger than 75 million years were never found, as if all coelacanths had become extinct at that time, very much like the dinosaurs. The bony structures in these fossil crossopterygians, especially their paired (pectoral and pelvic) fins, placed them close to the ancestor of the first amphibians and all other land vertebrates.
It is of no surprise, therefore, that the December 1938 find of a living coelacanth, when announced to the world by J. L. B. Smith in March 1939, caused disbelief and created one of the greatest biological sensations of the last century. Finding a living coelacanth—morphologically so similar to the fossil specimens left in rocks more than 75 million years ago—was as inconceivable as meeting a living dinosaur on a weekend walk.
The living coelacanth, sometimes known by the common name "gombessa," is a single advanced life form that has survived with relatively little change for nearly 400 million years. While some of the coelacanth's relatives became implicated in the ancestry of all terrestrial vertebrates, the aquatic descendants developed structural solutions to life absent in other animals. For example, instead of the calcified vertebrae that normally reinforce the axial skeleton, the coelacanths evolved a strong-walled elastic tube that is as far transformed from the notochord as are the vertebrae. Instead of a solid braincase, they evolved a two-part neurocranium with an intracranial joint that is operated by a special basicranial muscle. It is the only animal with that structure living today. This intracranial joint and other unique rotational joints in the head together with the rostral organs and the gular reticulate electrosensory system may explain the special suction feeding and head standing behavior observed by Hans Fricke.
In the synthesis of coelacanth evolution by P. Forey, a total of 83 species are recognized to have lived between 380 million years ago in the middle Devonian and 75 million years ago in the Upper Cretaceous. There is no fossil record of coelacanths from the past 75 million years. During that time coelacanths lived and died without leaving fossils. The highest diversity of coelacanths during their geological history was recorded in the Lower Triassic (16 species) and in the Upper Jurassic (8 species). While most fossil remnants were found in marine deposits, many were also found in freshwater deposits, especially in the Upper Carboniferous, Lower Permian, Upper Triassic, Jurassic, and Lower Cretaceous.
The first living coelacanth that came to the attention of scientists was named Latimeria chalumnae by J. L. B. Smith in 1939. Upon seeing a second specimen in 1952 and noting that it lacked the first dorsal fin, Smith considered it a new species and named it Malania anjouanae. When it was found later that except for the lack of the first dorsal fin all other structures were like the first specimen, it was concluded that this first dorsal fin was probably bitten off by a shark, and Malania anjouanae become a synonym of Latimeria chalumnae.
A second species of coelacanth, Latimeria menadoensis, was discovered near Sulawesi by M. V. Erdmann in 1998.
At the beginning of the twenty-first century, one family was recognized: Latimeriidae. It contains one genus (Latimeria) and two species: Latimeria chalumnae and Latimeria menadoensis.
The living coelacanth is often referred to as a "living fossil." A representative of an ancient group whose other members have all gone extinct, it has survived for millions of years with a virtually unchanged body form. Studies of the living coelacanth's soft anatomy and body fluids have shown various similarities with chondrichthyans. These characteristics are thus considered "primitive vertebrate features," but the coelacanth has also developed many specialized characteristics.
Coelacanths have several unique characteristics, the most obvious of which are the fleshy (lobate or pedunculate) fins. While these fins have some similarity with the lobate fins in fossil lungfishes, rhipidistians, and some polypterids, no other fish group has developed seven fleshy fins. The paired fins are supported by girdles that resemble the purported precursors of the pectoral and pelvic girdles of tetrapods. The axial skeleton of coelacanths evolved differently from that of other vertebrates, even those with a persistent notochord. Instead of developing vertebrae, the notochord of the living coelacanth developed into a tube, over 1.57 in (4 cm) in diameter in adults, which is stiffened by fluid under pressure.
The skull of coelacanths has an intracranial joint that divides the neurocranium into an anterior and a posterior half and that allows the mouth to open not only by lowering the lower jaw but also by raising the upper jaw. This increases the gape considerably, and by extending the buccal cavity creates a strong suction. No other living animal has this feature.
