There’s a cave-dwelling fish that doesn’t follow the light of the sun, and a reindeer that changes eye colour to find lichen.

Via Tumblr, the Wikipedia article for the cave-dwelling fish Garra andruzzii contains a fascinating fact:

This fish is the first animal discovered that does not adjust its biological clock with the light of the sun. It has an unusual internal clock, which measures the passage of time with an extremely long period (up to 47 hours). It is also completely blind to all light stimuli.

The phrase “first animal discovered” suggests that there are other animals with a biological clock that isn’t based on sunlight – that includes other cave-dwelling creatures, but it also includes the Arctic reindeer, who spend summer in perpetual light.

The eyes don’t have it

The references point to an Italian journal article. I couldn’t find a copy of the article text, but a summary page explains (via Google Translate) that the blind eyes and blind clocks aren’t related:

La perdita degli occhi non è tuttavia responsabile della cecità dell’orologio stesso. Le responsabili del fenotipo cieco dell’orologio in questo pesce sotterraneo sono le mutazioni a carico di due fotorecettori ampiamente espressi (le opsine).

The loss of eyes is not the cause of the clock’s own “blindness.” The factors responsible for the clock’s blind phenotype in this subterranean fish are mutations affecting two widely expressed photoreceptors (opsins).

It makes sense to me that lacking eyes doesn’t mean a creature is disconnected from a 24-hour clock. Plants don’t have eyes, but they do have internal rhythms. They’re still photosensitive and they adapt their processes to a 24-hour cycle.

I was also reminded of olms. When I visited Slovenia in 2023, I visited a vivarium in Postojna where they’re housed. These salamander-like creatures are adapted to complete darkness and they don’t have eyes, but there was no mention of an atypical circadiam rhythm. (A casual search suggests that they don’t have any daily rhythm, rather than one of unusual length.)

Eyes of gold and eyes of blue

At the other end of the optical spectrum, the article about reindeer circadian rhythms contains another remarkable fact:

In winter, food is scarce and […] the reindeer’s golden eyes turn bright blue. “If the colour of light in the environment is predominantly blue, it makes sense for the eye to accentuate the blue colour to ensure that the reindeer’s photoreceptors maximise these wavelengths,” explains Nathaniel Dominy, an evolutionary biologist at Dartmouth University and first author of a study published in the review i-Perception.

The article explains that certain lichens emit ultraviolet light, and turning their eyes blue allows reindeer to pick out the lichen from the snow.

Eyes are something we all take for granted, but if they didn’t exist, you’d only find them in the weirdest of sci-fi novels.