Cusk-eel

Image: NOAA/Monterey Bay Aquarium Research Institute

I'd like to call this thing a "fish-shaped lump of flab" but it's scarcely even fish-shaped!

At least it actually IS a fish. What it isn't, is any kind of eel. Or indeed, a Cusk, which is a whole other, unrelated, North Atlantic fish also known as a Tusk or Torsk. Apparently 'torsk' comes from an Old Norse word which also has something to do with the word 'thirst'. Why's a fish thirsty? This makes no sense!

Image: NOAA Office of Ocean Exploration and Research,
Leucicorus atlanticus

To make matters worse, Cusk-eels belong to a family called Ophidiidae. Ophis is Greek for 'snake'. Ophidion is a diminutive version, like 'little snake' or 'snakeling'.

But this Cusk-eel looks nothing like a snakeling! Nor even a snake. Some of them look more like what you'd get if you tried to make one of those 'rock in a sock' weapons but all you had was tissue paper. It's like they're talking about a completely different fish!

Image: Roberto Pillon
Snake Blenny (Ophidion barbatum)

Enter a completely different fish. It's Ophidion barbatum, a name which means 'bearded snakeling'. Although people actually call it a Snake Blenny. Which is odd because it isn't a blenny and there actually does exist a long, thin blenny also known as a Snake Blenny. It can never be simple, can it.

O. barbatum is a Cusk-eel. Perhaps even the first one to acquire the name. It does look a bit like a cusk, but it's long and thin like an eel. It reaches a length of 25 cm (10 in), which I guess one might consider snakeling territory. Also it comes from relatively shallow waters from England to the Mediterranean and down to Senegal. That means it was easy to find both for the people who thought it looked like cusk crossed with an eel, and for the early scientists who thought it looked like a little snake with a beard.

Video: Mike Bear

Speaking of beards, this marks one of the main differences between Cusk-eels and actual eels.

That difference is in the pelvic fins. Most fish have pelvic (or ventral) fins on their underside, where they help the fish make sharp turns and quick stops. Some fish have their pelvic fins way back towards the end of their body, others have them a little way behind the head, and others settle for somewhere in the middle.

Image: Ross Robertson

Eels? They don't have pelvic fins. Not a single one between the lot of them. Eels don't have a uniform as such, but what they do have, is not a pelvic fin. Cusk-eels, now they're not actually eels. They do have pelvic fins. It's just that they're not really fins.

Their pelvic fins have migrated all the way up to their chin, lost most of their fin-like properties and turned into what are essentially barbels. With these, a Cusk-eel can come out of its hidey-hole at night and search the ocean floor for crustaceans and small fish to nibble on.

Image: TANAKA Juuyoh (田中十洋)
Brotula multibarbata

Some Cusk-eels even supplement their sensory ex-fins with yet more barbels around their mouth, just like a catfish. If barbatum is bearded, then what can you call such a fantastically bearded creature but multibarbata?

But not all Cusk-eels have to come out at night. As it turns out, of the almost 250 known species, the majority live in the eternal night time of the deep sea.

Video: Inner Space Center

There they acquire all the characteristics that make them look like a denizen of the deep and not at all like a snakeling. Their eyes shrink, their muscles relax, they're drained of colour and soon they are another mole-eyed ghost slowly making their way through the darkness.

They do very well down there, too. In fact, one Cusk-eel might be the deepest living fish in the world! It's called Abyssobrotula galatheae and it was dredged up from a depth of 8,370 m (27,460 ft) in the Puerto Rico Trench in 1970.

Video: New Scientist

We have video evidence of Snailfish going about their business at a depth of 7,700 m (25,250 ft). The Cusk-eel was even deeper, we just can't be completely sure that they're really living their lives at such a depth. Maybe it was just one heroic individual going where no fish had gone before. Or maybe he got lost. It's pretty dark down there after all.

But no matter how deep or shallow the waters in which a Cusk-eel lives, it seems to be the case that every single last one of them lays eggs that float in a gelatinous mass at the water's surface. Deep sea Cusk-eels don't undertake a punishing journey to the surface, like a sea turtle lugging itself up a beach. Instead, it's a long trip for bunch of floating eggs. At least they're too young to complain about it.

When the eggs hatch, the creature that emerges is quite astonishing. Cusk-eel larvae are bizarrely elegant creatures. Clear as crystal, with decorative fins, spots of colour and, er... guts that dangle from their neck like an oversized feather boa.

Certainly that's one way to maintain a slim figure.

Image: NOAA/Monterey Bay Aquarium Research Institute
Giant Cusk-eel (Spectrunculus grandis)

Alas, all external guts must come to an end some day. Eventually the larval Cusk-eel loses its fantastical fins, takes on some drab colours and encloses its guts within its body in a manner both sensible and commonplace.

They probably start making 'dad-jokes', too. They look the type.