RESPIRATORY GILL SURFACE OF THE SERRASALMID FISH, PIARACTUS-MESOPOTAMICUS

Gill element dimensions of pacu Piaractus mesopotamicus were estimated and correlated to body mass (W), according to the power equation Y = aW(b). The filament number (b = 0.154) and length (b = 0.457) increase d with body mass, markedly influencing the respiratory gill surface ar ea (b = 0.769). The high filament number and length, associated with a high secondary lamellae frequency (a = 40.21), are typical of active fish species and may be an adaptation to its migratory movements durin g reproduction. The comparatively small dimensions of its secondary la mellae are found more commonly in less active species, and may be rela ted to the environmental conditions prevailing in lentic environments. where the species is normally found most of the year. Such features, together with its ability to compensate for oxygen reduction by means of a high ventilatory volume, and the use of aquatic surface respirati on (ASR), may account for its adaptative capacity to withstand hypoxic conditions, with a low respiratory energy cost. (C) 1997 The Fisherie s Society of the British Isles