Fish communicate with one another using urine.

A paper published in the journal Behavioral Ecology and Sociobiology by a team of researchers has shown that at least one species of fish communicates with others of its kind using chemicals in its urine. The researchers inserted a partition in the middle of a fish tank that prevented fish on either side from interacting physically with one another. In some scenarios, the barrier had tiny holes to allow water to pass between the sides, while in others it did not. Also, some barriers were opaque and others were transparent. In addition, the researchers injected the fish with a blue dye that allowed them to see and measure urine being expelled by the fish once in the tank. The researchers measured how much urine was expelled under a variety of situations—in which only one fish was in the tank; in which there were two but they could not see each other; in which there were two and they could see each other but were or were not able to communicate via urine through the barrier—the team also used a variety of fish sizes and noted fish behavior throughout each test. The researchers looked at their results and noted that when two fish saw one another in the tank, they raised their fins and approached each other in an aggressive manner, and both emitted more urine than when they were not able to see another fish. They found that only when the urine was allowed to move through the barrier was there a noticeable change in behavior of the fish—in such cases, the smaller fish generally reduced its aggressiveness, yielding to the larger one. The researchers also noted that when the urine was not able to pass through the barrier and the fish were able to see one another, both emitted more urine than in any other scenario, apparently aware that their message was not getting through.