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ScienceWeek
ZOOLOGY: A REMARKABLE NEW FAMILY OF FROGS
The following points are made by S. Blair Hedges (Nature 2003 425:669):
1) The discovery of a living coelacanth in 1938 captured public attention because it represented an ancient lineage of fishes thought to have been extinct for some 80 million years(1). Now, a living amphibian with unusually deep evolutionary roots has been discovered in India. Biju and Bossuyt(2) describe this odd-looking species of frog, which was collected in the Western Ghats Mountains of southern India. The characteristics that seem strange to a non-herpetologist -- a bloated body, stubby limbs, tiny eyes and protruding snout -- are not uncommon in burrowing frogs. However, its internal anatomy and DNA sequence data show that this species represents a deep branch in the family tree of frogs. Its closest relatives live in the Seychelles, 3,000 kilometers south of India, near Madagascar. Appropriately, the authors place their new species in a new family.
2) Just how significant is the discovery of another family of frogs? Only 29 families are known, encompassing the approximately 4800 known species(3). Most of these families were named by the mid-1800s, and the last discovery of a species of frog belonging to a new family, as opposed to merely a taxonomic rearrangement, was in 1926(3). All others date to the 1700s and 1800s, making this a once-in-a-century find. Moreover, according to fossils and evolutionary "clocks" devised using molecular data, families of frogs are about as ancient as orders and superorders of mammals, having diverged from one another during the heyday of the dinosaurs in the Mesozoic era (251-65 million years ago)(2,4).
3) This discovery also draws attention to our incomplete knowledge of biological diversity, even at the higher taxonomic levels. The home of this plump amphibian, the Western Ghats, is one of the eight "hottest hot spots" of biodiversity in the world(5), meaning that many species occur there that are found nowhere else. Like most other hot spots, it was a region once covered with tropical forests. But pressures from human activities, such as farming, have reduced the forests to less than 10% of their former extent(5).
4) Biologists are racing to survey and discover species in hot spots before they disappear. Unfortunately, fieldwork can be dangerous (diseases, guerrilla wars, venomous animals), and greater efforts are now required to reach unaltered habitats, such as the tops of mountains. Moreover, some governments are afraid of losing their country's genetic resources, and have been discouraging foreign scientists from collecting plants and animals. To complete a gloomy picture, taxonomy in general has become an unpopular career choice. Nonetheless, extraordinary discoveries such as this frog show that there is an urgent need for more biotic surveys.
References (abridged):
1. Thomson, K. S. Living Fossil: The Story of the Coelacanth (Norton, New York, 1992) 2. Biju, S. D. & Bossuyt, F. Nature 425, 711-714 (2003)
3. Frost, D. R. Amphibian Species of the World: An Online Reference Version 2.21 (15 July 2002). Available at http://research.amnh.org/herpetology/amphibia (2003)
4. Hedges, S. B. Nature Rev. Genet. 3, 838-849 (2002)
5. Myers, N., Mittermeier, R. A., Mittermeier, C. G., Da Fonseca, G. A. B. & Kent, J. Nature 403, 853-858 (2000)
Nature http://www.nature.com/nature
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EVOLUTIONARY BIOLOGY: TWO SPECIES OF LIVING COELACANTHS
The discovery of a single living coelacanth off the coast of South Africa in 1938 came as a surprise to the scientific community because the fish is a member of a lineage that was thought to have become extinct approximately 80 million years ago. In 1952, it was determined that the home of this single coelacanth (species: Latimeria chalumnae) was in the Comores Islands in the western Indian Ocean, and since that time more than 200 coelacanths have been captured in the vicinity of these islands. By 1994, the population of coelacanths in the Comores was thought to have dwindled to a few hundred animals. Then, in 1998, a coelacanth was discovered off the coast of Manado Tua Island, Sulawesi, Indonesia -- the first coelacanth recorded from a location outside the western Indian Ocean. Interviews with Indonesian fishermen have revealed a history of catches of coelacanths in the north Sulawesi area, and these facts and the general pattern of ocean current flow from north Sulawesi toward the Comores have implied that the Indonesian specimens cannot be strays from the Comoran population.
The following points are made by M.T. Holder et al (Proc. Natl. Acad. Sci. 1999 96:12616):
1) The authors report a molecular biological analysis of *mitochondrial DNA from the Indonesian specimen of a coelacanth. The authors report they have obtained the sequence of 4823 nucleotide base pairs of mitochondrial DNA from this fish, and that the Comoran coelacanth (L. chalumnae) and the Indonesian coelacanth (recently described as a new species, L. menadoensis) indeed appear to be separate species based on divergence of mitochondrial DNA.
