Using Behavior for Communication
Body language -- communicating with others through behavior -- is a vital part of the behavior repertoire of 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's important to get your message across it is best not to leave any room for ambivalence! An important facet of this is that the signals should be stereotypical -- each behavioral posture should be consistent, always having the same meaning to their intended receiver.
What do fish use body language for?
Much of the body language of fish is devoted to defending their own interests. When two fish such as damsels squabble, they display to one another, each holding out its fins rigidly 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 that they can be seen by a rival. Although there are similarities between species, individuals within the same species tend to use virtually the same body language to make sure that every "gesture" is clearly understood. This can be quite bizarre: some species of goby adopt a display posture with their head raised and their mouth gaping open. When threatened, lionfish adopt a head-down posture. Similarly, fish at reef cleaner stations assume certain poses to encourage the cleaner wrasse or shrimp to begin cleaning them and, simultaneously, to reassure them that they carry no threat to the smaller cleaners.
How do fish use behavior to communicate fright or submission?
As As well as informing the world at large that you are prepared to fight for your corner, there are also times when dangerous aggressors must 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. Therefore, a defensive fish will 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 behavior is the most important element in communicating submission and one reason why aggression in the aquarium can sometimes continue to the death. Since a fish in the aquarium can obviously only back off so far, the saltwater aquarist needs to look out for the signals and separate the fish before it comes to this.
Do fish use body language for courtship?
Female fish are often choosy, so males of many species have to work hard to impress their potential mates. One of the ways a male can achieve this is by displaying, helping the female to decide if he will make a good father. In bicolor damsels, the direct relationship between the ability of the male to display and his skills and diligence as a father allows females to be sure that they are doing their best for their offspring. Male damselfish produce extensive displays during courtship to lead the female towards the nest; climbing and diving in the water column are interspersed with hovering and dipping. But displaying does not just impress potential partners, it also attracts the attention of predators; male gobies react to this by dramatically reducing their displays when a predator is in the vicinity.