Marine Hatchetfish

The marine hatchetfish's body is compressed side-to-side into a thin, deep, silver blade, with the belly drawn out into a sharp keel, so that in side view it has the outline of a hatchet or axe head and in front view it is almost knife-thin. Two things drive this shape. First, the extreme flattening and mirror-bright silver sides make the fish nearly disappear when seen from the side in open water, because the mirror reflects the surrounding blue light and shows no features. Second, the deep belly houses and orients the rows of light organs used for camouflaging the fish from below, and the keel positions them along a single line so the emitted glow is even. The shape is not for swimming speed; hatchetfish are slow, hovering fish.

In the twilight zone, the only light comes faintly from directly above, so any animal is visible from below as a dark silhouette against that glow. Counter-illumination cancels the silhouette. The hatchetfish has a row of downward-pointing light organs (photophores) along its underside that produce a soft blue-green light of the same colour as the filtered daylight. Sensors in the fish measure how much light is coming from above, and it adjusts the brightness of its own glow to match, so that a predator looking up sees an unbroken field of dim light with no dark shape in it. Hatchetfish can even shape the glow so it looks right from a range of viewing angles. Many other twilight-zone animals, including lanternfish, some squid, and the cookiecutter shark, use the same trick.

Life in the twilight zone rewards looking up. Prey drifting overhead, and the larger predators hunting the hatchetfish, both appear as silhouettes against the faint surface light, so an eye optimised to look straight up and detect small dark shapes against a dim background is very useful. In many hatchetfish the eyes have become tubular — elongated cylinders topped with a large lens and a dense patch of retina — which sacrifices all-round vision for maximum light-gathering and acuity in that one critical direction. Some species also have a secondary strip of retina giving a little sideways and downward vision. The barreleye fish takes the same adaptation to an even greater extreme.

They are ecologically very important despite being small and rarely seen. Marine hatchetfish, together with lanternfish and other small twilight-zone fish, make up an enormous biomass — by some estimates the mesopelagic fish of the world ocean outweigh all the fish humans catch by an order of magnitude. Each night many of them migrate up toward the surface to feed and back down by day, physically transporting carbon and nutrients into the deep sea, and by day they are eaten in vast numbers by tuna, squid, dolphins, and diving seabirds, funnelling energy from plankton up to top predators. The species themselves are listed as Least Concern and are not targeted by fishing, but there is growing concern about proposals to trawl the twilight zone commercially, and about how ocean warming and deoxygenation will affect this layer.