Navigating the Reef: How Fish Become Familiar With Their Territory
Although a reef may look to us like a huge continuous environment, the reality for fish is much more complex. Their habitat is a patchwork of hiding places, feeding sites, and fiercely defended territories. Being able to orientate within the environment is therefore a crucial skill for fishes. It is vital to be familiar with the environment -- to know where food or danger may be found, where might be a good refuge, or where your territory is. With so much at stake, it is not surprising that coral reef fish are expert navigators.
How do fish find their way around?
Larval coral reef fishes have to find a suitable reef to settle on as they develop, yet it can be a daunting journey for such a tiny fish to make from the open seas where they spend the first few weeks of their life. Recent research suggests that they use the smell of other members of their own species to home in on likely reefs. In addition, reefs are noisy places, particularly because of the waves that break upon them, and evidence suggests that the fish also use these sounds to guide them towards the reefs. Adult fish are also thought to use these cues when migrating between different reefs that they may use for feeding and for hiding. In addition, they use familiar landmarks to orientate themselves along their route. Fish memorize landmarks on their home reef too.
Experiments in the nineteenth century showed how blennies could memorize a spatial map to find their way around. More recently, a study on clownfish showed that if fish were removed from their own anemone and released elsewhere on the reef, out of sight of the anemone, they quickly found their way home, even after considerable periods away. There are even suggestions that clownfish can remember such spatial information for periods of up to six months.
How good are they at navigating?
Given the costs associated with getting lost, it is perhaps not surprising that fish are good at navigating. Perhaps the most dramatic example of fish finding their way around is provided by frillfin gobies.These fish are confined to rock pools at low tide and can be vulnerable to predators, such as gulls, or to the pool itself drying out.The gobies respond to these risks by jumping to a different rock pool, even though they cannot actually see where that pool is before they jump. To achieve this extraordinary feat, the gobies must memorize the relative positions of rock pools at high tide and retain this information so that they know which way to jump in an emergency. This memory extends to encompass the locations of several different rock pools, allowing the fish to leap from one to another to another in sequence. Of course, the costs of getting it wrong are great; if they miss their target rock pool, they could be stranded and die. For this reason, individuals that are captured and put into a pool without being able to investigate its location beforehand will not jump if danger threatens.