Adult coelacanths have a minute brain (occupying only 1.5% of the cranial cavity) in common with many deep-sea sharks and the sixgill stingray (Hexatrygon bickelli). The pineal complex, which is involved with photoreception in many vertebrates, is relatively primitive and undifferentiated in Latimeria, whereas the basilar papilla in the inner ear has some similarity to that of tetrapods. The electrosensory systems in the head and the gular plates, in addition to the rostral organs, might be useful for locating prey.
The coelacanth has a spiral valve with unique, extremely elongate, nearly parallel spiral cones in its intestine. The valvular intestine is a shared character with ancestral jawed fishes (Gnathostomata), progressively reduced in actinopterygians and replaced by an elongated intestine in teleosts and tetrapods. The heart is elongate but is not simple; it is as complex as in other fishes, and far removed from the superficial earlier interpretation as an S-shaped embryonic tube. Bogart, Balon, and Bruton reported in 1994 that a Latimeria chalumnae specimen caught in April 1991 at Hahaya, Grand Comoro, has a 48-chromosome karyotype. This karyotype is unlike those found in lungfishes but is very similar to the 46-chromosome karyotype of one of the ancient frogs, Ascaphus truei.
The complex dermal canals known only from fossil jawless and jawed fishes are combined in L. chalumnae with the common pit lines of superficial neuromasts (lateral line) of extant fishes. Therefore, retention and specialization of ancestral structures, no longer present in other living fishes, is one of the most significant attributes represented, along with their evolutionary persistence, in this true "living fossil."
The coloration of Latimeria chalumnae is bluish grey with large whitish marks scattered on the body, head, and the fleshy fin bases. The white markings are specific to each individual, so each single fish can be distinguished. The white marks simulate white sessile tunicates on the walls of caves where coelacanths aggregate and on the substrate over which they drift, so that the animals blend perfectly with their background. On a dying coelacanth the bluish hue turns brown, the color of all dead specimens. The Indonesian coelacanth was brown when still alive with much golden glitter in the whitish markings.
Females grow to 74.8 in (190 cm), males to 59.1 in (150 cm) and 110–198 lb (50–90 kg). Individuals are 13.8–15 in (35–38 cm) long at birth.
Since 1998 the coelacanth distribution is known to be not only in the west Indian Ocean, but also 6,214 mi (10,000 km) east on the other side of the Indian Ocean. The specimen that was caught off the Chalumna River in 1938 was later thought to be a stray from the Comoran population around Grand Comoro and Anjouan. Captures near Malindi (Kenya) and at Sodwana Bay near St. Lucia estuary in South Africa extend the range of intermittent distribution along the East African coast. It has not yet been established whether these are discrete populations. Only the Mozambique specimen and the southwest Madagascar specimens were proven to be of Comoran origin. According to Victor Springer, Latimeria menadoensis in Indonesia is most likely isolated from the western population(s) by unsuitable habitats in the central Indian Ocean.
The extant coelacanths are tropical marine fishes inhabiting inshore water below 328 ft (100 m) depth. They seem to prefer steep sloping areas with little coral sand deposits. The hemoglobin of Latimeria chalumnae has the best affinity to oxygen at 61–64.4°F (16–18°C). This temperature coincides with the isobaths of 328–984 ft (100–300 m) in most localities inhabited by coelacanths. As there seems to be very little prey at these depths, the coelacanth is forced to ascend at night to more shallow waters in order to feed, risking some respiratory discomfort. For the daytime, coelacanths descend back into more comfortable temperatures and hide in groups under overhangs and in caves. A sluggish locomotion and drifting instead of fast active swimming probably help to save energy. If this is the case, then a fish hauled to the surface often with a water temperature far above 68°F (20°C) is under
such respiratory stress that its survival is uncertain even if it is released back into cooler waters.
At Grand Comoro most coelacanth catches have occurred over the newest lava flows of the periodically erupting volcano Kartala. These lava fields under water consist of more cavities where prey can hide, and more caves for daytime aggregations of coelacanths than other less steeply sloping shores.
Coelacanths aggregate in caves and overhangs about 328–656 ft (100–200 m) deep during the daytime. At Grand Comoro 19 adults were counted in one cave close together, gently moving their paired fins but never touching each other. Individuals distinguished by their specific white markings were found faithful to a particular cave for many months, although every day some strayed into other caves. At night the fish drifted individually close to the substrate.
