A bluefin swims with its mouth open, forcing water past the gills in a process called ram ventilation. Its gills have up to 30 times more surface area than those of other fish, and they extract nearly half the oxygen dissolved in the water. If the tuna ever stops swimming, it suffocates.
Tunas are unique among bony fish in their ability to keep key parts of their body warm. Rather than lose heat to cold water in the gills like most fish, tunas have heat-exchange systems that retain metabolic heat produced in the tissue. These systems are present in three areas (orange outline, below).
The bluefin is one of the fastest fish in the ocean, thanks to a combination of physical characteristics. Its large tail maximizes thrust, while the tapered shape of its body minimizes drag. For optimum streamlining, some fins retract into a groove. Others fold into a depression in the body.
Graphics: Fernando G. Baptista, Lawson Parker, and Daniela Santamarina, NGM Staff; Shizuka Aoki. Sources: Barbara Block, Stanford University; Bruce Collette, IUCN Tuna and Billfish Specialist Group; Don Stevens, University of Guelph