General Recognition: Not All Freshwater Fish Are the Same
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While to us it may be difficult to tell apart fish of the same species -- even sometimes of different species -- it is important for the fishes themselves to be able to do so. For example, fish that reacted in the same way to a predator as to a mate would quickly find itself in trouble. Beyond this, new research shows that fish are capable of some highly impressive recognition abilities.
What is recognition in fishes?
There are a few main ways in which a fish can recognize others. For example, many species of fish are preprogrammed by their genes to recognize predators and other fish that may be a threat to them or, in the case of species that look after their young, to recognize that the large fish near them (their "parent") is not a threat but that they should keep close by it.
Experiments on newly free-swimming convict cichlids have shown that the fry will approach and remain with a model that has conspicuous stripes like their parents. As well as this, they may form a "recognition template" in very early life -- many fish grow up mainly with conspecifics, so they follow a simple rule of "I am the same as the fish around me," much like the ugly duckling tale. Unfortunately, this is not foolproof -- swordtails that are raised with guppies prefer to associate with guppies in later life. Recognition abilities may also be learned; for example, if a fish sees a predator attacking another fish, it can learn that the predator represents danger and can take action to avoid it.
What and who do they recognize?
Fish are capable of both a general kind of recognition, say, the ability to recognize members of their own species or predators, and a much more specific recognition of individual fish. In fact, fish are capable of quite a few different levels of recognition. To illustrate this, imagine walking down the street and seeing a figure at a distance. You would probably work out first that they were from the same species as yourself -- a human -- and as you got nearer you might recognize various other characteristics, maybe their height age, and sex. Next you might hear them talking and recognize a regional accent so you have an idea where they are from. Only when you get pretty close can you work out if you have met them before. A similar process occurs with fish -- they recognize species, size, gender and population and, in some cases, they can recognize detail minute enough to know exactly who they are dealing with.
What other things can fish recognize?
It is in fishe's interests to recognize some other characteristics as well. Members of the carp family, such as barbs, can recognize which fish are poor competitors and show a preference for associating with them, presumably knowing that when it comes to feeding time, they themselves will have the upper hand. They can also recognize which individuals are behaving oddly -- these are important to avoid if you are a schooling fish as they can attract predators, Cyprinids also recognize fish that are obviously ill or parasitized and use this information to decide whether to associate with them or not because it is in their own interests to stay away if the condition is catching.