Aggressive behaviour in Betta splendens as a bio-indicator of freshwater pollution

Male Betta splendens are known to be highly territorial and aggressive towards conspecifics. The influence of two metal pollutants, mercuric chloride (HgCl₂) and sodium azide (NaN₃), on specific aggressive behaviours of the male Betta species was investigated. The aim was to determine if aggressive behaviour can be used as a metal pollution bio-indicator. For this purpose, two males were placed in an aquarium (60 x 25 x 25 cm) that was divided into two equal compartments by a clear, perforated Plexiglas wall. The aquarium was filled with untreated water as control, and with polluted water under experimental conditions. After 24 h, one male (intruder) was placed in the other male's (resident) half of the aquarium. The behaviour of both males, particularly, air gulping rate, latency to first bite and to first opercular expansion, were recorded. The results indicate that the males under treatment conditions were less aggressive than those under control conditions. Thus, the male Betta splendens aggressive behaviour can be potentially used as a biological indicator of pollutants in freshwater.