Fish Can Tell The Time

Experiments have revealed that fishes have a well-developed sense of time and soon learn to gather at the appropriate place if fed at the same time each day. Automatic feeders are often a good example of this and will often have a queue waiting before each feed. As they soon learn that the approach of their owner often means food is coming at set times, most inhabitants are often seen rushing to the water's surface to await their diner!

Studies at the University of Plymouth have proven how fish can learn to tell time. One such experiment involved Goldfish (Carassius auratus) that were placed in an aquarium, in which they were fed only when they pressed a lever. The fish rapidly learned that pressing the lever produced a food reward. Once the fish were fully 'trained', researchers set up the lever to work for just one hour each day. The fish then soon became aware of this and learned to press the lever at the same time every day. The activity of the fish around the lever increased enormously just before the set hour when their food was dispensed. If no food came out, they stopped pressing the lever when the hour was up. This demonstrates that the goldfish not only had an awareness of time but also remembered their prior experience and knew that there was no point to keep pressing the lever!

Internal clocks, known as circadian rhythms, also help fish to adapt their daily activities to the cycle of day and night. These clocks do not always follow a precise 24-hour schedule, so to help keep synchronised with the natural world, they get reset on a daily basis by signals such as daylight, or more appropriately in an aquarium, when the lights get turned on & off. However, one exception to this phenomenon is the Blind Cave Fish (Astyanax mexicanus). This is a species that has spent millions of years underground isolated from the evidence of day and night, yet has still been proven to have a working biological clock, albeit an unusually distorted one.