Relations Between Gill Structure and Activity in Fish

From well over 200 papers quantitative data concerning gill structure, oxygen consumption and activity in fish were compilated. These data were corrected, whenever necessary, for a body weight of 200 g and a surrounding temperature of 20° C, which enabled us to compare various species. The standard metabolism (expressed as the standard-oxygen-uptake) appeared to be a fairly good quantitative measure for the general activity of a fish species and was used as such. Comparing a series of species increasing in activity (in regard with behaviour), one finds that oxygen consumption does rise more slowly than the quotient of gill surface and membrane thickness. According to FICK'S law, this is due to the fact that under standard conditions in active fishes a considerably lower difference is maintained between the oxygen in the water and that in the blood than in slow-moving fishes. Partially this may be due to the greater resistance of the gills of active fishes, partially to the way of breathing of the very active species (ram-ventilation). In the hypothetical standard fish of 200 g, the gill surface and the thickness of the gill membrane appeared to be linear inversely proportional. Probably the gill membrane must have a minimum thickness in order to be equal to the defence against noxious particles, while the density of the secondary lamellae is limited by the membrane thickness. Data covering all of the examined parameters were known for only 9 species. The quantitative relations inferred from these were neither completely confirmed nor contradicted by comparing them with insufficiently documented species. If required, the quantitative relations may be expressed by the following formulas: in which: D = thickness ofgill membrane (μ) A 200 = gill surface of a standard fish (mm 2 /g) dP = difference in partial oxygen pressure over the gill membrane (atm) M 200 = oxygen consumption of a standard fish (mg/kg/hr) Applying our results to benthic species, it is conceivable that, while slowness of locomotion in an environment with a large supply of slow moving or even sessile food formed no hindrance, the sequence of causes and effects during gill evolution is as follows. Basic was a considerable thickness of the gill membrane, indispensable as defence against scouring particles. This room-consuming quality of the membrane limited the number of secondary lamellae, which put a limit to the disposable surface. And this limited surface, in its turn put a limit to the oxygen absorption and, thereby, to the activity of the fish.