Japanese Pygmy Squid

5 Apr

Idiosepius paradoxus

The Japanese pygmy squid — one of the world’s smallest cephalopods with a mantle length of just 16 mm (0.6 in) — hunts crustaceans up to twice its size. It paralyses them, then slips its mouthparts inside their exoskeletons to slurp out their insides, leaving their shells intact.

Sailors of yore feared the Kraken; a monstrous squid that could embrace an entire ship with its many arms, dragging the vessel and all its crew to the depths of the sea. This was a myth that proved to be real, if exaggerated. The giant squid, an actual species from the deep seas, grows to a total length of 15 metres (50 ft) and the colossal squid, a different species from the Southern Ocean, can become nearly as big. Neither sank ships nor ate sailors, but the rare sighting of a dinner-plate-sized eye and a mass of streaming arms could doubtless send imaginations to work and tongues wagging, spreading stories about behemoth sea monsters.

Anti-Kraken

In the coastal waters of the western Pacific, from Japan to South Korea to northern Australia, dwells a much more miniature monster known as the Japanese pygmy squid. It lives amidst algae and beds of seagrass, sticking to surfaces with a special adhesive organ. At around 16 millimetres (0.6 in) long, it's among the smallest cephalopods — that is, squid, cuttlefish, or octopuses — in the world.¹

In many ways, the pygmy squid is the opposite of the monstrous Kraken. It lives in the shallows along the coast, rather than the deep open ocean. Its body is larger than its tentacles, giving it a big-headed, babyish appearance, rather than that of an eldritch monster. And, being about the size of a rice grain, it's far too tiny for a Kraken. But, despite all of that, it's far more monstrous than its colossal cousins.

Micro Monster

A Japanese pygmy squid eating a skeleton shrimp (Caprellidae spp.).

It was said that the Kraken could swallow a shipful of sailors, however, we know today that giant and colossal squids feed primarily on deep-sea fish and other squid, catching prey with two elongated tentacles and tearing it apart with their sharp beaks before ingesting it.

The Japanese pygmy squid prefers crustaceans such as grass shrimp, gammarids, and mysids.² The squid's well-developed eyes spot its target, a floating shrimp. It approaches stealthily — possibly using its colour-shifting abilities for camouflage — until it's about a body length away. Then it considers its next move.

Like all squid, the pygmy carries in its body a sac of ink. Most squid use ink to evade predators, squirting it out to distract an attacker or obscure themselves before escaping. The Japanese pygmy found that ink works just as well for acquiring prey.³ It can squirt its ink off to the side — a move that would divert its target's attention — but instead, it creates a dark cloud directly between itself and its prey. Then, with great speed, it shoots through this smoke screen, trailing darkness as it emerges like an arrow from the other side. It seizes the confused shrimp with its stubby, but strong, arms, sliding its eight appendages between the joints of the shrimp's armour in just the right way to dose it with paralyzing cephalotoxin. Its grasping arms pull its victim towards its mouth, but here the squid has a problem; the shrimp it's caught is almost twice its own size. So, instead of trying to swallow this oversized prey, the squid injects a part of itself inside the shrimp's body.

The buccal mass structure and radular apparatus of a squid.

The pygmy squid's buccal mass — all of its mouth parts, including the beak and muscles that control it — is stretched to the length of one of its arms and inserted beneath the shrimp's exoskeleton. With its mouth on the inside, the squid has access to its victim's soft parts, but it doesn't just eat it alive, it releases enzymes to partially digest it first. Only then does it devour the semi-dissolved flesh, wriggling its mouthy mass around to slurp up every bit of organ and muscle. The shrimp is helpless, paralyzed in the grasp of its tiny captor, as its insides are slowly liquified and ingested. When the pygmy squid is done feeding, all that's left is the shrimp's exoskeleton; sucked of all life, with virtually no flesh left, but undamaged — not even chipped — as if the shrimp had simply moulted, rather than met a horrific end. The pygmy squid occasionally hunts fish as well. Like its armoured prey, the squid paralyzes the fish before ingesting all of its muscle mass, leaving the fish's skeleton perfectly preserved. If it catches a large fish — too large to fully paralyze — it will only eat part of it before retreating.

The myths of giant squid can hardly match the cruelty of this pygmy.

Speedy Squid

The smart sailor feared the female Kraken most, for she was the largest of all. The same is true of real-world giant and colossal squids, and, indeed, applies to most cephalopods, including the Japanese pygmy squid — although "largest", in this case, means just under 19 millimetres (0.7 in).

