Everyone knows octopuses, those invertebrate molluscs that live in shallow waters, often in coral reefs. But did you know that these animals have cousins living in the deep sea? They are called dumbo octopuses.

Dumbo octopuses belong to the Cephalopoda class. This group includes the subclasses Nautiloidea (nautiluses) and Coleoidea. This second group includes: the Decabranchia, which have ten ‘arms’ (these are the orders of cuttlefish and squid) and the Octobranchia, which have eight ‘arms’ (corresponding to the order of Vampiromorphidae, of which the deep-sea vampire squid is the only known representative to date, and to the order of Octopoda: octopuses). The Octopoda are divided into two suborders: the Cirrata and the Incirrata. The Cirrata suborder is characterised by an internal shell, an absence of an ink sac, a reduction or absence of a radula and two fins on the head1,2, which octopuses belonging to the Incirrata suborder do not have. The Cirrata suborder takes its name from the small filaments under their tentacles, resembling cilia (cirri).

Dumbo octopuses are Cirrata. They belong to four genus: Cirroteuthidae, Cirroctopodidae, Opisthoteuthidae, and Grimpoteuthidae2,3. Here we will focus specifically on the genera Grimpoteuthis, which includes 17 species, and Opisthoteuthis, which includes 25 species.

Some Dumbo octopuses

They can be found all over the world. They first appeared at the beginning of the Mesozoic era, around 242 million years ago4. They are descendants from distant ancestors living in the Cambrian period, around 540 million years ago5. They evolved, with the protective outer shell of these organisms becoming an internal “bone”, an organ dedicated to buoyancy 1,2,6,7, also serving as a support structure for fins1,8. Recent phylogenetic studies have confirmed the relationship between the nautilus group and the Cirrata 4. As soft tissues are difficult to fossilise, there are very few fossils of these organisms.

Their name is misleading. They do not belong to the same group as octopuses, and the protrusions on either side of their eyes are not ears but small fins! These fins are used for propulsion, a more energy-efficient method than the jet propulsion used by most other octopus species1. Like other octopods, they can change the colour of their mantle, and some groups are even suspected of being bioluminescent 2!

Their eight arms are connected by a membrane, helping them to move around. They are small in size and live in deep waters (abyssal zones): between 3,000 and 7,000 metres deep2, making them the octopuses that live furthest below the ocean's surface. They are pelagic species: they live close to the sea floor3.

The group of dumbo octopuses is one of the least known to researchers: to date, we have no observations of their predatory behaviour and very little information about their reproduction or their vision capabilities in deep waters2,9. Moreover, most species have only been described based on one or two observed specimens, and new species are still regularly being discovered 8,9. Recent studies show that females do not lay thousands of eggs, as is the case with octopuses, but carry a few at different stages of maturation. They release one or two at a time to develop (without following a particular season). Unlike other octopod species, they do not ‘brood’ the eggs and the young are born independent 1,3.