Marine Hatchetfish

FishCosmopolitan

Oct 26

Sternoptychidae spp.

Perhaps no other creature in nature looks so anguished to exist as the marine hatchetfish. Its giant, protruding eyes glisten, as if on the verge of tears, and its gaping mouth is downturned into a sorrowful frown. It looks like Munch's 'The Scream' painting brought to life, as a fish. But, to be fair, the hatchetfish does lead a dour, and awfully gloomy, life.

Hatchetfish

Firstly, what is a hatchetfish? It would be more accurate to refer to them in the plural form, since there are some 45 species found around the world; in the Atlantic, Indian, and Pacific Oceans. They belong to a family with a mouthful name — Sternoptychidae — which also includes such charming fish (in name, if not appearance) as bottlelights, pearlsides, and constellationfishes.¹

These are creatures of the deep. Hatchetfish can live at depths of 1,500 meters (about 5,000 feet), in a realm of darkness called the midnight zone (or bathypelagic zone), where no sun from the surface ever reaches. Perhaps it's for the best; they are not very pretty fish. Apart from their expressions of torment, their bodies are oddly proportioned, with extremely shrunken back ends and bulbous front ends, giving the appearance of distended bellies. Many have skins like gleaming metal with rows of slicing scutes along their bellies and, combined with their concave shapes, they often resemble the head of a hatchet — hence the name.

Most hatchetfish, however, would make for pretty tiny hatchet-heads. The largest species is the giant hatchetfish. "Giant" is a stretch, since it only reaches a maximum length of 12 cm (4.7 in), but relative to other hatchetfish, it is indeed quite giant. The smallest species, Polyipnus danae, grows to a max length of only 2.8 cm (1.1 in). The rest — such as the half-naked hatchetfish at 5 cm (2 in) and lovely hatchetfish at 7 cm (2.8 in) — fall on a spectrum in between.

Abyssal Adaptation

The deep sea, abyssal realm, an unfathomable chasm — it repels us with its vastness and darkness, with the horrors that dwell in those crushing waters, while, at the same time, drawing us in to gawk and marvel at the grotesque miracles that it spits out. For the creatures that live in the abyss, life is often simple, stripped to its bare essentials; eat and don't get eaten, then reproduce and probably die. But the ways in which they achieve these goals can be surprisingly innovative and complex.

You're probably familiar with the blobfish ² — one of the most famous deep-sea denizens, although perhaps not for the best reason. Its mucus-covered, big-lipped, droopy-nosed, "blobby" physique has graced all corners of the internet since 2003. You may also know that its dishevelled physique is the result of it being a fish out of very deep water.

Its home is anywhere from 600 to 2,800 metres below the surface (2,000 to 9,185 ft), where it floats placidly just above the sea floor. The water pressure down here can be over 100 times greater than the air pressure at sea level, and the blobfish's body is specifically adapted to cope with the crushing mass of water above it. Observed in the depths, the blobfish looks more like a regular fish, with a somewhat bulbous head. But bring it to the surface and its body — which is scaleless, and composed mostly of fat and water — slumps into a woeful, gloopy mess.

I bring up "Blobby" the blobfish to illustrate how extreme the effects of pressure can be on an animal's physiology and to highlight how tough hatchetfish, and many other deepsea animals, are. These odd-shaped fish take part in some of the most extreme migrations in the world.³ As night falls in the world above, hatchetfish ascend to shallower waters. The deepest-living species rise from their ever-dark, midnight world. They're joined by species from the realm of twilight — the mesopelagic zone, at depths of 200 to 1,000 metres (650 to 3,300 feet). Their bodies withstand the changing pressures as they swim upwards, as if they really were made of tempered steel. Reaching the shallows, they feast on a floating banquet of zooplankton; gulping tiny crustaceans, like seed shrimp and copepods, and the larvae of other fish. But daybreak soon banishes them back to the abyss.

Glowing to Stay Hidden

Sunlight from the surface, weak as it can be at depth, is often enough to be fatal. It casts its rays onto swimming fish, revealing them as dark silhouettes on a canvas of light blue — revealing them to predators that lurk in the dark below.

The photogenic organs of the marine hatchetfish.

Although it may sound counterproductive, the hatchetfish of the twilight zone actually produce their own light to conceal themselves from the light above. This strategy, known as counterillumination, is employed by myriad deepsea creatures — from fish to sharks to squid — in myriad forms. A hatchetfish uses rows of light-producing organs that line its belly. These photophores usually shine with pale blue light, but a hatchetfish can adjust the brightness and colour of its bioluminescent structures to better vanish into the water column.

You might think that in the deepest depths, in the midnight zone where sunlight can't create silhouettes, this ability would be useless or possibly dangerous — attracting hungry predators to the hatchetfish like a beacon in the dark. After all, that's what deepsea killers, like anglerfish and viperfish, use their photogenic organs for. But deepsea hatchetfish have been moulded by their abyssal habitat to survive in the pitch blackness and evade the dangers that lurk within it. The hatchetfish's bioluminescent organs scatter light — including light used by predators to hunt — in a pattern that blends with the surrounding darkness, creating an invisibility cloak made of light, to hide them amidst the deepest dark.

Even if a hatchetfish can evade a violent end — can avoid the lure of a viperfish, elude a darting tuna, and escape the jaws of a lancetfish — it can't hide for very long. For most hatchetfish, the grim reaper comes swiftly. After a relatively short life, usually lasting less than a year, a hatchetfish is dragged into the unfathomable abyss of death. Perhaps that's why they look so sad. Maybe they lament their brief lifespans; a brief wink of blue in the vast deep, snuffed out forever all too soon.

