Gill Surface-Area and Its Components in the Yellowfin Bream, Acanthopagrus-Australis (Gunther) (Pisces, Sparidae)

Abstract

As head length (HL) of Acanthopagrus australis increases, fewer gill filaments are added, total gill filament length (TFL) increases linearly, and total gill surface area (TSA) increases exponentially. Changes in surface area components [surface area per lamella (SA), distance between adjacent lamellae (DBT) and number of lamellae per zone on one side of the filament (NPZ)] with increasing HL were examined in the distal, middle and basal zones (relative lengths 3:10:1) of a corresponding filament on the external hemibranch of the four gill arches. SA was the smallest in the basal zone and largest in the middle zone; DBT was greatest in the distal zone and smallest in the basal zone; SA and DBT increased at similar rates in different gill arches within the distal and middle zones but at different rates in each zone within an arch; growth of NPZ in the middle zone differed among gill arches but did not in the distal zone. Surface area of lamellae in the middle zone contributed most to increasing TSA. Compared with other fish of intermediate activity, the estimate of TSA for A. australis was smaller than expected; although DBT and TFL were as expected, SA was smaller than expected; possible reasons include method of measurement in situ, shrinkage caused by fixation, and absence of a weighting factor. The method employed enables large numbers of lamellae and filaments in a large number of fish to be measured, and enables regression equations to be derived that relate surface area per filament zone to head length and filament length in order to estimate the loss of surface area caused by ectoparasites.