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MEET THE THUNNINI TRIBE
So what exactly makes a tuna a tuna? Taxonomically
speaking, tunas are predatory ocean-going fish
belonging to a “tribe” called Thunnini, a subgroup of the
mackerel family (Scombridae). The tribe encompasses
the eight “true tunas” of the genus Thunnus — albacore,
bigeye, longtail, yellowfin, blackfin and three species
of bluefin — and seven other tuna species including
skipjack, slender and little tunny.
All tunas, from the smaller bullet and frigate species
to the giant bluefin, have streamlined, spindle-shaped
bodies with a crescent-shaped tail fin, two dorsal fins,
pectoral fins that can be tucked in for high-speed
swimming and triangular “finlets” running along the
upper and lower sides of the tail. Skin coloration
varies from species to species but generally fades from
metallic blue on top to silvery-white underneath, often
with a splash of yellow accents.
Peeking under the hood, we find one of the most
impressive pieces of biological machinery in the entire
ocean. “When you look at a tuna, you’re looking at an
animal that’s built for very, very high performance,” said
Randy Kochevar, a marine biologist who works with
Block at Stanford University’s Hopkins Marine Station.
Indeed, every aspect of tuna physiology seems to
push the limits of what’s physically possible, from
ultra-high-efficiency gills to a massive heart that can
beat up to 200 times per minute. What makes these
animals truly special is not just their size, speed or
strength but the unique evolutionary adaptations,
honed over hundreds of millions of years, that make
such incredible performance possible.
JUST KEEP SWIMMING — OR ELSE
The name Thunnus comes from the ancient Greek
for “to dart” or “to rush” — an apt descriptor for these
efficient, powerful swimmers capable of tremendous
bursts of acceleration. That power comes at a price
though. With high oxygen demands and a muscular
physique that’s denser than seawater, a tuna can never
stop swimming during its lifetime.
Tuna require five to 10 times greater flow of water
over their gills than most fishes, which they achieve by
swimming with their mouths perpetually open, called
“ram ventilation.” They fly through the water like fighter
jets, their lack of buoyancy offset by lift generated by
their pectoral fins. And, like an aircraft, if a tuna stops
producing forward thrust, it will sink like a stone.
To keep themselves in perpetual forward motion,
tuna have evolved a compact swimming style
consisting of short, powerful strokes of the tail with
very little head movement. Bundles of deep red,
oxygen-burning “slow-twitch” muscle running along
the core of the fish enable them to sustain this unique
form of locomotion over vast distances.
THE “WONDERFUL NET”
All the constant swimming tuna are forced to do
generates a lot of excess heat. For most fish, that
energy would be lost, dissipated quickly into the colder
surrounding seawater.
But, as we’ve established, tuna are not “most fish.”
One of their remarkable evolutionary adaptations is
the rete mirabile (Latin for “wonderful net”), a network
of blood vessels that lets them retain body heat and
use it to warm their muscles, brain and eyes above the
ambient ocean temperature. Members of the genus
Thunnus can even heat their stomachs to aid digestion
after swallowing a meal of ocean-chilled seafood.
That’s right — tuna are endothermic, or “warm-
blooded,” like humans. No other bony fish can claim
that distinction. While their cold-blooded cousins are
generally limited to a narrow range of temperatures,
tuna can move freely around the globe through the
dark depths and the sunlit shallows, the sweltering
tropics and the frigid higher latitudes.
Different species inhabit various regions around the
globe, with some favoring tropical latitudes (yellowfin,
skipjack), while others prefer cooler regions (albacore,
Atlantic bluefin, Pacific bluefin, southern bluefin).
Bigeye in particular are champion divers, known to
travel to depths of 3,300 feet or more in pursuit of
food, making roller-coaster descents and ascents that
would make a divemaster blanch.
A WANDERING LIFE
Tuna don’t turn into these hulking, supercharged
speed demons overnight. Almost all tuna species
spawn in warm tropical regions, where their larvae
start life floating with the currents and dining on
zooplankton until they begin to resemble miniature
versions of their parents.
They continue to grow rapidly as juveniles, shifting
their focus to new prey as they get larger. Tuna are
opportunistic feeders and will happily consume a
wide variety of prey over the course of their lives
depending on their age, location and season. A highly
abridged list of tuna prey includes crustaceans, squids
and countless small fish species such as anchovies,
sardines, menhaden, hake and mackerel.
Groups of tuna will often attack bait balls — huge,
swirling schools of smaller fish — sometimes working
in concert with other predators such as dolphins and
seabirds as they drive their prey into ever-tighter spaces.
