Bluefin and bigeyes are the only tuna with a warm-blooded circulatory system that allows them to recycle oxygenated blood one more time before sending it to the gills for re-aeration. The difference is, bluefins use a complex of blood vessels lying in close proximity — the ‘rete mirabile’ — to warm them in cold waters, and as a result are the only tuna that regularly inhabits cold water. Bigeyes use the rete mirabile to feed in oxygen-depleted cold water beyond the scattering layer, and are the only tuna that can do it.
This deep ocean water has oxygen concentrations down to one-half part per million; other fish would suffocate, but bigeyes go down there and back up all night long. When they come up to warm surface water, they not only warm up but recharge their blood with oxygen before repeating the deep feeding dive.
The same adaptation allows both tunas to exploit different habitats and food sources closed to other tunas.
Be the first to comment