Living things interpret the world around them by tuning in to the planet’s natural rhythms, from a honeybee seeking pollen to a killer whale below the waves
Words Amy Grisdale
Crabs keep tempo with the tides
Most coastal areas experience two tidal cycles in a 24-hour period, and the animals that live on the waterfront synchronise with the 12-hourly ebb and flow in order to survive. Fiddler crabs emerge from their burrows to forage, fight and socialise when the water is at its lowest, while shore crabs prefer to come out during high tide to hide from predators under the waves.
Nerve tissue in a crab’s eyestalks called the medulla terminalis gathers information about the peaks and falls in the ocean’s journey up the beach. The eye’s activity is kick-started by an internal pacemaker in the brain that essentially works like an alarm clock that chirps when the tide is on the move. This phenomenon is still poorly understood, but laboratory experiments have demonstrated that wild-captured crabs maintain their tidal rhythms hundreds of miles away from their home beach.
CIRCADIAN RHYTHMS
The word ‘circadian’ is a blend of two Latin terms, circa and diem, which mean ‘approximately’ and ‘day’ respectively. Circadian rhythms are the 24-hour daily cycles that most organisms experience. Generated internally, they are regulated by sunlight and other factors in the environment.
Changing with the tides
The lunar cycle causes fluctuations in the height of the tide that crabs have to keep up with
Cavefish clocks are slow in the absence of light
Because the Sun rises every morning, most animals experience 24-hour days. However, those that sequester themselves deep underground have lost touch with the solar cycle. Somalian cavefish have been in the dark for so long they no longer have eyes and have developed a daily rhythm that lasts around 47 hours.
Two eye proteins mutated during the evolution of this species while the eyes were being phased out. These photoreceptive opsins are responsible for the body clock’s continued presence even though the eye no longer exists. The proteins have changed so much over millions of years that they don’t respond to light any more but cause the brain to experience a very long day in the dark.
Behind the times
Scientists have proven that the human internal clock can slow down through extreme experiments
In 1972, French geologist Michel Siffre spent more than 60 days underground to research rocks but ended up discovering something we didn’t know about how humans keep time.
He chose to live in the dark, sleeping and eating only when his body told him to. Each time he contacted his team on the surface he attempted to count out 120 seconds. Rather than taking just two minutes, it took five. Siffre planned to stay in the cave for two months from 16 July until 14 September. He had to be informed by his cre