After the first observations in 1987 from the submersible GEO, Hans Fricke noted that at night, all individuals took advantage of up- or down-wellings and drifted slowly with the current. The paired fins stabilized the drifting fish so that "all individuals seemed perfectly oriented in that they avoided obstacles in their environment, apparently detecting them well in advance." Fricke further commented that "all individuals irregularly performed a curious headstand, lasting up to 2 minutes."
During swimming, the coelacanth very slowly moves its paired fleshy pectoral and pelvic fins alternatingly in the manner of a trotting horse (left pectoral and right pelvic simultaneously and then right pectoral and left pelvic together). This pattern is also common to lungfish and a few other bottom-dwelling fishes and, of course, most tetrapods. The unpaired fleshy second dorsal and anal fins are sculled in synchrony from side to side and are the main organs for forward propulsion. This explains their similar shape and exact juxtaposition. The nonfleshy first dorsal fin is usually folded and flush with the dorsal surface in undisturbed fish, but when spread it appears to be used as a sail and/or for lateral display when the fish feels threatened. The large caudal fin (in fact the third dorsal, epicaudal, and second anal) is held rigid during drift-swimming as in weakly electric fishes (to enable interpretation of the electric field distortions), but provides powerful propulsion during rapid forward bursts. The small epicaudal lobe is bent to and fro when the coelacanth is swimming, drifting, or standing on its head, and may be implicated in electro-reception together with the rostral organ and the reticulate organs. The GEO team was able to induce headstands in the coelacanth by emitting weak electric currents from electrodes held in the submersible's remotely controlled arm.
Fin coordination probably developed to stabilize the bulky body of the coelacanth, but could in its extinct ancestors have facilitated the eventual transition to locomotion on land. When coelacanths have been observed coming in contact with the substrate, the paired, fleshy fins were not used for locomotion. The coelacanth probably never walks.
Feeding ecology and diet
Latimeria chalumnae is an opportunistic nocturnal bottom drift feeder. Prey items identified in several studies are benthic or epibenthic dwellers like some lanternfishes, deepwater cardinal fishes, cuttlefishes, snappers, cephalopods, and even a swell shark. Most of these are known to hide in bottom cavities.
The coelacanth prefers fresh lava rocks with cavities not filled by coral sand. Being a sluggish swimmer with a low metabolic rate, it regularly performs intermittent head stands during its nocturnal drifts along the bottom. Latimeria is able not only to move its lower jaw but, thanks to the intracranial joint, to lift its upper jaw. This feature, unique among extant vertebrates, allows for a considerable increase in the oral gape. In addition to the rostral organ the fish has a distinct reticular system in the gular bones under the head that probably also functions as an electro-sensory system.
The cranial morphology of Latimeria chalumnae suggests that it is a gape-and-suck predator whose anatomical specializations appear to permit it to extract prey from the crannies and cavities where it takes shelter.
Until 1975 Latimeria chalumnae had been considered to be an egg laying (oviparous) species because a 64.2 in (163 cm) long female caught at Anjouan in 1972 was found containing 19 eggs the size and color of oranges. But then another female, 63 in (160 cm) long, previously caught at Anjouan in 1962, preserved, and kept as an exhibit at the American Museum of Natural History (AMNH), was dissected in 1975. The curators of this museum were persuaded to cut open the specimen in order to sample needed tissue of some inner organs. The curators discovered in the female's oviduct five well-developed embryos, with a length of 11.8–13 in (30–33 cm), each with a large yolk sac. This finding meant that the living coelacanth is a livebearer (viviparous).
Later, John Wourms and Jim Atz studied these embryos and their mother's oviduct in detail and found that the heavily vascularized yolk sac surfaces were in very close contact with the equally densely vascularized oviduct walls, thus forming a simple placentalike structure. It seems, therefore, that in addition to the yolk, the embryos have a second, more direct, way to receive nutrients from the mother. A third way of obtaining nutrients suggested itself when more females were dissected. One 66-in (168-cm) long female contained 59 eggs the size of chicken eggs, another female had 65 eggs, and yet others had 62, 56, and 66 eggs. All these females produced more eggs than their oviduct would be able to accommodate as embryos. While the five embryos from the AMNH female still had large yolk sacs, the 26 fetuses from the female caught near Mozambique were close to term and had only a scar on the belly where the yolk sac once was. Both groups of embryos/fetuses had well-developed alimentary tracts and dentitions. It is thus possible that additional nutrient delivery occurred through the debris from the supernumerary eggs. After all, it is known that in some shark species the fetuses feed on eggs and siblings, so that at the end only one large predator is born. It is possible that such oophagy, as it is called, also occurs in Latimeria.
