WEGNER, N. C.*; SEPULVEDA, C. A.; GRAHAM, J. B.; Scripps Institution of Oceanography, University of California, San Diego; Pfleger Institute of Environmental Research; Scripps Institution of Oceanography, University of California, San Diego: Gill specializations of the shortfin mako, Isurus oxyrinchus, a lamnid shark.

The tunas (family Scombridae) and the lamnid sharks (family Lamnidae) demonstrate a remarkable convergence for high performance swimming. Increased aerobic muscle power, continuous swimming, and regional endothermy, characteristic of these two groups, mandate the specialization of respiratory structure and function to meet increased oxygen demands. This study provides the first comprehensive analysis of lamnid gill structure in comparison to that of tunas. Vascular plastic replica casts of mako shark gills reveal microvascular modifications to increase lamellar gas transfer. Mako lamellae have diagonal blood-flow channels (previously documented only in tunas and billfishes) that are thought to manage blood pressure drop across long lamellae. Blood is delivered to these oblique channels through an additional outer marginal channel on the lamellar lateral edge, and blood is collected by inner marginal channels. Mako diffusion distances are also quite short; lamellar thickness and lamellar wall width (the blood-water barrier distance) are comparable to measurements obtained for tunas and are among the smallest recorded for comparably sized fish. These microvascular specializations were not found in the blue shark (a non-lamnid). This study also confirms that mako gill surface areas are greater than non-lamnid shark species, but significantly less than those of tunas. The basic elasmobranch respiratory design (which exhibits a much more tortuous water pathway than that of teleosts) may limit lamnid gill surface areas and serve as a bottleneck preventing these sharks from reaching the scope of sustainable aerobic performance achieved by tunas.