Timed feeding synchronizes circadian rhythm in vertical swimming activity in cave loach,Nemacheilus evezardi

Cavefishes in Chronobiological Research: A Narrative Review

Cavefish are vertebrates living in extreme subterranean environments with no light, temperature changes, and limited food. Circadian rhythms in these fish are suppressed in natural habitats. However, they can be found in artificial light-dark cycles and other zeitgebers. The molecular circadian clock has its peculiarities in cavefish. In Astyanax mexicanus, the core clock mechanism is tonically repressed in the caves due to the overactivation of the light input pathway. A lack of functional light input pathway but rather the entrainment of circadian genes' expression by scheduled feeding were revealed in more ancient Phreatichthys andruzzii. Different evolutionarily determined irregularities in the functioning of molecular circadian oscillators can be expected in other cavefish. The unique property of some species is the existence of surface and cave forms. Along with the ease of maintenance and breeding, it made cavefish a promising model for chronobiological studies. At the same time, a divergence of the circadian system between cavefish populations requires the strain of origin to be indicated in further research.

Characterization and comparison of activity profiles exhibited by the cave and surface morphotypes of the blind Mexican tetra, Astyanax mexicanus

Departure from normal circadian rhythmicity and exposure to atypical lighting cues has been shown to adversely affect human health and wellness in a variety of ways. In contrast, adaptation to extreme environments has led many species to alter or even entirely abandon their reliance upon cyclic environmental inputs, principally daily cycles of light and darkness. The extreme darkness, stability and isolation of cave ecosystems has made cave-adapted species particularly attractive systems in which to study the consequences of life without light and the strategies that allow species to survive and even thrive in such environments. In order to further explore these questions, we have assessed the rhythmicity of locomotion in the blind Mexican tetra, Astyanax mexicanus, under controlled laboratory conditions. Using high-resolution video tracking assays, we characterized patterns in locomotor activity and spatial tank usage for members of the surface and Pachón cave populations. Here we demonstrate that cavefish have a higher overall level of activity and use the space within the trial tank differently than surface fish. Further, Pachón cavefish show circadian rhythmicity in both activity and spatial tank usage under a 12:12 light/dark cycle. We provide further evidence that these cavefish retain a weakly light-entrainable, endogenous circadian oscillator with limited capability to sustain rhythms in activity, but not spatial tank usage, in the absence of photic cues. Finally, we demonstrate a putative behavioral "masking effect" contributing to behavioral rhythms and provide evidence that exposure to constant darkness during development may alter behavioral patterns later in life.

Synchronized Circadian Bioluminescence in Cave-Dwelling Arachnocampa tasmaniensis (Glowworms)

Larvae of the genus Arachnocampa, known as glowworms, are bioluminescent predatory insects that use light to attract prey. One species, Arachnocampa flava, is known to possess true circadian regulation of bioluminescence: light:dark cycles entrain the rhythm of nocturnal glowing. Given the absence of natural light as a cue in caves, we addressed the question of whether cave populations of Arachnocampa tasmaniensis, a species known to inhabit caves as well as epigean environments, are rhythmic. We found that the major dark-zone cave populations of A. tasmaniensis maintain a high-amplitude 24-hour rhythm of bioluminescence, with the acrophase during external daylight hours. Populations of A. tasmaniensis in caves many kilometers apart show similar, but not exactly the same, timing of the acrophase. Systematic investigation of colonies in the dark zone of a single cave showed that some smaller colonies distant to the main ceiling colony, also in the dark zone, glow in antiphase. Periodic monitoring of a single colony over several years showed that the acrophase shifted from nocturnal to diurnal some time between October 2008 and January 2009. Prey availability was investigated as a possible zeitgeber. The acrophase of prey availability, as measured by light trapping, and the acrophase of bioluminescence do not precisely match, occurring 3 hours apart. Using in-cave artificial light exposure, we show that after LD cycles, cave larvae become entrained to bioluminesce during the foregoing photophase. In contrast, epigean larvae exposed to artificial LD cycles after a period of DD become entrained to bioluminesce during the foregoing scotophase. One explanation is that individuals within colonies in the dark zone synchronize their bioluminescence rhythms through detection and matching of each other ’s bioluminescence.