Tuesday, November 22, 2016

Latimeria chalumnae "fish with legs from Indian Ocean" the mysterious living fossil reveals its secrets

In 1938, the world zoologists was shaken by a discovery without precedent: in the Indian Ocean was living creature amazing - a fish, no doubt, but one very strange fin provided with a kind of "paws", reminiscent of limbs terrestrial vertebrates.

 Mysterious animal has become one of the "stars" of the living world, where scientists have realized that they were dealing with a species surviving in a group of fish ancient considered missing ago 65 million years . And recently, the strange "fish with legs" back onto the world stage: geneticists have managed to decipher its genome and exciting source of new information about this "living fossil", about its relationship with other creatures and the evolution of the living world.

The discovery "fish with legs" in 1938 in response to a desire biologists old - to find the "missing link" between fish and tetrapods animals - four States - who colonized the terrestrial ago about 395 million years.

Even if further investigation questioned the new-found that fish were direct ancestor of tetrapods, discovery remains one extraordinary landmark in the history of zoology. Here, briefly, the story:

At December 23, 1938, Hendrik Goosen, captain of the fishing vessel Nerine, returned to port South African East London, after an expedition ocean fishing between the mouth of the river Chalumna and Ncera, on the coast west of the country . Captain customary when you have to catch fish most interesting to them and told a friend, Marjorie Courtenay-Latimer, who worked at the museum in East London. He called and this time, telling him that she had saved for a very special fish.


Marjorie Courtenay-Latimer was happy to receive the odd fish but although searched all the books they had available, failed to realize what species have to do. L has called one of her friends, chemistry professor James Leonard Brierley Smith, but it was left for Christmas vacation. Marjorie Courtenay-Latimer could not keep the whole fish in good condition, so it has entrusted to a taxidermist to be naturalized ( "stuffed"). 

After your holiday, Professor J. L. B. Smith immediately realized it had to do with something extraordinary: a representative group of ancient celacanţilor (Coelacanth) group considered extinct for 65 million years. Fish received scientific name of Latimeria chalumnae, after the name of Marjorie Courtenay-Latimer and the river Chalumna, and the news has gone around the world, putting them on fire biologists and paleontologists and fueling hopes of finding "missing link" in the chain of evolutionary what tie fish tetrapods.

Latimeria chalumnae photo: pinterest
A very special fish

After three quarters of a century of research, scientists have come to know something about Latimeria chalumnae; very little, yet enough to realize it's a very unusual creature.

Celacanţii now appeared approx. 400 million years ago and was thought to have disappeared at the end of the Cretaceous, now about 65 million years, in large extinction that took place then, that the dinosaurs disappeared. Indeed, the vast majority of species of this group have disappeared and is known only from fossil remains. But behold, two, at least, they have withstood time and catastrophes, surviving until today. In total, it has been described about 80 species of CELAC (including them and the two current).

Latimeria chalumnae is what is called in familiar language, a living fossil, and a newer term, "Lazarus taxon", named after the biblical character brought back to life. A Lazarus taxon is a species or taxonomic group (such as celacanţii) disappears at some point in the fossil record (and is therefore considered extinct), but then reappears after a long time. It is believed that these fish, celacanţii are extinct and yet they are still living, participating in biodiversity by two representatives known world.

Latimeria chalumnae fully deserves its fame it enjoys among biologists. It is a creature with many unusual features. For example, although he lives in the ocean, yea depths large, without leaving the surface, it is more akin to fish lung and tetrapods than fish actinopterigieni group of bony fish "ordinary" which constitutes 99% world fish fauna.


It has many unusual features of the skeleton, a heart made up differently than the other fish, a sort of vestigial lung, filled with fat, and other strange features that distinguish it from most of the fish world.

No its not like the eggs of other bony fish: almost all lay eggs "and what not" - a large number of eggs small size compared to adult fish. Instead, Latimeria chalumnae "factory" eggs the size of oranges, a huge fish (even a big fish like him, because can reach 1.8 meters in length). The species is ovoviviparity, that fertilized eggs develop in the mother's body - gestation is about. 1 year - coming out of the egg here, and the female, eventually eliminates hatched chicks ready ("give birth to live young" after the current expression).


And outside, the most striking feature are "paws" - muscular stalks that are attached to each of the fins and which give so strange appearance of "fish with legs".

But do not use these "legs" to walk on the ocean but swim or get carried currents, using their fins 8 for precise steering maneuvers. These fish live at depths of 100-500 meters; They are predators, feeding on other fish; day stay hidden in the cracks of rocks submarine or underwater caves and night out for food.

Latimeria chalumnae is considered an endangered species (like the Latimeria menadoensis), although they know so little about celacanţii lcururi living today, that the true status of the species is difficult to determine.


Before the scientific world to discover the fish and show interest in it, Latimeria chalumnae not have trouble living areas (Kenya, Tanzania, Mozambique, South Africa, Madagascar, Comoros). It was sometimes caught accidentally by local fishermen, but which threw him back into the water, do not consumed, because his flesh has an unpleasant taste and is somewhat toxic; It contains large amounts of fat, urea, esters of fatty acids and indigestible other substances that can cause diarrhea. But since biologists began marching after CELAC, fishermen do not take these fish, but looking to sell, a behavior that could lead to reduced populations of Latimeria chalumnae. Both this species and "sister" her Latimeria menadoensis, are today the subject of conservation programs aimed at maintaining balance populations. No longer need to stress how important it is to protect these species - "windows" distant past come to life on earth.

