The scaling and potential importance of cutaneous and branchial surfaces in respiratory gas exchange in larval and juvenile walleye

Measurements were made of the surface areas (As) of the skin and gills of larval and juvenile walleye Stizostedion vitreum with a body mass (M) of between 2mg (1 day post hatch) and 2.3g (98 days post hatch). The skin, with a relative surface area (As/M) of approximately 8500mm2g-1, accounted for more than 99.9% of the total surface area (skin + gills) at 1 day post hatch. The relative area of the skin decreased as fish grew at an allometric rate of b-1=-0.32&plusmn;0.01 (mean &plusmn; s.e.m., where b-1 is the specific-mass exponent in the allometric equation YxM-1=aMb-1, in which Y is surface area and a is a constant). The relative surface area of the gills (filaments + lamellae) increased in a hyperbolic fashion from very low levels (approximately 5mm2g-1) at 1 day post hatch to reach a maximum of approximately 1100mm2g-1 at a body mass of approximately 200mg. Thereafter, relative gill area declined at an allometric rate of b-1=-0.19&plusmn;0.10 (mean &plusmn; s.e.m.). Gill area, because it declined at a slower relative rate, finally exceeded skin area at a body mass of approximately 700mg. The relative surface area of the skin and gills combined (total surface area) decreased at a more-or-less constant allometric rate of b-1=-0.21&plusmn;0.01 (mean &plusmn; s.e.m.) throughout the experimental period. On the basis of the allometric rates of expansion, the structural capacity to supply oxygen (b-1=-0.19; total gill area, this study) and metabolic demand for oxygen (b-1~-0.13; mean literature value for routine and resting metabolism) appear to remain fairly closely matched in postlarval walleye (>300mg). The two parameters do not display the same degree of concordance during larval development. In larvae, total respiratory surface area declines on a mass-specific basis at roughly the same rate (b-1=-0.21) as gill area does in older fish but, unlike in older fish, metabolic demand for oxygen does not change (b-1~0.0). This results in a progressive decline in effective respiratory surface area (As/M.O2) but does not affect O2 uptake, probably because larvae are so small that surface area is not the limiting factor in gas exchange. Analysis of data from the literature suggests that surface area typically becomes limiting at a body mass of approximately 100mg. The major function of gills in smaller larvae (<100mg) appears to involve ionoregulation or related aspects of acid&shy;base balance rather than respiratory gas exchange.