The Longspine squirrelfish, Holocentrus rufus, has such large eyes because it’s mostly active at night. During the day, it hides in crevices in the reef- and can make sounds to defend its territory.
Squirrelfish vibrate their swim bladders to make a variety of clicking and grunting noises. It’s even thought that they make different sounds depending on the level of threat. For example, a single grunt can indicate low threat while a series of clicks means danger. They can use these sounds to alert nearby squirrelfish.
Who knew fish could communicate? Sea hares, on the other hand, are rather silent, but they make their presence known when they spray a cloud of ink when threatened. The ink not only warns predators, but can also be sensed by other hares. Teamwork!