Friday, October 7, 2011

Gastric Brooding Frog (Rheobatrachus silus)

Another victim of the amphibian disaster was a fascinating little frog from Australia that was only discovered in 1973, yet by 1981, it had vanished without a trace. 

The gastric-brooding frog was a small species; females were around 5 cm long, while males were smaller, at approximately 4 cm. It lived in forest streams and rocky pools, and for much of the time, it would hide beneath rocks on the bed of these water bodies, but when it left these rocky refuges and moved out into the fast-?owing water, it showed itself to be a very accomplished swimmer. Its powerful hind-limbs terminated in feet that were almost completely webbed, and these were used with good effect to propel the frog through the water. The big, protruding eyes of this frog were positioned well on top of its head, and this allowed it to survey what was going on in the air and on land, while its body was out of sight beneath the water. Although it was very well adapted to an aquatic existence, the gastric brooding frog would often leave the water to hunt or to seek out a new stretch of stream. 

Its favored prey were small invertebrates, such as insects, but unlike many types of frog, the gastric brooder did not have a long, sticky tongue to secure its prey; instead, it waited until its food was within range and simply lunged at it with an open mouth. With its prey partially trapped, the frog would shove the rest of the victim’s body into its mouth using its forelimbs. Even though this frog was a capable predator, it was very small, and it was a tasty morsel for a range of predators. Herons and eels were partial to this amphibian, but it did have a useful defense if it was grabbed by one of these animals: mucus. All amphibians have skin glands that produce mucus to keep their skin moist as well as for protection. The gastric brooder could produce lots of very slippery mucus, which made it very hard for a predator to get a good grip. 

In most respects, the gastric-brooding frog was like most other frogs, but what set it apart was the way it reproduced. Mating was never observed in this species, but it is known that the female laid between 26 and 40 eggs and that these were then fertilized by the male. Again, this is the normal amphibian approach when it comes to breeding as fertilization in all these animals is external. It is not completely clear what happened next as it was never actually seen, but at some point after the eggs were fertilized, either when they were still eggs or when they had hatched into tiny tadpoles, the female swallowed as many of her offspring as she could. To the uninitiated, this may have looked like maternal cannibalism, but in fact, this was part of this frog’s unique reproductive strategy. The eggs or small tadpoles slipped down their mother’s throat and ended up in her stomach, and this is where they grew. In all animals, the stomach is the organ that plays a major role in digestion. Cells in the lining of the stomach produce very strong acid that breaks down food into its component fats, proteins, and carbohydrates so that enzymes can begin their digestive work. This harsh, acidic environment is hardly ideal for developing offspring, but over millions of years, these frogs evolved a couple of tricks that turned the stomach into a snug little capsule for their developing brood. It seems that the eggs and the tadpoles of this frog secreted a type of chemical known as a prostaglandin. This chemical blocked the cells of the stomach lining from secreting acid, and the walls of the stomach thinned. The young frogs turned the stomach into a cozy crèche. After six to seven weeks of developing in their mother’s alimentary canal, 6 to 25 tiny but fully developed frog lets clambered out of their mother’s mouth to begin their own life in the big wide world. Throughout this whole brooding period, with her stomach effectively shut down, the female frog was unable to feed, so after the departure of her young, her first consideration was probably finding some food. 

In fewer than 10 years after its discovery, the gastric-brooding frog disappeared. Extensive searches of the mountain streams in the early 1980s failed to turn up a single specimen. When the species was first discovered in 1973, it was considered to be quite common, but by 1981, not a single specimen was to be found—it was as though it had been spirited away. Like the golden toad of Costa Rica, exactly what happened to the gastric-brooding frog is unknown, but there have been several explanations, some of which are more plausible than others. Pollution of the mountain streams by logging companies and gold panners has been cited as a reason for the disappearance of this species, but tests on the stream water failed to show any significant pollution. Habitat destruction has also been mentioned, but the areas where this frog was found have been pretty well protected. With pollution and habitat destruction largely ruled out, we arrive at the specter of disease. The chytrid fungus has caused the deaths of amphibians all over the world. The fungus latches on to the body of an amphibian and takes root in its skin. The fungus forms cysts within the deeper layers of the skin and breaks down keratin, a protein in the cuticle of many vertebrates, including adult frogs and toads. The skin of an amphibian infected with this fungus begins to break down, and in severe cases, the disease can eat right into the deeper tissues. In these cases, digits, and even limbs, can be eaten away. This in itself is not fatal, but the ability of the skin to transport gases and prevent the entry of other harmful micro-organisms is probably impaired, and the victim dies a slow and probably very painful death.

Source: Wikipedia


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