Grotto Salamander

The grotto salamander's larvae live in surface streams and near cave entrances, where being able to see is still useful for finding food and avoiding predators, so they hatch with small but functional eyes. As the salamander undergoes metamorphosis and shifts to a permanently lightless life deep in caves, vision becomes worthless, and maintaining eyes and the brain circuitry to process sight would be a pointless energy cost. During metamorphosis the eye tissue degenerates, the lens breaks down, and the eyelids grow together and fuse shut over the shrunken eyes, so the transformed adult is effectively blind. This pattern, keeping eyes for a light-exposed juvenile stage and discarding them for a dark adult stage, is unusual and makes the species a favourite example for studying how and why animals lose complex traits.

The adult grotto salamander navigates and hunts using its other senses, particularly touch and smell, along with detection of vibrations and water currents. It moves slowly through wet cave passages and along the edges of cave streams, sensing small invertebrate prey such as springtails, isopods, and worms by direct contact and by chemical cues in the water and on surfaces. Because food is scarce in caves and the salamander does not need to chase fast-moving prey, a slow, thorough, contact-based foraging style works well, and a very low metabolic rate lets it survive long stretches between meals.

In many of the caves where the grotto salamander lives, there is no sunlight and therefore no plant growth to form the base of a food web, so the energy that sustains the cave ecosystem is imported from outside, and one of the most important sources is bat guano, the accumulated droppings of bats that roost in the cave. Fungi and bacteria break down the guano, small invertebrates feed on that, and the salamander feeds on the invertebrates. This makes the salamander indirectly dependent on healthy bat colonies, and threats to cave bats, such as the disease white-nose syndrome, can ripple down the food web and reduce the prey available to grotto salamanders.

It is one of several unrelated or distantly related salamanders around the world that have independently adapted to cave life by reducing or losing their eyes and pigment, a group that includes the Texas blind salamander of the United States and the olm of Europe. What sets the grotto salamander apart is that most of those other species are permanently aquatic and keep larval features such as external gills for life, whereas the grotto salamander completes metamorphosis, loses its gills, and becomes a terrestrial, lung-breathing adult, and its eyes are present and working in the larva before being lost at transformation rather than being poorly developed from the start.