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Troglomorphism is the morphological adaptation of an animal to living in the constant darkness of caves, characterized by features such as loss of pigment, reduced eyesight or blindness, and attenuated bodies or appendages. The terms troglobitic, stygobitic, stygofauna, troglofauna, and hypogean or hypogeic, are often used for cave-dwelling organisms.^[1]

Troglomorphism occurs in molluscs, velvet worms, arthropods, 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.^[2]

Troglomorphic species must adapt to unique elements of subterranean life, like continual darkness, reduced season queues, and limited food availability.^[3] The reduction of characteristics like eyes and pigmentation is generally considered to be an evolutionary tradeoff in troglomorphic species. While these characteristics, 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, due to limited resources, these species tend to have low metabolic and activity rates to maximize the little energy they can obtain.^[3][2]

While general trends are maintained, troglomorphic species can be highly variable. While some species, like the Mexican tetra, trend towards eyelessness, many still retain their eyes even in darkness, and some retain pigmentation whose function is not well understood. Additionally, traits such as the reduction of scales in some troglomorphic fish remain poorly 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.^[4] 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.^[5]

Changes in this troglomorphic morphology have been directly tied to changes in the expression of key developmental genes, altering the expression of particularly vision-associated genes entirely. In species such as the Mexican tetra, expression of the pax6 gene, which regulates many eye-associated genes during development, is strongly suppressed by other genetic signals. A current theory holds that beneficial traits often come with negative associations with the genes underlying them, resulting in a double positive for cave dwellers that would otherwise be selected against in surface populations.^[6][2]

These genetic linkages may explain the loss of otherwise unrelated traits, such as scales, or the maintenance of pigmentation 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.^[2] Alternatively, lacking linkages in the genome might explain why some species can 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 play roles in advancing reductive changes (e.g., smaller eyes) and constructive changes (e.g., larger claws), thereby subjecting troglomorphic adaptations to strong forces that shape an organism's morphology.^[7]

One point of contention in the discussion of troglomorphism is the ultimate evolutionary implications of adaptation to cave life. Scientists have debated whether adaptation to cave life will ultimately lead to evolutionary stagnation, or a point at which evolutionary change becomes minimal. Some literature suggests that once species adapt to cave life, there is a limit to the diversification and adaptation they can undergo.^[2] 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.^[8] Another example of this type of ancestral state outside of caves would be the famous Coelacanth, which greatly resembles fossils of the same lineage.^[9]

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 have endured periods of climatic instability, such as 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.^[10][11] 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.^[10]

• List of troglobites

• "The Olm and Other Troglobites"