Subterranean environments are characterized by distinct ecological conditions, the most important of which is the absence of light. Sunlight can penetrate only the entrance areas, creating a gradient towards the deepest zone, where the microclimate is very stable. Darkness prevents colonization by most primary producers, creating a peculiar food web that, with a few exceptions (e.g. systems based on chemoautotrophy), relies on the input of organic matter from the surface. Consequently, food in most subterranean habitats is scarce and only occasionally available.
The animals that live in these environments show peculiar adaptations. Among the most striking features is the loss of eyes and pigmentation, which is almost ubiquitous in various groups of cave animals. The extent of eye and pigment loss is regularly studied and even used to ecologically categorize species into strictly or facultatively subterranean. These two traits have been capturing the interest of researchers since Charles Darwin, and have recently been the subject of heated debates about the evolutionary mechanisms of their origin. Other common morphological changes include the enhancement of non-visual sensory systems and associated structures, such as the elongation of chemoreceptor- or mechanoreceptor-bearing appendages in arthropods, or the increase in the number of neuromasts and taste buds in fish. Physiological adaptations include lower metabolic rate, increased fat deposition, increased resistance to starvation, reduced number of offspring, enlarged eggs, and altered longevity. Behavioral adaptations include changes in activity patterns, feeding behavior, and social behavior, among others. Compared to morphological traits, these adaptations are less well known because the study of physiology and behavior requires live animals that are difficult to observe in their natural environment or, when brought into the laboratory, the survival (and possibly behavior) of the species is usually strongly influenced by the artificial conditions. As a result, most of our knowledge of these adaptations comes from the study of only a handful of species, and many of the commonly cited subterranean adaptations have been demonstrated in only a few animals (often vertebrates).
Another major gap in our knowledge is the mechanistic understanding of the evolution of cave adaptations at the molecular level. Few molecular mechanisms have been discovered to date, and studies in both model and non-model species can provide important information and open new frontiers of research. Therefore, this Research Topic aims to fill in the gaps regarding the extent of occurrence as well as the molecular and evolutionary mechanisms behind different adaptations in diverse groups of cave-dwelling animals. We would also like to stimulate studies on lesser-known traits such as behavior and physiology. In addition, we would like to emphasize the importance of evaluating adaptations in facultative cave-dwelling species, as they may exhibit early-stage changes that are no longer apparent or are enhanced in evolved cave species. We also invite reviews that summarize current knowledge on the occurrence of different adaptations in a given species or animal group or, conversely, the occurrence of a given adaptation in different groups of cave-dwelling animals.
Subterranean environments are characterized by distinct ecological conditions, the most important of which is the absence of light. Sunlight can penetrate only the entrance areas, creating a gradient towards the deepest zone, where the microclimate is very stable. Darkness prevents colonization by most primary producers, creating a peculiar food web that, with a few exceptions (e.g. systems based on chemoautotrophy), relies on the input of organic matter from the surface. Consequently, food in most subterranean habitats is scarce and only occasionally available.
The animals that live in these environments show peculiar adaptations. Among the most striking features is the loss of eyes and pigmentation, which is almost ubiquitous in various groups of cave animals. The extent of eye and pigment loss is regularly studied and even used to ecologically categorize species into strictly or facultatively subterranean. These two traits have been capturing the interest of researchers since Charles Darwin, and have recently been the subject of heated debates about the evolutionary mechanisms of their origin. Other common morphological changes include the enhancement of non-visual sensory systems and associated structures, such as the elongation of chemoreceptor- or mechanoreceptor-bearing appendages in arthropods, or the increase in the number of neuromasts and taste buds in fish. Physiological adaptations include lower metabolic rate, increased fat deposition, increased resistance to starvation, reduced number of offspring, enlarged eggs, and altered longevity. Behavioral adaptations include changes in activity patterns, feeding behavior, and social behavior, among others. Compared to morphological traits, these adaptations are less well known because the study of physiology and behavior requires live animals that are difficult to observe in their natural environment or, when brought into the laboratory, the survival (and possibly behavior) of the species is usually strongly influenced by the artificial conditions. As a result, most of our knowledge of these adaptations comes from the study of only a handful of species, and many of the commonly cited subterranean adaptations have been demonstrated in only a few animals (often vertebrates).
Another major gap in our knowledge is the mechanistic understanding of the evolution of cave adaptations at the molecular level. Few molecular mechanisms have been discovered to date, and studies in both model and non-model species can provide important information and open new frontiers of research. Therefore, this Research Topic aims to fill in the gaps regarding the extent of occurrence as well as the molecular and evolutionary mechanisms behind different adaptations in diverse groups of cave-dwelling animals. We would also like to stimulate studies on lesser-known traits such as behavior and physiology. In addition, we would like to emphasize the importance of evaluating adaptations in facultative cave-dwelling species, as they may exhibit early-stage changes that are no longer apparent or are enhanced in evolved cave species. We also invite reviews that summarize current knowledge on the occurrence of different adaptations in a given species or animal group or, conversely, the occurrence of a given adaptation in different groups of cave-dwelling animals.