THE ANIMAL FILES

Pacific halibut

Hippoglossus stenolepis

One eye crosses to the other side during growth.

Pacific halibut begin as free-floating larvae and develop the asymmetric form of a bottom-living flatfish. The left eye moves to the right side before settlement. Adults also move between shallower feeding areas and deeper seasonal spawning grounds.[1]

Follow the evidence ↓
Pacific halibut
Image: Jlikes2Fish · Commons · Public domain
Two eyes above
Both eyes lie on the upper right side.[1]
A larval transformation
Eye migration precedes the bottom-dwelling phase.[1]
Seasonal depth changes
Mature fish spawn in deeper water.[1]

Two eyes above

The adult has a diamond-like body and dark upper side, with a much paler underside. An arched lateral line rises above the pectoral fin and the tail is crescent-shaped. These features accompany the bottom-dwelling adult form.[1]

A larval transformation

During the drifting period, the left eye shifts across the head. Currents carry larvae before juveniles enter shallower nursery habitat. The transformation changes orientation and body organisation; it is not an injury that has pushed an adult eye out of position.[1]

Seasonal depth changes

Mature fish move from shallower summer feeding grounds to deeper spawning areas near the continental-shelf edge in winter. This is separate from the current-driven dispersal of larvae into nursery grounds.[1]

A COMMON MIX-UP

Are both eyes on one side from the beginning?

No. The left eye migrates during larval development.[1]

TRY A LITTLE REASONING

Why should a floating larva not be expected to behave like a settled adult?

Reveal the explanation

It is in a dispersing developmental stage before eye migration and settlement establish the adult form.[1]

Sources & editorial notes

Adapted and translated from the cited Wikipedia revisions, with source-linked reasoning. Image credit and licence are listed separately above.

Adapted and translated from Wikipedia contributors; exact revisions are linked below. CC BY-SA 4.0

  1. Wikipedia contributors — Pacific halibut (cited revision)
  2. NCBI Taxonomy — Hippoglossus stenolepis

Last fact check:

One discovery leads to another

SwordfishA slashing bill works alongside locally warmed eyes.Xiphias gladius ↗Allmouth goosefishThe fishing rod is part of the fish.Lophius piscatorius ↗GuppyColour can attract a mate and catch a predator’s eye.Poecilia reticulata ↗