Communicating Through Behavior
Body language and behavioral cues are vital for communication between all visual animals, and fish are no exception. A wide variety of signals can be transmitted through body language, some are relatively subtle but others are very deliberate and obvious; when it is important to get your message across it is best not to leave any room for ambivalence.
What do fish use body language for?
Much of the body language of fish is devoted to defending their own interests. When two neon tetras squabble, they display to one another, each rigidly holding out its fins to show the other that it is both large and in great shape. This flank display is sometimes augmented by spreading the gill covers, again to impress the rival with size. Fish are usually very exaggerated and stereotypical in their aggressive displays; some enact a very rigid, almost robotic, slow swimming pattern, making absolutely sure they can be seen by a rival. Although there are similarities between species, individuals within the same species tend to use virtually identical body language to make sure that everyone is "speaking the same language". This can be quite bizarre -- some species of goby adopt a display posture with their head raised and their mouth gaping open.
How do fish communicate fright or submission?
As well as informing the world at large that you are prepared to fight your corner, there are also times when dangerous aggressors have to be appeased. Perhaps not surprisingly, one of the best ways to do this is to adopt a posture that is the opposite of the usual threat display. A defensive fish will therefore often fold its fins and do everything it can to convince the aggressor that it poses no danger, including swimming with its head up and backing off. This latter is the most important element in communicating submission and is one reason why aggression in the aquarium can sometimes continue to the death -- a fish in the aquarium can obviously only back off so far, so the aquarist needs to look out for the signals in order to separate fish before it comes to this.
Do fish use body language for courtship?
Female fish tend to be choosy, so males of many species have to work hard to impress their potential mates. One of the ways that males can achieve this is by displaying, helping the female to decide if he will make a good father. Female guppies are harassed almost continuously by males, both in the aquarium and in the streams of their home in Trinidad. To avoid this, the females move into deeper, faster-flowing water where the smaller males find it hard to follow, even risking predation to rid themselves of the incessant attentions of the males. If a male is to mate successfully, he can invest in a display, which, if it impresses, will allow him the chance to mate successfully without the female darting off.
Males that build nests need to advertise the presence of their constructions to females. The proud owner will patrol an area around his nest intercepting any females that swim past. The male can then use a wide variety of displays to tempt the female back to his nest. Each species has its own display -- for instance, male sticklebacks swim in an exaggerated zigzag to attract the attentions of their mates.