Page "Amphibian" Paragraph 80

At hatching, a typical salamander larva has eyes without lids, teeth in both upper and lower jaws, three pairs of feathery external gills and a long, somewhat laterally flattened body and tail with dorsal and ventral fins.

The forelimbs may be partially developed and the hind limbs are rudimentary in pond-living species but may be rather more developed in species that reproduce in moving water.

Pond-type larvae often have a pair of balancers, rod-like structures on either side of the head which perhaps prevent the gills from getting clogged up with bottom sediment.

Some members of the genera Ambystoma and Dicamptodon have larvae that never fully develop into the adult form but this varies with species and with populations.

The northwestern salamander ( Ambystoma gracile ) is one of these and, depending on environmental factors, either remains permanently in the larval state, a condition known as neoteny, or transforms into an adult.

Neoteny occurs when the animal's growth rate is very low.

This is usually because of adverse conditions such as low water temperatures which may inhibit the production of thyroxine or prevent the larva from reacting to the hormone in the normal way.

Other factors that may inhibit metamorphosis include lack of food, lack of trace elements and competition from conspecifics.

The tiger salamander ( Ambystoma tigrinum ) also sometimes behaves in this way and may grow particularly large in the process.

The adult tiger salamander is terrestrial but the larva is aquatic and able to breed while still in the larval state.

When conditions are particularly inhospitable on land, larval breeding may allow continuation of a population that would otherwise die out.

There are fifteen species of obligate neotenic salamanders including Necturus, Proteus and Amphiuma, and many examples of facultative ones that adopt this strategy under appropriate environmental circumstances.