In the mid-1990’s school children found frogs with missing or extra legs while on a field trip to a pond in Minnesota. Following the event, other ponds were surveyed and similar malformations of the amphibians were found. Deformities are a normal state of nature and occur in about five percent of individuals naturally. However, these individuals are usually preyed upon by predators or do not live long after birth. The malformations found in Minnesota were in percentages far higher than the normal five percent. In time malformations were found to stretch from ocean to ocean in the United States and north into Canada. Many species of frogs are affected, including Pacific Tree Frogs and Leopard Frogs.
The most common malformations were extra limbs, including extra legs, feet, and digits. Some frogs had missing eyes (although one frog with a missing eye was found to have the eye inverted to the inside of its mouth!) or webbing connecting their lower leg to their upper leg in strange positions. Tadpoles, the larval form of amphibians, were seemingly not affected; just adult frogs showed malformations.
Hypotheses were quickly formed to suggest why the frogs were malformed. Leading causes included exposure to increased UV radiation and pesticide run-off. The latter seemed very feasible, given that many of the malformed frogs were found in agricultural areas, particularly in California, the Midwest of the United States, and southern Canada.
In time the culprit was found. The cause of such malformations was a small organism called a trematode or a fluke. Trematodes have complex lifecycles that includes several hosts and even a free-swimming life stage. Water birds, such as herons, feed on fish or frogs that are infected with trematodes. Trematodes lay their eggs, which are removed from the bird in their feces (which is usually dropped in the water, where the birds spend much of their time wading and feeding). Aquatic snails feed on the feces and take up the egg, where it hatches and grows. It later leaves the snail in a free-swimming organism that twists and turns in the water. This life stage burrows into a fish or frog and then begins the cycle anew.
For the Riberoia trematode, it was discovered to affect amphibians. Fish are common hosts. However, population sizes of the trematdoe had increased and frogs became additional hosts. The trematodes burrows into the area of the tadpole where the hind limbs will form, causing the cell (which would become a leg) to partially split (much like conjoined twins). Thus, multiple limbs form where one limb should have developed. Frogs that were missing limbs likely suffered cell damage from the trematode and the limb did not form correctly.
In time, it was learned that, in fact, pesticides did play a role. The run-off of these chemicals was providing extra nutrients in the water. This had led to algae blooms, which provided more food for the snails. This led to an increase in snail populations, and therefore, an increase in trematode populations. This is a classic example of how complex nature really is!
UV radiation has been shown in the laboratory also to cause deformities. However, such deformities are not what are most commonly seen in the wild and very high levels of radiations were needed to induce the deformities. It is likely that some frogs are deformed by this cause in some places, but it is not as widespread as those affected by the trematodes.
Malformed frogs are still being found in the wild, although research unfortunately has been reduced because of a lack of funding. It is suspected that malformed frogs are playing their role in the worldwide amphibian declines. Frogs with extra or missing limbs are easier prey for predators, including raccoons, birds, and some fish. If populations have a high incidence of malformations, predators wipe out the frogs and numbers are reduced.
Some Department of Natural Resources (DNR) offices continue to keep track of amphibian malformations. If you find a malformed frog, it is best to take pictures and report the sighting, along with information on the place, date, and time of your find, to your local DNR agent. This is particularly important if you see a number of deformed frogs at one location. To help further, consider making a contribution to an organization working to study and conserve amphibian species around the world. Much is unknown as to why these important and amazing critters are declining and time is very limited to save them.
Nature is very complex and it took time to understand what caused the amphibian malformations. Like many things, the media moved on to other topics and tragedies and many people likely forgot about the malformed frogs. It is important to follow up on such environmentally important topics and then share the information with friends and family members so we all can make an effort to preserve the planet and make the Earth a better place for all organisms to live.
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