A squid's courtship display, in which the smaller male tries to impress a larger female, involves a dance of contorting arms and fluttering fins accompanied by a colourful light show across the male squid's skin. After his performance, the male squid extends a hectocotylus arm — an arm specifically modified for delivering his spermatangia (sperm packet) — which he inserts directly into the female.

But the male pygmy squid isn't much of a romantic; if he can copulate without the courtship, he'll attempt to do so. This hurried, "let's just get to it" attitude can make for awkward mistakes. Trading good judgment for speed, a male pygmy squid will sometimes mistake another male for a female and implant his spermatangia into that male — who probably feels confused, flattered, and aggravated, all at the same time.

A female Japanese pygmy squid laying her eggs.

If, however, the male has successfully identified a squid of the opposite sex and given her his sperm packet, she can go off and begin to lay her eggs. For more than a month she produces them industriously, like a living egg factory, laying 30–80 eggs every 2–7 days, and sticking them to her chosen surface in a flat mass. The minuscule eggs hatch into equally minuscule squid that float through the water column like plankton. In only 1.5–2 months, they'll be mature enough to breed themselves, and, in only 150 days, almost all of them will be dead.

Generations of Heat and Cold

Two generations of pygmy squid are born and die every year. Necessarily then, the two generations experience their breeding seasons at different times of year, facing different conditions that change the physiology of one generation compared to the next. Those that breed in late spring and early summer, the warm season, mature faster and reach a smaller body size, while their children grow larger over the winter but don't sexually mature and breed until the cool season, in early spring. So one after another the generations cycle with the seasons; a small-bodied generation for the warm and plenty of summer, followed by a big-bodied generation to survive winter's cold.

Cold conditions encourage larger body sizes. In warm-blooded land animals, this is referred to as Bergmann's rule, which states that animals at higher latitudes have larger bodies that retain more heat. In the cold-blooded creatures of the sea, we see the superficially similar observation known as the temperature-size rule (TSR); organisms tend to mature faster and reach smaller sizes in warmer waters, while those in colder waters mature more slowly and reach larger sizes. This holds between different generations of the same species, as demonstrated by the Japanese pygmy squid, but also between different species. The Japanese pygmy squid, the most northern of all pygmy squid species, is also the longest-lived — likely a result of its slower growth rate in colder waters. 150 days seems all too short, but the other pygmy species don't even get that much.

Pygmy Squids

The pygmy squids — of which there are nine known species, so far — are the smallest of all squids and the smallest cephalopods in the entire world. Searching for the smallest of something is, for obvious reasons, harder than searching for the biggest, and the rankings are prone to being reshuffled by the discovery of a new speck of a species hiding out in a crevice somewhere. Prior to 2023, there were seven pygmy squids. Now there are nine.

The two new species were discovered in the seagrass and coral reefs around the islands of Okinawa in southern Japan. One of the species, the Ryukyuan pygmy squid, gets its common name from the Japanese archipelago to which Okinawa belongs — the Ryukyu Islands, which stretch in an arc from Japan's southern main island of Kyushu to Taiwan — while its scientific name, Idiosepius kijimuna, is more whimsical, referring to the red-haired fairies that are said to live within the banyan trees on Okinawa. The Kijimunā might be tree spirits, but like the pygmy squid, they're also known to be excellent fishermen who subsist entirely on seafood. Octopuses, however, the fairies avoid at all costs — making it somewhat amusing that their name was given to a species of cephalopod.

The second new species was named Hannan's pygmy squid, after the underwater photographer Brandon Ryan Hannan, who discovered the squid suctioned beneath a coral. Distinct from all other pygmy squid, it was placed in its own genus, Kodama, named after another mythological being from Okinawa — this one a round-shaped tree spirit whose presence is said to signify a healthy forest, just as the pygmy squid signifies a healthy coral reef. This pygmy squid swims in a bobbing fashion with its arms splayed out to grasp its shrimp prey, and so was given the specific name jujutsu, after the grappling martial art.

The Ryukyuan pygmy squid (*Idiosepius kijimuna*).

The jujutsu or Hannan's pygmy squid (*Kodama jujutsu*).

The the Mozambique pygmy squid (*Idiosepius minimus*).

