Saying, “This is my territory” with squid ink: The unique ink-spitting behavior of male spear squid

by the University of Tokyo, Atmosphere and Ocean Research Institute
October 26, 2026

Squid and octopus ink is generally used to obstruct the vision of predators or to create a false image of themselves to deceive them. However, in recent years, there have been numerous reports of cephalopods using ink for purposes other than defense, such as predation, courtship, and mating (related press release), revealing that cephalopods utilize ink more skillfully than previously thought.

In the Japanese spear squid (Heterololigo bleekeri), females lay egg masses in crevices of rocks and structures, while males form territories around these spawning grounds and exclude other males that intrude. In this study, we observed in detail the territorial behavior of such male Japanese spear squid in a large tank at the Hakodate International Fisheries and Marine Research Center. As a result, it became clear that males that have formed territories (territorial males) release ink at rival males and females, and even when there are no targets nearby. Furthermore, the ink released by territorial males was all in small amounts and highly diffusive, dissolving instantly and becoming invisible, which was significantly different in nature from the viscous, clumpy ink that Japanese spear squid use for defense. From this, it was suggested that the ink release of Japanese spear squid discovered in this study is a characteristic behavior specific to the situation of territorial behavior, and that it may play a role in territorial males appealing to other individuals with their strength and attractiveness. Furthermore, considering its high diffusivity and the fact that it can be used even when there is no target present, it was considered highly likely that this ink functions more as a chemical signal mediated by its components than as a visual signal directly shown to the target.

Keep reading (in Japanese): https://www.aori.u-tokyo.ac.jp/research/news/2026/20261026.html

Read the Ecology paper: https://esajournals.onlinelibrary.wiley.com/doi/10.1002/ecy.70576