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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 Troglobite, stygobitic, stygofauna, troglofauna, and Hypogeal or hypogeic, are often used for cave-dwelling organisms.
Troglomorphism occurs in , , , Cavefish, amphibians (notably ) and reptiles. To date no mammals or birds have been found to live exclusively in caves. are classed as either , or possibly . The first Troglobiont to be described was Leptodirus.
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.
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.
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.
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