UC San Diego

This study reports gill morphometrics for the three thresher shark species (genus Alopias) to determine how metabolism and habitat influence respiratory specializations for increased gill diffusion capacity. Thresher sharks have high gill surface areas, short water- blood barrier distances (1.60-2.55 [mu]m) and thin lamellae (12.50-14.29 [mu]m). Their high gill surface areas are derived from long total filament lengths and large bilateral lamellar areas, a morphometric configuration documented in other active elasmobranches (i.e., Lamnidae), which optimally augments respiratory surface area while limiting branchial resistance to ventilatory flow. The bigeye thresher, A. superciliosus, has the largest gill surface area documented in any elasmobranch species to date, which may be associated with prolonged exposure to hypoxia during diel vertical migrations. The common thresher shark, A. vulpinus, a red muscle endotherm, has gill surface areas comparable to A. pelagicus, despite expected higher aerobic requirements required for the conservation of metabolic heat. Additionally, A. vulpinus has a significantly longer water -blood barrier distance (2.55 ± 0.48 [mu]m) and thicker lamellae (14.29 ± 1.39 [mu]m) than A. pelagicus (1.61 ± 0.37; 12.51 ± 1.40 [mu]m) and A. superciliosus (1.60 ± 0.31; 12.50 ± 1.14 [mu]m). This likely reflects the cold, well-oxygenated habitat of A. vulpinus relative to the other Alopias species