Fighting cichlids size up opposition with lateral line

Abstract

When sizing each other up before a confrontation, most males depend on all of their senses to learn more about their opponent – listening, watching and sniffing out strengths and weaknesses. However, Karen Maruska and Julie Butler, from Louisiana State University, USA, explain that fish also explore their surroundings with an additional sense: the lateral line system responds to the ebb and flow of water surrounding the fish's body. Could fish also use this ‘touch at a distance’ system to learn more about a foe? Maruska and Butler explain that fish adversaries often waft water around while posturing in preparation for an encounter and add, ‘The lateral line is poised to provide crucial information for opponent assessment’. However, no one had ever tested whether fish resort to the specialised sense in preparation for combat. ‘Astatotilapia burtoni males use a variety of aggressive behaviors, such as lateral displays, border fights, biting and mouth fights to defend their spawning territory,’ says Butler, so the scientists selected this aggressive species to test their theory.First, the duo investigated the structure of the feisty males’ lateral line system and clearly saw the sensory units that detect disturbances in water distributed on the fish's head and along the side of the body. Then they selected dominant males that were keen to defend their territory and deactivated the animals’ lateral line systems by immersing them in a solution of cobalt chloride and severing the posterior lateral line nerve. Next, they paired combinations of males, with and without active lateral line systems, in a duel over new territory and filmed the encounters.Butler explains that for each contest there were three possible outcomes: the fish did not fight; one fish capitulated to the other before full hostilities began; or the fish battled for supremacy. Analysing the outcomes of each confrontation, the duo quickly saw that the lateral line system was an essential component of a successful non-confrontational outcome.Almost 75% of the pairs where both fish had an intact lateral line system battled. However, when a dueller with a lateral line system was pitched against an opponent that had lost the sense, the duo saw a large increase in the proportion of fish that backed down before a confrontation – up from 5% in two males with intact lateral line systems to 30% where one combatant had lost the use of the lateral line. ‘This decreased motivation was likely due to an impaired ability to adequately assess the opponent’, say Butler and Maruska, adding that the disabled fish spent more time in close proximity to their opponents than the intact fish and that fish that had lost the lateral line system failed to instigate combat with the fin-flaring display that intact fish perform before a skirmish. And when Butler and Maruska analysed the battle manoeuvres, they saw that the fish with a functional lateral line system avoided contact with their opponents, while the fish that had lost the system bit and rammed each other more.Butler and Maruska were impressed to find that the fish use the additional sense to avoid damaging confrontation and they say, ‘To our knowledge, this is the first study to implicate the lateral line system as a mode of social communication necessary for assessment during agonistic interactions’.