Genetics a living fossil

Recently, Latimeria chalumnae experienced a comeback to the forefront of research. An international group of scientists has succeeded in sequencing the genome of this creature, deciphering and "reading" the sequencing of genes that contain the genetic heritage of this animal and that could explain so some features of amazing and mysteries deeper evolution forms life on Earth.

Genome sequencing process itself was challenging in many respects. Chalumnae are rarely captured, threatened, so procuring tissue samples and extracting genetic material from them were difficult stages, not to mention the sequencing itself. But the effort, which involved experts from several countries and has united towards this common goal, and the international nature of this research project is one of its most valuable aspects, I think the scientists involved.

Since its discovery in 1938, biologists have wondered how survived Latimeria chalumnae until today, unchanged for many millions of years, and some have hypothesized that this fish is evolving unusually slowly, that its genes are "conservative" and undergoing changes at a pace slower than other species of creatures.

Latimeria chalumnae photo: commons.wikipedia.org

And indeed, analyzing genome species, scientists have confirmed this assumption:

"We found that, in general, genes [species Latimeria chalumnae] evolving significantly slower than with any fish and any vertebrate land that I studied," said Jessica Alföldi, the Broad Institute, one of the authors published in the journal Nature.

Genome sequencing has enabled researchers to investigate several issues that dated back a long time.


For example, celacanţii have certain characteristics reminiscent oddly specific than the animals living on land, including "paws" fins that resemble the limbs of tetrapods. Another strange group of fish present (called longfish, or long fish), and they have similar fin (picture below). It is likely that in one of these species of ancient fish "legs" to have evolved early amphibian tetrapods that came out of the water and stepped ashore, but until now, researchers had not been able determine which of the two groups - celacanţii or dipnoii - is the most promising candidate.

Now, in addition to whole genome sequenced (almost 3 billion letters - nitrogenous bases - the DNA of Latimeria chalumnae), researchers have studied the RNA CELAC both species compared with that of the longfish. This information allowed the comparison of homologous genes associated with the development and functioning of the brain, kidneys, liver, spleen and intestines from CELAC, lungfish and other 20 species of vertebrates. And the results showed that the genetic tetrapods are closer than fish longfish.

Therefore, the "missing link" between fish and tetrapods celacanţii not seem to be, they are not the direct ancestors of land vertebrates with four limbs.

However, celacanţii key pieces remain in the study process was essential that the conquest of land by vertebrate animals.

Even if dipnoii are closer to tetrapods land than celacanţii genome lungfish still remains a mystery: having 100 billion nitrogenous bases, is simply that too large for scientists to be able sequencing, assemble and analyze the means available now.

Instead, smaller genomes of African celacantului (comparable in size to the human one) suitable deciphering methods available today and provides valuable clues on the genetic changes that allowed tetrapods thrive on land.


Scientists have sought, on the one hand, to find out what genes have lost vertebrates when they took the life on land and on the other hand, what regulatory elements (parts of the genome that control where, when and to what extent are activated certain genes) have acquired.

And the findings were interesting as possible. Here are a few:

The sense of smell

Numerous regulatory changes have influenced genes involved in olfactory perception and detection of odors from the air. The scientists believe that when vertebrates had conquered land, they needed new ways to detect chemicals in this new environment.


Immunity

By comparing the genome with the genomes of terrestrial animals African celacantului, it was discovered a significant endorsement number of regulatory changes related to immune function, and scientists believe that these changes could be related to the body's response to new pathogens found in the terrestrial environment.


Evolutionary development
photo: rationalrevolution.net


Researchers have identified several key areas of the genome that could have been "co-opted" to control body tetrapods innovations, such as the formation of limbs and fingers or placental mammals. One of these areas, called HoxD contains a specific gene sequence that is common celacanţilor and tetrapods; it is likely that this sequence have been made by tetrapods, "in service" training and rear limbs.

Urea cycle

Fish body eliminates excess nitrogen from ammonia in water excretând; instead, terrestrial vertebrates (including humans) have a different mechanism of "management" of nitrogen: ammonia rapidly converted into urea, less toxic, through a succession of chemical reactions that constitute the urea cycle. The study compared the genomes, the CELAC and other vertebrates have been identified indications of evolution that led to the development of this mechanism, essential if living permanently outside the water: the researchers found that the most important genes involved in this cycle were tetrapods modified to allow them to solve the problem of excess nitrogen in the conditions of life on land.

But this is just the beginning discoveries. Celacanţilor genome could hold many other important clues to the evolution of tetrapods researching.


Much more great things to be learned about these fish; experts anticipated that future studies on immunity, physiology, respiration and other aspects of the biology of these creatures will lead us, finally, to a deeper understanding of the phenomenon of "great passage" from life in water at the shore - one of most fascinating episodes in the history of life on Earth.



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The above post is reprinted from materials provided by Descopera . Note: Materials may be edited for content and length.

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