The two new pygmy species — neither larger than 12 mm (0.5 in) — joined the ranks of the other pygmy squid; including the southern pygmy (Xipholeptos notoides) from the waters around Australia, the Mozambique pygmy squid (Idiosepius minimus), the only pygmy squid from the coasts of Africa, and the Thai pygmy squid (Idiosepius thailandicus), perhaps the smallest of the small, with a maximum mantle length of only 10 mm (0.4 in) — this latter species was itself only discovered in 1991.

Idiosepida — the pygmy squids — with nine species in all, is an exceptionally small order of animals. Their range, on the other hand — from the waters of Japan, south throughout the Indo-Pacific region, all the way to Australia and Tasmania, as well as west to Mozambique — is large. There are likely more pygmy squids out there to be discovered; unknown species hiding among the coastal grass beds, along the underexplored coastlines of the world, or even the explored ones, so small as to go unseen. Perhaps we've yet to find the smallest of all cephalopods.

¹ 16 millimetres is the Japanese pygmy squid's average mantle (the sheath that covers its body) length, but its stumpy tentacles don't add much more length — unlike the giant squid, which is mostly tentacles, with a max mantle length of only 2.25 metres (7.4 ft).

² Gammarids are a family of amphipods that resemble long-limbed rolly pollies (pill bugs) and mysids are lanky crustaceans that are alternatively known as opossum shrimps because females have pouches in which they rear their larvae.

³ The use of ink as an offensive weapon for hunting is rare, even in Japanese pygmy squids. Out of a total of 322 predation events, the squids only used ink on 13 of those occasions, with the squids' success rates varying from 75% with one species of shrimp, 11% with another, and one shrimp species on which they never used ink. Curiously, the use of ink didn't seem to affect the squids' hunting success rates.

**Where Does It Live?**

⛰️ Along coastlines around algae and beds of seagrass.
📍 Western Pacific, from Japan to South Korea to northern Australia. ¹

‘Data Deficient’ as of 28 March, 2009.

IUCN

Size // Tiny
Length // 16 mm (0.6 in) average mantle length
Weight // 10–796 mg
Activity: Nocturnal 🌙
Lifestyle: Solitary 👤
Lifespan: 150 days
Diet: Carnivore
Favorite Food: Small crustaceans 🦐
Class: Cephalopoda
Order: Idiosepida
Family: Idiosepiidae
Genus: Idiosepius
Species: I. paradoxus

This tiny squid has a large range, found in the coastal waters of the western Pacific, from Japan to South Korea to northern Australia — it lives amidst algae and beds of seagrass, sticking to surfaces with a special adhesive organ.

The Japanese pygmy squid uses chromatophores to rapidly change the colour of its body, allowing it to hide from predators or sneak up on prey.

Like all squid, this pygmy can squirt ink when escaping from danger, but, unusually, it also uses ink to catch its prey (small crustaceans and fish). It either squirts its ink off to the side, diverting its target's attention, or it creates a dark cloud directly between itself and its prey, then bursts through to grab it.

Once it has its prey, the squid slides its stubby but strong arms between the joints of its armour, paralyzing it with a cephalotoxin.

The squid can consume prey up to twice its own size by stretching its buccal mass — the muscular structure containing its beak and mouthparts — to the length of one of its arms and inserting it beneath its prey's exoskeleton. It then releases enzymes that partially digest its prey's insides before devouring them from the inside out. When the pygmy squid is done feeding, all that's left is its prey's pristine exoskeleton.

If the pygmy squid catches a large fish, one too large to fully paralyze, it will only eat part of it before retreating.

To fertilise a female, a male pygmy squid uses his hectocotylus arm — an arm specialized to deliver his sperm packet — which he inserts directly into the female. A male pygmy squid sometimes mistakes another male for a female and implants his spermatangia into that male.

Once fertilised, a female will industriously produce and lay 30–80 eggs every 2–7 days for more than a month. Pygmy squids become sexually mature only 1.5–2 months after hatching and live for around 150 days.

Two generations of pygmy squid are born and die every year, each living and breeding in different conditions: there is a small-bodied generation for the warm and plenty of summer, followed by a big-bodied generation to survive winter's cold.

So far, we know of nine pygmy squid species (in the genus Idiosepida), with two new species — the Ryukyuan and jujutsu (or Hannan's) pygmy squids — being discovered around Okinawa, Japan, in 2023. The smallest of the small, with a maximum mantle length of only 10 mm (0.4 in), is the Thai pygmy squid.