A metal-blue torpedo with dark stripes on the belly, passing a drop-off at speed and gone. Everything about it is built around one constraint: it cannot stop. Skipjack have lost the ability to pump water over their gills and must swim with the mouth open to breathe, from the day they hatch until the day they die.
They also run warm. A network of blood vessels in the muscle works as a countercurrent heat exchanger, trapping the heat their own swimming produces instead of losing it through the gills, so the fish is several degrees warmer than the water around it. Warm muscle is faster muscle, and that is how a fish keeps up a cruising speed no cold-blooded animal of the same size could.
The cost is enormous. It must eat around a tenth of its body weight a day, grows at a rate that would be absurd in a reef fish, and rarely lives past four or five years.
This is also the most heavily fished tuna on earth - the one in the can, several million tons a year. On this coast it appears as a fast shape in the blue, usually over the drop-off, usually once.
The species. Katsuwonus pelamis (Linnaeus 1758) - from pelamys. ETYFish
- streamlined dark blue back, silver sides with four to six dark stripes along the belly
- no scales except on the shoulder and along the lateral line
- row of small finlets between the dorsal fin and the tail
- never still - it cannot stop swimming
- length up to 110 cm
