Troglomorphism

Troglomorphism is the morphological adaptation of an animal to living in the constant darkness of caves, characterised by features such as loss of pigment, reduced eyesight or blindness, and frequently with attenuated bodies or appendages. The terms troglobitic, stygobitic, stygofauna, troglofauna, and hypogean or hypogeic, are often used for cave-dwelling organisms.

Troglomorphism occurs in molluscs, velvet worms, arachnids, myriapods, crustaceans, insects, fish, amphibians (notably cave salamanders) and reptiles. To date no mammals or birds have been found to live exclusively in caves. Pickerel frogs are classed as either trogloxenes, or possibly troglophiles. The first Troglobiont to be described was Leptodirus hochenwartii.

Morphology of Troglomorphism
Troglomorphic species must adapt to unique elements of subterranean life, like continual darkness, reduced season queues, and limited food availability. The reduction of characters like eyes and pigmentation is generally considered to be an evolutionary tradeoff in troglomorphic species. While these characters, which are no longer useful to them in continual darkness, begin to be selected against, improved secondary sensory structures are selected for. Many troglomorphs display impressive and exaggerated sensory elements, like greatly elongated antennae, that allow them to navigate in this unconventional setting. Additionally, as a result of poor resource availability, these species tend towards low rates of metabolism and activity, to maximize what little energy input they are able to achieve.

While general trends are maintained, troglomorphic species can be highly variable. While some species like the Mexican tetra trend towards eyelessness, there are still many that maintain their eyes even in darkness, or even still some that retain pigmentation that are not well understood. Additionally, there are traits like reduction of scales in some troglomorphic fish that have yet to be well explained.

Additionally, troglomorphism can vary within a species. In species like the Mexican tetra, some populations may retain their eyes, while others have varying stages of eye loss, and can interbreed with one another. Other species like the cave amphipod also display this relationship of surface and subterranean populations retaining a species relationship, adding to the complexity in understanding this unique evolutionary phenomenon.

Mechanisms of Troglomorphism
Changes in this troglomorphic morphology have been directly tied to changes in expression of key developmental genes, altering the expression of particularly vision associated genes entirely. In species like the Mexican tetra, expression of the pax6 gene which regulates many of the eye associated genes in development, is greatly suppressed by other genetic signals. A current theory holds that beneficial traits that have been selected for, also often come with negative associations for these genes, resulting in a double positive for cave dwellers that would otherwise be selected against in surface populations.

These genetic linkages may be a potential explanation for the loss of otherwise unrelated traits like scales, or the maintaining of pigment in some species. Some of these trait losses or gains may be due to these associations with genes that are actually selected for, rather than any evolutionary benefit to the organism. If being eyeless and scaleless are linked in the genome, pressure to become eyeless will result in scaleless organisms, even if that brings them little benefit- assuming that any detriment from losing scales does not outweigh the benefit of losing eyes. Alternatively, lacking linkages in the genome might explain why some species are able to adapt to cave life without the loss of traits like eyes and pigment.

A 2012 study by a team from the National University of Singapore found that reductive changes in freshwater cave crabs evolved at the same rate as constructive changes. This shows that both selection and evolution have a role in advancing reductive changes (e.g smaller eyes) and constructive changes (e.g larger claws), making troglomorphic adaptations subject to strong factors that affect an organism's morphology.

Caves as Evolutionary "Dead Ends"
One point of contention in the discussion of troglomorphism, is the ultimate evolutionary implications to adaptation to life in caves. Scientists have debated if adaptation to cave life will ultimately lead to evolutionary stagnation, or a point at which evolutionary change becomes minimal. Some literature has suggested that once species adapt to cave life, there is a limit to the diversification and adaptation that they can undergo. Genera like the whip spider genus Paracharon point to the ability for species to remain mostly the same as their ancestral state, by taking to cave life. Another example of this type of ancestral state outside of caves would be the infamous Coelacanth, which greatly resembles fossils of the same lineage.

This evolutionary break however, has also been suggested to instead act as an evolutionary time capsule, an advantage to the survival of species. Due to the relatively stable nature of caves, some species have been suggested to endure periods of climatic instability, like the Pleistocene, before readapting to surface life when conditions are favorable. This would suggest that caves are highly influential in the persistence of species, and the preservation of biodiversity. In fact, many of these lineages show similar rates of speciation and diversity even within these smaller habitats, as uniquely specialized colonists of another environmental niche, rather than an evolutionary trap.