Further studies of these unborn fetuses revealed exceptionally wide gill-cover membranes full of cells capable of absorbing uterine milk (histotrophes) secreted by the oviduct walls. This type of nutrient transfer is known in some fishes. Finally, the carotenoid pigments in the yolk are also implicated in oxygen delivery. While more investigation is needed, it is clear that the coelacanth is a fish with a very advanced and complex style of reproduction. This is not surprising, given that the Jurassic coelacanth Holophagus gulo was probably a live-bearer, and the Carboniferous Rhabdoderma exiguum, although still oviparous, had eggs of relatively large size.
Circumstantial evidence suggests that the gestation time of the living coelacanth is very long (about 13 months), that the females become mature for the first time when older than 20 years (as in some sturgeons), and that a female does not deliver young every year but several years apart. Scientists do not know how the internal fertilization of a female is achieved and where the young live right after birth and in subsequent years. No young were noticed from the submersibles either drifting or in caves, and only one or two have been collected free swimming.
After the catch of the "second living coelacanth" became known to science in 1952, the Comoro Archipelago (then a French colony) was recognized as the "home" of the coelacanth. Soon, national ownership was declared for all subsequent specimens, and the second one declared stolen from its "rightful" owners. Only the French were allowed to collect them. J. Millot, a spider specialist on Madagascar, moved to Paris and with J. Anthony started detailed anatomical descriptions of the coelacanth. After close to 80 specimens were accumulated, some were used as diplomatic gifts. Eventually, other nationals joined the frenzy of working on the prestigious animal. Several special expeditions converged onto the
Comoros, but luckily the beast could not be caught at will by the methods employed.
The Japanese imported larger fiberglass boats called japavas to supplement the small single log dugout outriggers (galavas), and built a fishing school on Anjouan in the early 1980s. At the same time, rumor was started that the fluid from the "notochord" tube when ingested prolonged life. Soon, in addition to the demands for coelacanths for museum exhibitions, a black market for fresh or frozen specimens for "medicinal" purposes was started. The price soared to $3,000 or more per fish, especially during the rule of the white mercenary who called himself Colonel Baku.
The first deployment of Fricke's submersible GEO and the first sighting of coelacanths in their natural habitat coincided with the urgent need for conservation. The Coelacanth Conservation Council (CCC) was established by Eugene Balon, Mike Bruton, Christine Flegler-Balon, Hans Fricke, and Rafael Plante when they met in Moroni (Grand Comoro), the capital of the Federal Islamic Republic of the Comoros, in 1987. The CCC was inspired by the Desert Fishes Council that led to the most progressive conservation law in the world, the Endangered Species Act of the United States. Within the next two years, members of the CCC managed to have the coelacanth included in Appendix I of CITES.
Subsequent dives by Fricke and his team with the new submersible JAGO at Grand Comoro revealed a serious decline in the number of coelacanths in each previously surveyed cave. Thus the initial estimates of the numbers of adults (200–500) became potentially invalid. In spite of the discovery of additional individuals off Sulawesi in 1998 and lately at Sodwana Bay (South Africa), the living coelacanth remains unique and highly vulnerable because of its narrow habitat range and very specialized physiology and life style. Although the 2002 IUCN Red List does not list Latimeria menadoensis, it lists L. chalumnae as Critically Endangered.
Significance to humans
Before the coelacanth's value for science was recognized in the mid-twentieth century, it was occasionally consumed for its presumed antimalarial properties. Because of its high oil content, the meat tastes foul and rancid and causes severe diarrhea when eaten. Since 1952 its interest to science has remained extremely high.
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South African Coelacanth Conservation and Genome Resource Programme. South African Institute for Aquatic Biodiversity, Somerset Street, Private Bag 1015, Grahamstown, 6140 South Africa. Phone: +27 (0)46 636 1002. Fax: +27 (0)46 6222403. Web site: <http://www.saiab.ru.ac.za/coelacanth>
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Eugene K. Balon, PhD