Ink defence works by blocking a predator's view or leaving a decoy shape in the water. Both depend on light. In the deep sea, where the only illumination is bioluminescence, that defence loses most of its value, and the cirrate octopuses, including Grimpoteuthis, lack a functional ink sac.
The group is unusual in other respects. Rather than jetting through a funnel like shallow-water octopuses, they flap a pair of fins that emerge from the mantle above the eyes, and they use the webbing between their arms to make slow, medusa-like pulses. Their arms carry cirri, thin filaments alongside the suckers, thought to help move small prey towards the mouth. They swallow prey such as copepods and polychaete worms whole.
Most of what is known comes from a small number of specimens; observations from remotely operated vehicles have recorded individuals below 4,000 metres, and one record extends near 7,000 metres. Species-level identification within Grimpoteuthis remains difficult, and how many species exist is unclear.