世界最小のイカであるヒメイカ。英名：Japanese pygmy squid 、学名：Idiosepius paradoxus 。
Don’t get me wrong, Giant Squid are quite impressive and Vampire Squid are certainly not without their charm, but I would have to posit that the Pygmy Squids (Idiosepius spp.) are the strangest of all squidkind. These compact cephalopods boast a bevy of unusual morphological traits and behaviors not seen in any of their ten-armed brethren, and new data has increasingly argued that this small genus (totalling just seven species) likely represents the sister group to all other squids and cuttlefishes. These are itty bitty living fossils!
Normally we would expect to find the last living survivors of an ancient evolutionary group in some deep, inaccessible part of the ocean. Think of the coelacanth… hagfishes… sea lilies… Gardineria… or even the aforementioned Vampyroteuthis, all restricted to deeper waters. But not so for Idiosepius, which occur abundantly in shallow, coastal habitats of the Indo-Pacific. Specimens can be found virtually everywhere, from the temperate waters of Russia and South Africa, to the tropical climes of the Coral Triangle, but it takes a keen eye to spot them in their natural habitat.
Fully grown, the Pygmy Squids live up to their name, topping out at a meager 10-18mm in total length—roughly speaking, the size of a fingernail. These are the very smallest of squids and, among the Cephalopoda, only a few octopodes can compete with their absurd level of diminution. No doubt, it’s this peculiar propensity for petite proportions which has allowed the group to flourish into the present day, enabling them to make use of an ecological niche which might otherwise be unfilled.
In many ways, the behavior of Idiosepius matches that of the Bobtail Squids (Sepiolidae), and, owing to their similarities, the two have long been lumped together as a single group. Both have rather stubby arms and small bodies, and both are associated more with the benthos than their pelagic cousins in the Teuthida (the “true squids”). However; these similarities apparently belie a deep evolutionary divergence between the two, as seen in recent molecular studies that place the Pygmy Squids as basal among all decapods (squids, cuttlefishes, etc.).
Cephalopods use inking as a defensive tactic, but researchers observed ink also used for predation. Japanese pygmy squid (Idiosepius paradoxus) use ink during hunting attacks in two different ways: 1- the squid releases ink between themselves and the prey and then attack through the ink cloud, 2 – the squid releases ink away from the prey and then attack from another direction.
Idiosepius are largely ambush hunters, attaching themselves to seagrasses and macroalgae while they wait for their crustacean prey to come into reach. And they do quite literally attach themselves—a glandular organ present on their backs secretes a gluey substance that secures them in place. No other cephalopod does this, and, interestingly, these intrepid invertebrates have even been seen to make use of the floating refuse that builds up in the ocean. As the saying goes, one man’s trash is another Pygmy Squid’s treasure.
Once prey has been successfully ententacled, it is then injected with a paralyzing venom and consumed in a rather macabre fashion. Digestive enzymes are apparently excreted directly into the unlucky arthropod, and, much like with a spider, the innards become liquified and are then slurped up with nary a bite needed. All that’s left behind is the withered husk of an exoskeleton, drained of all the precious bodily fluids. In aquarium studies, small fish have been offered as food, resulting in a perfect little fishy skeleton when dinnertime was over. This is a highly unorthodox feeding behavior among squids, as most are well-known for the beaklike jaws used to vigorously masticate prey; it’s only in certain juveniles that we see a similar strategy of liquefaction employed.
You might think that Idiosepius would make for an excellent aquarium subject given its sprightly size, and you’d largely be right. These are abundant in the wild, easily collected, and suitable for even the smallest of fish tanks… but there is one major drawback: the normal lifespan is just 80-90 days. Alas, the brightest cephalopod stars do burn out fastest! Still, the ease with which they can be made to breed in captivity has endeared them to developmental biologists, and some spectacular footage of their embryology can be seen in the video above.
There is still much left to be learned about these lilliputians. For example, we still don’t fully understand their true biodiversity. Not all described species seem to be legitimate, and there’s at least one species (from Eastern Australia) said to be in need of description. The fossil record is, unsurprisingly, entirely unknown for this lineage, despite the hundreds of millions of years since they split off from their calamari cousins. But most fascinating about Idiosepius is that it offers us a window into what the very first squids may have looked like—tiny, inconsequential bottomdwellers that would eventually evolve and diversify to become some of the most fearsome predators in the ocean. It’s a long road from Idiosepius to Architeuthis.
Our Photo Pro took this video of a pygmy squid hunting and feeding on a squat lobster. The pygmy squid gets up to 15mm, this individual was about 10mm long. Watch how the squid “drinks” the squat lobster like a juice box and if you look carefully, you can even see how the liquid moves into the squid’s body.
- Kasugai, T., Shigeno, S. and Ikeda, Y., 2004. Feeding and external digestion in the japanese pygmy squid Idiosepius paradoxus (Cephalopoda: Idiosepiidae). Journal of Molluscan Studies, 70(3), pp.231-236.
- Lindgren, A. and Anderson, F., 2017. Assessing the utility of transcriptome data for inferring phylogenetic relationships among coleoid cephalopods. Molecular Phylogenetics and Evolution, https://doi.org/10.1016/j.ympev.2017.10.004