Wednesday, 28 January 2009

Water monsters of Mexico

In Twilight World here at Bristol, can be seen a strange, newt like creature which visitors often walk straight past. This is a pity, because the axolotl is one of the strangest of amphibians, and is on the endangered species list.

Native to Lake Xochimilco, the axolotl (the name translates as “water dog” or “water monster”) is a member of a group of salamanders, the ambystomids or mole salamanders, which are native to North America. Several species are known, many with subspecies (which may be better treated as full species) and as a group the mole salamanders, like most salamanders, breed in water, laying eggs which hatch out into newt-like tadpoles. When they reach full size, which in some species can take 2 years, the tadpoles lose their external gills, start breathing air, and leave the water.

In the axolotl however, this does not take place. Instead the tadpole reached full sexual maturity and breeds while still in the larval state, a process called neoteny. This probable occurred because the climate around Lake Xochimilcho is quite severe, and it makes more sense for an amphibian to stay in the water all year.

Unfortunately, these days even the water is unsafe for the axolotl. Introduced Tilapia prey on them, and pollution and collection for food have also reduced their numbers. As a result, while they are well established in captivity, and have been since 1858 when they were first taken to Paris, in the wild they are endangered.

You might think that such a strange creature is unique, but in fact there are several similar forms of mole salamander known. The North American Tiger Salamander is known to produce occasional neotenous forms, especially in mountains or extreme environments, and these can still breed successfully with the metamorphosing form.

In addition, there are pure neotenous species known from Mexico. Anderson’s salamander comes from Laguna de Zacupa, and was collected in 1967 from a stream running into the lake. It appears to prefer moving water, rather than the still waters used by the Axolotl. Even more extreme is Taylor’s salamander from Laguna Alchichica, where it like deep water below 30m. Even stranger, Laguna Alchichica is quite brackish, with a salinity of 8.5g/l, making it quite a hostile environment for an amphibian.

One further point to note – it is not just amphibians which are neotenous. Many of the features which distinguish modern humans from other apes have been interpreted as neotenous – bare skin, a skull which looks at 90 degrees from the backbone instead of forwards along the axis of the spinal column, and a reduced extension of the facial bones. Adult humans look more like infants than for example adult gorillas look like babies, and human and gorilla infants are much more like each other than the adults of our two species are. So remember, a Mexican water monster may have more in common with you than you might think!

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