Troglobites are small cave-dwelling animals that have adapted to their dark surroundings. Troglobite species include spiders, insects, fish and others. They live permanently underground and cannot survive outside the cave environment. Troglobite adaptations and characteristics include a heightened sense of hearing, touch and smell.[1] Loss of under-used senses is apparent in the lack of pigmentation as well as eyesight in most troglobites. Troglobite insects also exhibit a lack of wings and longer appendages.
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Troglophiles, trogloxenes and troglobites are often confused with one another. A troglophile is a cave-dwelling animal that may complete its life cycle in a cave, but can also survive in above ground habitats.[1] Troglophiles usually maintain some of their senses such as partial pigmentation and are usually only partially blind. A trogloxene is an animal that uses caves for shelter but does not complete its life cycle in them; for example, bats. A troglobite is different in that it cannot survive outside its cave environment. Troglobites may descend from troglophiles over time, through adaptation.[1] Stygophiles, stygoxenes and stygobites can also be confused. Stygophiles are aquatic troglophiles that are partial to caves; they live in them, but are not restricted to them. Stygoxenes are aquatic trogloxenes. They visit caves but do not complete their life cycle in them. Stygobites are aquatic troglobites that complete their entire life in the cave environment.
Troglobites usually live in moderate cave regions.[2] The overall climates of these caves do not significantly change throughout the year. Humidity in such caves is generally high ranging from 95 to 100 percent; evaporation rates are low. Terrestrial troglobites are seldom found in tropical caves. Conversely, aquatic troglobites, or stygobites, are more commonly found in caves with tropical and subtropical climates.[3]
The cave ecosystem in which troglobites reside can be divided into four zones: entrance, twilight, transition and deep cave.[4] The entrance zone is where the surface and underground environments meet. Light becomes scarcer in the twilight zone. The transition zone is almost completely dark; however some outside environmental effects can still be felt. Finally the deep cave zone is completely dark, relatively stable, and exhibits no evaporation. Troglobites are found in the deep cave zone.
Troglobites have adapted to the limited food supply and are extremely energy efficient. Food is found from "twigs, leaves, bacteria and epigean animals (including zooplankton)."[5] Food is also found from trogloxene carcasses, egg deposits and feces. Bat guano is another source of food for troglobites.[5] Stygobites feed on plankton, bacteria and plants found in streams.
Troglobites often have very low metabolisms to survive in a low-oxygen environment where food is scarce. Because of their low metabolisms and sedentary lifestyles, troglobites are able to live longer than other species. The crayfish Orconectes australis of Shelta Cave in Alabama illustrates this point, as it may reproduce at 100 years, and live to 175.[6]
Reproduction varies depending on troglobitic species. Reproduction in troglobites is infrequent.[2]
Troglobites have evolved in isolation.[6] Stratigraphic barriers, such as rock walls and layers have prevented the dispersal of these animals; consequently species often consist of just a handful of individuals in one cave, or one room of one cave. Fluvial barriers, such as rivers and streams prevent terrestrial troglobites from spreading.[2]
Troglobitic species include representatives of many animal groups, including turbellarians, gastropods, millipedes, spiders, pseudoscorpions, harvestmen, isopods, amphipods, decapods, collembolans, diplurans, beetles, fishes and salamanders.[7]
Troglobites can be found worldwide.[3] Turbellarian species can be found in the United States, Europe and Japan.[7] Troglobitic gastropods are endemic to the U.S and in Europe; they are mostly concentrated in the northeastern Mediterranean regions. Stygobite gastropods are found in the U.S, Europe and Japan.[7] Troglobite scorpions are mostly found in Mexican caves.[7] Troglobitic spiders are found more widespread in the U.S., Europe, and Japan.[7] However, they are also found in Mexico, Congo, Cuba and the Philippines. Terrestrial troglobite gastropods are found not only in Europe and the U.S but also in Cuba, Mexico, Burma, Congo and Venezuela. Aquatic troglobite isopods can be found in similar areas and also in Madagascar. Troglobite amphipods found in similar areas as isopods but can also be found in Africa, Australia and Brazil. Troglobite collumbolans are found in the U.S, Europe, Japan, Mexico and Cuba. Troglobite salamanders are found in Europe and the U.S.
Roughly 7,700 species of troglobite are known to date.[6] Approximately 90% of caves worldwide remain undiscovered due to lack of visible entrances. Consequently, many troglobite species that may be living in these caves have yet to be discovered.
Carabid beetles of the subfamily Trechinae are the most common type of terrestrial troglobites.[7] These particular troglobitic beetles range from 5 to 7 mm (0.20 to 0.28 in).[2]
In general, troglobite beetles are predators and feed on other troglobites rather than bacteria, twigs and guano. They are often found in wet places, near streams. Troglobite beetles have reduced eyes and wings; however, some species have neither. They are not typically found in tropical caves.[2] Troglobitic millipedes however, are the most common terrestrial troglobite found in tropical caves. Troglobite spiders, like beetles, are not well represented in tropical caves. Terrestrial troglobites are almost completely restricted to the deep cave zone where there is neither light nor evaporation but a stable environment.[3]
Francis G. Howarth hypothesized on other possible terrestrial troglobite adaptations to cave environment, postulating that "terrestrial troglobites have lost many of the water conservation mechanisms of surface relatives, and more nearly resemble permanently aquatic arthropods in water balance mechanisms, including cuticular permeability."[4] Troglobites thrive in a humid environment and when a "chamber is too dry ... animals display either agitated or comatose behavior".[4] This may be part of the reason terrestrial troglobites are not typically found in tropical cave environments. Tropical caves are warmer and consequently have a higher level of evaporation due to higher temperatures.
Aquatic troglobites are often called stygobites. Stygobites are generally more wide-ranging geographically than troglobites.[8] In general, stygobite species occur more often than troglobite species in counties across the U.S. Florida and Texas in particular contain more stygobitic species in relation to troglobitic species.[8] This may be due to the fact that troglobites and stygobites have different extinction risks.
Stygobites generally feed on plants, algae and bacteria. Species include aquatic gastropods, crayfish and fish among others.
Cave biology can be dated back to the late 18th century, when one of the first discovered aquatic troglobites, the Proteus Salamander, more commonly referred to as an "olm", was discovered.[6] These blind white salamanders were thought to be baby dragons by the locals of the time. They are the largest troglobites known to date and can measure up to a foot long.
In more recent times, more troglobite species are being identified. In Sequoia and Kings Canyon National Parks, in the Sierra Nevada mountains of California, scientists recently discovered 255 new caves, and 30 undescribed invertebrate species – "an extraordinary number for such a small area".[6]
Floodwaters can be detrimental to troglobite species. Consequently, stygobites can be swept out of the cave. Floods accompany a drop in temperature and a slight raise in pH. Troglobites tend to be temperamental and these conditions are not easily sustainable by them.[8] Extreme winter temperatures are also intolerable. Birds and bats in caves prey on troglobites. Troglobites also compete with each other for survival.
Humans also pose a threat to troglobites. Pesticides and sewage from cities poison troglobite communities.[6] Scientific cave explorations can also harm these creatures.