Baby halibut are born symmetrical, then one eye travels across the head
A young halibut looks like an ordinary fish, with an eye on each side. At about six months old, one eye migrates over the skull to join the other, and the fish settles into life lying flat on the seabed. The adults become the world's largest flatfish, some exceeding two metres.
Three species carry the name: Atlantic, Pacific and Greenland halibut, all members of the right-eye flounder family. The word combines haly, meaning holy, and butte, a flatfish, because it was a favourite food on Catholic holy days. A 2018 study of genes and anatomy found that the Greenland halibut split from the lineage that later produced the Atlantic and Pacific species.
The eye's journey happens during larval metamorphosis and stops once the skull hardens into bone. The side left without an eye stays white, while the upper side turns dark brown. This countershading hides the fish from above against the sea floor and from below against the bright surface. Its scales are so small they are invisible to the naked eye. Atlantic and Pacific halibut vie for the title of largest flatfish; a fish caught off Norway in July 2013 weighed 234 kilograms and measured 2.62 metres, while a 219-kilogram Pacific halibut from Alaska in 2014 was disqualified from records because it had been shot and harpooned before landing.
Halibut eat almost anything that fits in their mouths. Juveniles pick at small crustaceans; adults take octopus, crab, salmon, cod, herring and even other halibut. They live from a few metres down to hundreds, usually near the bottom but rising to feed, and sit near the top of most marine food chains. In the North Pacific their predators include sea lions, killer whales, salmon sharks and people.
Curiously, the species determine sex differently. Atlantic halibut use an XX/XY system, like humans, that arose between 0.9 and 3.8 million years ago, while Pacific halibut evolved a ZZ/ZW system around 4.5 million years ago in which females carry the odd chromosome. The North Pacific commercial fishery dates to the late 19th century and is among the region's most valuable, mainly using long lines of baited hooks stretching for miles along the bottom.
Source: Halibut