¹ Marine hatchetfish (in the family Sternoptychidae) are not to be confused with freshwater hatchetfish (comprising the family Gasteropelecidae). The latter are nine species of small (3–9 cm or 1–3.5 in) fish that live in rivers, swamps, and pools throughout Central and South America — in freshwater, rather than salt, as their name implies. The shared name between these two disparate groups comes from a morphological convergence; naturalists thought both families of fish looked like hatchet heads.

A species of freshwater hatchetfish — marbled hatchetfish (*Carnegiella strigata*) — a popular aquarium pet.

The underside of a freshwater hatchetfish is convex to the extreme, with a bulging chest and abdomen, while its upper half looks more or less like a regular little fish (not a nightmarish ghoul, like its deepsea counterparts). Another quirk of the freshwater hatchetfish is its massive pectoral fins and the formidable muscles that control them — which altogether make up a quarter of the fish’s body weight.

Freshwater hatchetfish can also fly! Sort of. To escape predators, a freshwater hatchetfish uses its powerful pectoral fins to provide an initial push out of the water, it then begins to whip its tail up to 40 times a second (producing a buzzing noise), and uses both fins and tail to control its trajectory. Its flight can reach a few metres in height and cover distances of ten metres (33 ft). While the more famous marine 'flying fish' use their fin-wings to glide, these hatchetfish come close to ‘motorised’ flight — the only fish known to do so. They can't, however, sparkle blue like their marine equivalents.

The blob sculpin (*Psychrolutes phrictus*) out of deep water (top) and in its deepsea home (bottom).

² The "blobfish" is more formally known as the blob sculpin (Psychrolutes phrictus). It belongs to a family (Psychrolutidae) of tadpole-shaped fish alternatively known as blobfish, toadfish, or fathead sculpins. The blob sculpins genus name, Psychrolutes, is Greek for "someone that has a cold bath", and given that it can live some 2,800 metres (9,185 ft) below the surface of the North Pacific, its "bath water" is probably pretty cold.

The blob sculpin doesn't have a swim bladder — a gas-filled organ used by many fish to stay afloat in the water column — instead relying on its gelatinous constitution, of mostly fat and water, to float along the deep ocean floor where it hunts for invertebrates including crustaceans (such as crabs), cnidarians (sea pens), and molluscs (of the gastropod, snail/slug, variety).

³ Diel vertical migration (DVM) — or the daily movement of zooplankton up and down the water column — is described as the greatest migration on Earth. Every night, several billions of tiny fish, shrimps, crustaceans, and jellies migrate from the deep to the surface. Although the individual creatures are mostly minuscule, their collective mass is so great that these migrations are known to create turbulence in the water.

The tiniest zooplankton rise to feed on food sources like algae; doing so at night when their risk of predation is lower. They're followed by small predators, who are followed by larger predators, and so on, all the way up to organisms as large as sharks and whales. The marine hatchetfish are tiny participants in this vast parade that swarms to the surface for a nighttime feast.

**Where Does It Live?**

⛰️ Tropical, subtropical and temperate waters; mostly between 200–600 metres (655–1,970 ft) and as low as 1,500 meters (~5,000 feet) — ocassionaly found at depths of 2,400 metres (7,875 ft).
📍 Atlantic, Pacific and Indian Oceans.

Size // Tiny
Length // 7 cm (2.8 in) to 12 cm (4.7 in)
Weight // N/A
Activity: Nocturnal 🌙
Lifestyle: N/A
Lifespan: Less than a year
Diet: Carnivore
Favorite Food: Planktonic fish and crustaceans 🦐
Class: Actinopterygii
Order: Stomiiformes
Family: Sternoptychidae
Subfamily: Sternoptychinae

There are some 45 species of marine hatchetfish found around the world; in the Atlantic, Indian, and Pacific Oceans.

The largest species is the giant hatchetfish, with a maximum length of 12 cm (4.7 in). At the other end of the scale is Polyipnus danae, which grows to a length of only 2.8 cm (1.1 in). The other species fall on a spectrum in between — such as the half-naked hatchetfish at 5 cm (2 in) and lovely hatchetfish at 7 cm (2.8 in).

Most species live between 200–600 metres (655–1,970 ft), in the twilight (mesopelagic) zone. But some can live as deep as the midnight (bathypelagic) zone, at depths of 1,500 meters (~5,000 feet) and are occasionally found as deep as 2,400 metres (7,875 ft).

Hatchetfish participate in diel vertical migrations — or the daily movement of zooplankton up and down the water column. They rise to shallower waters during the night to feed, then retreat to the depths at dawn.

A hatchetfish's diet consists of tiny planktonic crustaceans, such as seed shrimp and copepods, and floating fish larvae.

Predators such as viperfish, lancetfish, and tuna hunt and eat hatchetfish.

The bioluminescent organs along the hatchetfish's belly conceal it from predators — a strategy known as counterillumination. They typically glow a pale blue, but a hatchetfish can adjust the brightness and colour of these structures to better hide in the water column.

In shallower waters, the photogenic organs hide the hatchetfish from predators below by matching light coming from above and obscuring its silhouette.

In the dark depths, it uses these structures to scatter light — including light used by predators to hunt — in a way that diffuses with the surroundings, hiding the hatchetfish.

The lifespan of a hatchetfish is short; usually less than a year.

Marine hatchetfish belong to the family Sternoptychidae — which also includes such charming fish (in name, if not appearance) as bottlelights, pearlsides, and constellationfishes.

Marine hatchetfish are not to be confused with freshwater hatchetfish, which comprise the family Gasteropelecidae. They look superficially similar — in that both are vaguely shaped like hatchet heads — but they aren't closely related. The freshwater varieties live in rivers, swamps, and pools throughout Central and South America and have evolved the ability of near-motorised flight.