|MadSci Network: Zoology|
Dear Linda: Thank you for your important question. In 1995 a group of students in Minnesota found a series of deformed frogs. This discovery worried scientists and the public alike, as frogs are considered to indicate the overall health of an ecosystem. Scientists use frogs like coal miners of the last century used canaries: when the canary died, the coal miners knew that the air in the mine was bad. Likewise, when a series of frogs are found to be deformed, scientists are worried that something is wrong in the environment. Nobody knows for sure why the frogs are becoming deformed, but there are four theories which I will tell you about. The theories are: parasitic infection, chemical contamination of water, UV irradiation and predation. There are two good websites you should check out on this topic, from which I drew most of my information. They are: Hartwick College Deformed Amphibian Research and Minnesota Pollution Control Agency
A scientist named Pieter Johnson has found that a type of parasite known as a trematode can cause all of the malformations seen in other types of frogs in Pacific Coast treefrogs (Johnson PTJ, Lunde KB, Ritchie EG, Launer AE. 1999. "The effect of trematode infection on amphibian limb development and survivorship." Science 284:802-804). Trematodes are small flatworms that can use frogs for one stage of their life cycle. The trematode larva, known as cercariae, can infect the developing limbs of frogs and cause the frogs to have multiple hind limbs. The effect of the trematodes appears to be mostly mechanical, as the Hartwick College group has implanted microbeads into the limbs of developing tadpoles and seen the same effect. The Hartwick group feels strongly that parasitic infection is the explanation for the deformed frogs, but the Minnesota Pollution Control Agency has found no correlation between parasitic infection and malformation in the wild. In other words, the trematodes can cause the malformations seen in the wild, but malformed frogs in the wild do not always have a trematode infection.
Chemical contamination of water
A second bypothesis is that there are chemicals in the water that are causing the frogs to develop abnormally. It is known that chemicals such as retinoids can alter embryonic development by altering gene expression. A possible source of retinoids in water is methoprene, an insecticide used to control mosquitoes and fleas. Retinoids can cause limb duplications, but they are not exactly like the ones seen in the wild. Due to this reason, the Hartwick group discounts the chemical contamination theory. The Minnesota Pollution Control Agency, on the other hand, has found that water taken from sites that have malformed frogs can cause malformations of developing frogs in the lab. This water has no parasites in it, so a non- retinoid chemical may be having the effect, and both the chemical contamination theory and the parasitic theory may be valid.
It is well known that UV irradiation can damage genetic material, leading to mutations. It is also known that extreme levels of UV irradiation can kill developing embryos, and UV irradiation in the lab can cause some frogs to have no limbs. It is unknown, however, what natural levels of UV irradiation do to developing frogs. UV irradiation can play a role in two indirect ways, however. UV irradiation can cause the breakdown of chemicals into potentially toxic components, causing harm to developing frogs, and UV irradiation can suppress the immune system, potentially making developing frogs more susceptible to parasitic infection or chemical insult.
Finally, some scientists believe that predation can explain why some frogs are missing limbs. The Hartwick group has observed fish attacking tadpoles and eating limbs in the lab, so they believe this to be the cause of most of the frogs observed without limbs in the wild. Predation, however, cannot explain the existence of multilegged frogs.
Nobody knows for sure why frogs are turning up deformed. The theories I've related above give us a hint why this might be happening, but more research needs to be done. It may also be the case that two or three of the factors listed above could be working together to cause frog malformations. Another question that is unresolved is: why now? What has changed in the frogs' environment to increase the number of malformed frogs? At this point, we don't know, but many scientists are busy trying to figure it out.
I hope this helps with your question. Take care- Ingrid Dodge, Mad Scientist
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