How Reef Fish Recognize Other Fish
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The ability to recognize others is vital to fish. They have to know whether the individuals they encounter are predators, competitors, or members of their own species that might potentially be a breeding partner. This recognition can be performed in a number of different ways, but the patterns and colors of coral reef fishes help to establish the identity of different species and to convey information about age, sex, and dominance. As well as these visual cues, reef fish also use smell and sound to identify one another.
Some of the most remarkable discoveries about fish in recent years have centered on their clear ability to recognize particular individual fish. Incredible as this may seem, this capacity to distinguish specific individual identities has now been demonstrated in a huge variety of fish.
What and who do they recognize?
Fish are extremely good at recognizing what is, and what is not, a potential predator. Obviously, they also have to recognize members of their own species (a "conspecific"), perhaps to school with them, or when choosing a mate. How they actually do this is not known for certain. Having spent their early life among the plankton, reef fishes may very well not even meet a conspecific until they are juveniles. Larval fish home in on the smell of adult conspecifics when settling on a reef. Fish can also tell generally what is a competitor and what is not by its behavior (for example, grazing fish all feed in a similar way) and by its shape and color.
But fish can also be flexible and can learn about types of fish that they have never seen before. In an experiment on this, damselfish were introduced to tilapia, a freshwater fish that they would never normally encounter. The tilapia had been acclimatized to saltwater and then trained to eat either algae (making them direct competitors to the damsels) or to eat invertebrates. Damselfish that encountered algae-eating tilapia quickly learned that they were competitors and responded aggressively, whereas those that met carnivorous tilapia simply ignored them.
What other things can fish recognize?
Fish are capable of learning when there is a free meal in prospect. For example, it has long been known that many species of groupers keep an eye out for octopuses and moray eels, then follow them when they go hunting. In this way, the groupers can snap up prey fish that are startled out of their hiding places by the other predators. Until recently, it was thought that this was very much a one-way process and that the groupers were simply exploiting the octopuses or eels. However, it now seems that the groupers might be earning their keep by guiding the eels to the hiding places of different fish. Both coral groupers and lunartail groupers have been observed swimming up close to resting moray eels in the Red Sea and displaying to them in an unusual manner. In half the cases observed, the eel would leave its cave and swim side-by-side with the grouper, which was apparently guiding its new partner to their prey's hiding place.
How do we know that fish recognize particular individuals?
For fish that live in monogamous pairs, such as many butterflyfish and gobies, recognizing your mate is clearly essential. Butterflyfish reinforce their pair bonds regularly by producing a series of different sounds.
Clownfish, too, have to recognize those individuals with whom they share their home anemone. Their ability to do this is extremely impressive. After spending just one hour learning an individual's identity, they are apparently capable of recognizing that fish again after one month apart because they behave very differently towards this individual than towards a total stranger. And it is not only their mate's identity that they learn; they also learn the identities of other fish within their social group, enabling them, for example, to perform appeasement behavior to dominant individuals. However, learning identities is likely to go beyond this, even extending to members of other species, particularly when certain individuals are encountered again and again. This enables dominance patterns to be established and maintained across the reef.
Can fish recognize one another on sight?
Vision is the most important sense for diurnal (day-active) reef fish, so it is appropriate that it is used for individual recognition.
Tests on recognition in clownfish showed that they are extremely good at recognizing their mates and other territory members. The fish respond very differently to these familiar individuals than to strangers (they are extremely aggressive to strangers). How do they tell each other apart? Some ingenious experiments showed that the fish are using visual cues -- recognizing individuals by their color patterns. To find this out, researchers gave the fish made-to-measure coats that obscured their color patterns. They were then able to show that the fish in the coats were no longer recognized by their mates and were treated in the same way as strangers. Subsequent tests have shown that the stripes around the head are especially important in allowing individual recognition in these fish.
Are the other senses used for individual recognition?
Fish do use their other senses to enable recognition. Their sense of smell is called into play when assessing potential mates, most especially in pair-forming fish. By sniffing their mate they are able to tell whether that fish is related to them. This is possible because each individual animal's genetic make-up gives it a subtly distinctive odor and the more similar prospective mates smell, the more likely they are to be related, meaning that they are less suitable as breeding partners. Some territorial damselfish are also able to distinguish between their neighbors simply by listening to the sounds they make, allowing them, for example, to tell the difference between an adjacent territory holder and a newcomer.