2) The authors suggest that given the apparent biological and geological restrictions to dispersal of coelacanths, it is perhaps not surprising that the Comoran and Indonesian populations represent distinct evolutionary lineages of Latimeria.
3) The authors conclude: "This finding clearly does not preclude the existence of further living species of coelacanths in the area between Sulawesi and the Comores or elsewhere, and genetic studies of any such population will help unravel the mysteries of coelacanth distribution and evolution."
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Notes:
mitochondrial DNA: Mitochondria are double-membrane enclosed organelles of cells that are involved with several important biochemical pathways, including electron transport and oxidative metabolism. Various types of eukaryotic cells may contain from a few to several thousand mitochondria in each cell type. The mitochondria are relatively large cylindrical structures up to 10 microns long and up to 2 microns in diameter, and they are believed to have originated as organisms that became symbiotic with eukaryotic cells (i.e., cells containing membrane-bound organelles such as a nucleus). In biology, "symbiosis" is an intimate and protracted association of individuals of different species. Mitochondrial DNA (sometimes denoted as mtDNA), found in the mitochondria of all eukaryotes, is believed to evolve in parallel with nuclear DNA, but since sperm lose their mitochondria, it is inherited only in the maternal lineage in animals.
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EVOLUTION: A NEW COELACANTH DISCOVERY
The coelacanth is a primitive *teleost fish that apparently first appeared in the *Devonian Period, with the most recent fossil specimens dating from the *Cretaceous Period. The group was assumed to be extinct, but a living specimen was caught in 1938 in the mouth of the Chalumna River in South Africa and named Latimeria chalumnae. The 1938 specimen was 2 meters in length and weighed 40 kilograms. More specimens have since been caught, all off the coast of South Africa and near the island of Madagascar (Comoros archipelago) in the Indian Ocean.
The following points are made by M.V. Erdmann et al (Nature 1998 395:335):
1) The authors report the capture and observation of a live coelacanth specimen near the island of Manado Tua (north Sulawesi) in Indonesia on 30 July 1998. The Indonesian specimen was caught in a net at a depth of 100 to 150 meters. The specimen is 1.24 meters in length and weighs 29.2 kilograms, and the authors report it was observed by them live for more than 3 hours before the carcass was deep frozen and tissue samples collected for molecular analysis.
2) The authors suggest that interviews with the local fishermen, and the vast distance from the Comoros archipelago to Indonesia (approximately 10,000 kilometers), strongly support the idea that the Indonesian coelacanths (which are evidently well known in the region and called "raja laut" or King of the Sea) are part of an established north Sulawesi coelacanth population and not simply strays. Like its Indian Ocean counterpart, the Indonesian coelacanth was found in the vicinity of oceanic volcanic caves, its presumed habitat.
In a companion article concerning the M.V. Erdmann et al paper, P. Forey (Nature 1998 395:319) makes the following points:
1) Before the 1938 discovery, the date of the youngest coelacanth fossil recovered was approximately 80 million years ago. This raises the question of how the lineage survived for that time without leaving any trace in the fossil record.
2) The theory of relationships prevalent in the late 1930s held that the coelacanth was a direct descendant of Devonian fish-like ancestors of land-living vertebrates (tetrapods), but from modern evolutionary analysis it seems that coelacanths are more distantly related to land-living vertebrates, and that lungfishes are the closest living relatives to the tetrapods.
3) Significant in the coelacanth are paired fins that move in a manner unlike that seen in most fishes but in a manner identical to limb movement in vertebrates. There are also sensory structures in the coelacanth that are apparently precursors of structures responsible for hearing in air.
4) The author concludes: "The new discovery underlines how little we really know about the coelacanth in particular and oceanic life in general."
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Notes:
teleost: In general, this refers to any of the bony fish, the most advanced in terms of evolution and the largest group of fish. Besides the calcified internal skeleton, the most obvious uniform characteristic of the teleost fish is their tail, with upper and lower halves of about equal size, whereas in cartilaginous fish the tail has two lobes of unequal size. Almost all sport, commercial, and ornamental fish are teleosts.
Devonian Period: From approximately 400 million to 345 million years ago. Sometimes called the Age of the Sea, since more of the Earth was underwater than is now.
Cretaceous Period: The geological period ranging approximately from 146 million years ago to 65 million years ago.
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