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Some species of fish use "ram ventilation" to pass water over the gills while swimming. Hydrodynamic analysis provides estimates of the pressure losses as the water passes through the gill spaces, and of the resistance force that must be overcome by the swimming muscles. For a 44-cm skipjack tuna (Katsuwonus pelamis), swimming at its basal speed of 66 cm/sec, the gill resistance is estimated to be 1090 dynes or 7% of the total swimming resistance. If oxygen consumption doubles, gill resistance increases to 27% of the total. The energetic cost of respiration lies between 1 and 3% of the total metabolism at basal swimming speed.

In tuna gills, blood flows obliquely to the long axis of the secondary lamellae rather than with it as in other teleosts. This pathway greatly shortens the length of the small diameter respiratory vessels; it shortens them by a factor of 4 in a 1.67-kg skipjack tuna (Euthynnus pelamis) and of 9 in a 272-kg bluefin tuna (Thunnus thynnus). Total blood-pressure drop across the entire gill varies as the square of the length of these vessels, so the drop is reduced by a factor of 16 in the skipjack and of about 80 in the bluefin tuna. Blood-pressure drop also varies as the inverse of the cube of the vessel diameter, and several teleosts we examined snowed a compensating increase in diameter as length increased with fish size. Because of the shorter vessel lengths, the diameters seen in tunas are relatively small.Vessel diameter determines, considerably, the thickness of the secondary lamellae. The specialized pathway of the tunas, in addition to reducing blood pressure drop, allows the secondary lamellae to remain relatively very thin. For a given lamellar spacing, this maximizes the interlamellar water channel thickness and minimizes resistance to water flow. The specialization is a major change in "design strategy" and its advantage appears to lie with the balance between the energy requirements of moving water and blood. Some such change may be essential to the attainment of high levels of activity in fish of large size.

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