Blind fish have a circadian rhythm of nearly 2 days

There is a blind fish living in a cave in Somalia but they all know what time they are in the day. However, their "day" is twice as long as our day.

Most animals have biological clocks in their bodies - or circadian rhythms. The biological clock lasts about 24 hours and is changed according to the daylight cycle of a day.

However, a group of international researchers have shown that blind fish living in caves have a circadian rhythm that lasts for nearly 2 days. This study was published in PloS Biology.

Fish that live in caves - also known as Phreatichthys andruzzii - have grown nearly 2 million years in isolated dark caves beneath the Somalia desert.

Millions of years of evolution in the dark have made Somalia blind fish
loss of eyes, ability to identify size and pigmentation

Professor Nick Foulkes from the Karlsruhe Institute of Technology in Germany said the particular fish was chosen "because it is a good example, isolated from the long cycle of night and day".

During evolution, this species has lost its eyes, its ability to distinguish colors and scales - unnecessary things in them in the pitch-black environment of underground cave systems.

However, it seems that the absence of day and night has caused a much deeper change in the circadian rhythm of this fish.

Light is detected primarily by the eyes, but most of the cells in the body have some reaction to light levels. In mammals like fish, peripheral detectors play a more important role.

This means, although the fish in the cave have lost their eyes during evolution, their bodies can still react to the change of light.

However, when comparing the biorhythic response of fish living in caves to " normal " zebrafish, blind fish does not react to the change of external light like zebrafish.

After 2 million years in the dark, fish in the cave do not need to react to the light and their biological clock has changed forever to reflect this.

However, the blind fish still has a biological clock and it is reset by other triggers, not by light.

Feeding fish at certain times shows that both zebrafish and cave fish react by the biological rhythm reset.

In addition, when the fish cave was re-established the biological clock in the natural rhythm, the researchers discovered their " 47 days" long.