Drifting through tropical open ocean, the blanket octopus is famed for its extraordinary size difference between the sexes and its ability to wield the stinging tentacles of the Portuguese man o’ war.
This genus of pelagic octopus exhibits one of the most extreme examples of sexual dimorphism, with the female growing up to 10,000 times the male’s size, and weighing 40,000 times more.
There are four species, that spend their entire lives wandering tropical and sub-tropical waters and are occasionally seen by divers during ‘black water’ dives (such as the example above, photographed in 2020 at Anilao in the Philippines).
The remarkable female, which can reach two metres (6.5ft) in length, has a long, fleshy cape which can change colour.
The cape can be dramatically unfurled between its rear arms as the octopus swims along, and it can be discarded if the octopus is threatened.
The membrane is formed from webbing between the dorsal arms and contains chromatophores, allowing rapid colour change for both camouflage and display.
When fully extended, it increases the animal’s apparent size significantly, a useful deterrent in the open ocean where cover is scarce.
Blanket octopuses are immune to the sting of the Portuguese man o’-war and have been reported brandishing broken-off tentacles from the stinging siphonophore as a defence against predators, and possibly to stun their own prey.
The immunity is thought to be due to specialised skin chemistry that prevents nematocysts (stinging cells) from discharging. Observations suggest the octopus may selectively handle the tentacles in ways that avoid triggering undischarged stinging cells.
Unlike most octopuses, which are primarily benthic, Tremoctopus species are fully pelagic and spend their lives in the upper layers of the open ocean.
They are strong swimmers and are often encountered at night, when many pelagic organisms migrate vertically towards the surface to feed. Their diet is believed to consist mainly of small fish, crustaceans and other planktonic organisms.
When mating, the walnut-sized male fills one of its arms with sperm, breaks it off and gives it to the female. The arm, known as a hectocotylus, detaches completely and is transferred to the female during mating. The male dies shortly afterwards.
The female stores the arm in her mantle cavity until she is ready to breed. She lays up to 100,000 eggs, which she then fertilises with the male’s sperm.
The eggs are carried in a gelatinous mass until they hatch into planktonic young, which drift in the open ocean before developing into juveniles.
More from DIVE’s Miscellany of Marine Curios:
- Marine Curios #21 – Hooded or lion nudibranch, Melibe leonina
- Marine Curios #20 – Sailor’s Eyeballs, Valonia ventricosa
- Marine Curios #19 – Torpedo ray, Torpedo spp
- Marine Curios #18 – Lampert’s Sea Cucumber, Syanptula lamperti
- The complete collection of DIVE’s Miscellany of Marine Curios
