Joachim G. Frommen
Aggressive interactions are ubiquitous throughout the animal kingdom. Animals fight for example to obtain or defend resources like food, mating partners or high-quality territories. Such overt aggressive interactions are usually costly not only in terms of increased risk of injury or death, but also due to opportunity costs and energy expenditure. Accordingly, animals are expected to keep such fights as short as possible or to avoid them completely. To that aim individuals often communicate their strength and aggressive motivation prior to fights. Here, they use different means of communication in various sensory modalities, i.e. visual, acoustic, chemical, mechanosensory and electric cues. Thus far, most studies on signalling during aggressive encounters have focussed on visual or acoustic cues, most likely as these are the two modalities predominantly used by humans. However, depending on the species’ ecology, visual or acoustic cues might play a minor role. In aquatic systems, for instance, visual communication is often hampered due to high levels of turbidity or limited light conditions, and thus alternative modalities such as chemical, mechanical or electrical cues play a more prominent role. For example, different crustaceans and fishes communicate their strength using chemical cues that are excreted via the urine, and some fishes possess special organs that allow them to assess changes in water pressure or electric discharges caused by their opponents. In this review, I provide an overview of different modalities used during aggressive communication in aquatic organisms. I highlight the importance of studying the role of multimodal communication during aggressive encounters in general and discuss the importance of understanding aquatic communication in the light of conservation and animal welfare issues.