Swordfish

A heater for the brain

Swordfish are believed to swim down to 2,100 feet (650 m) or deeper, where the water may be just above freezing. One adaptation that lets them swim in such cold water is a “brain heater”: a large bundle of tissue associated with one of the eye muscles. It warms the eyes and brain up to 10 to 15 degrees C above the water around them.

One study showed that in fast-swimming predatory fish such as the swordfish, warming the retina improves temporal resolution, and so the detection of rapid motion. Depending on dive depth, it can be more than ten times higher in these fish than in fishes whose eyes match the water temperature.

Eggs by the million A female swordfish may carry from 1 million to 29 million eggs, and estimates vary a lot.

The sword

The bill is long, even for a billfish, and in cross section it appears as a flattened oval. Adults have no teeth and no scales. Young swordfish have scales, but they lose them by about 3 feet (1 m) long. Swordfish feed mostly on pelagic fishes (fish of the open sea) and occasionally on squid.

The sword is apparently used to catch prey. Squid and cuttlefish pulled from swordfish stomachs often carry slashes, and in one study most large fish prey had been slashed, while small prey were eaten whole.

Size and lifespan

Swordfish reach a maximum of 177 inches (455 cm) and 1,400 pounds (650 kg). Nearly all of those over 300 pounds (140 kg) are female. The ones caught by the North Atlantic fishery average 50 to 200 pounds, and NOAA says North Atlantic swordfish live about 9 years. Females there can reproduce between 4 and 5 years of age.

Predators of adult swordfish include killer whales. Swordfish also host 49 species of parasites, from tapeworms in the gut to flukes on the gills.

Where they roam

Swordfish live in the Atlantic, Pacific and Indian Oceans, from about 60 degrees north to 45 degrees south. They migrate, generally moving to warmer water in winter and cooler water in summer, and they often turn up in frontal zones, where ocean currents collide and productivity is high.