Endothermy (the metabolic production and retention of heat to warm body temperature above ambient) enhances physiological function, and whole-body endothermy generally sets mammals and birds apart from other animals. Here, we describe a whole-body form of endothermy in a fish, the opah (Lampris guttatus), that produces heat through the constant "flapping" of wing-like pectoral fins and minimizes heat loss through a series of counter-current heat exchangers within its gills. Unlike other fish, opah distribute warmed blood throughout the body, including to the heart, enhancing physiological performance and buffering internal organ function while foraging in the cold, nutrient-rich waters below the ocean thermocline.

This comparative study of the gill morphometrics in scombrids (tunas, bonitos, and mackerels) and billfishes (marlins, swordfish) examines features of gill design related to high rates of gas transfer and the high-pressure branchial flow associated with fast, continuous swimming. Tunas have the largest relative gill surface areas of any fish group, and although the gill areas of non-tuna scombrids and billfishes are smaller than those of tunas, they are also disproportionally larger than those of most other teleosts. The morphometric features contributing to the large gill surface areas of these high-energy demand teleosts include: 1) a relative increase in the number and length of gill filaments that have, 2) a high lamellar frequency (i.e., the number of lamellae per length of filament), and 3) lamellae that are long and low in profile (height), which allows a greater number of filaments to be tightly packed into the branchial cavity. Augmentation of gill area through these morphometric changes represents a departure from the general mechanism of area enhancement utilized by most teleosts, which lengthen filaments and increase the size of the lamellae. The gill design of scombrids and billfishes reflects the combined requirements for ram ventilation and elevated energetic demands. The high lamellar frequencies and long lamellae increase branchial resistance to water flow which slows and streamlines the ram ventilatory stream. In general, scombrid and billfish gill surface areas correlate with metabolic requirements and this character may serve to predict the energetic demands of fish species for which direct measurement is not possible. The branching of the gill filaments documented for the swordfish in this study appears to increase its gill surface area above that of other billfishes and may allow it to penetrate oxygen-poor waters at depth.