[7] T. rhombus is not an active swimmer and propels itself slowly using its triangular fins, although the species is able to make powerful contractions of its mantle to escape predation.

[9] T. rhombus has a relatively low population density, which has led to a unique situation among squids, namely monogamy, where male and female couples of the same size remain together from their juvenile stage until death.

[10] The diamond squid was found to be largely inactive or even die at depths of 0–100m due to sudden drops in temperatures below 15 °C.

[13] As adults, the stomach contents of the Diamond squid were found to consist mostly of nonactive fishes at water depths of 400 to 650m.

[17] Mating occurs in a head-to-head position, in which the male uses its hectocotylus to attach to the female's buccal membrane to transfer its spermatophores.

[18] Females will produce secretions of gel-like substance from nidamental glands, similar to the Japanese flying squid, during spawning that will enter the water and swell.

The fused cord exits through into the water through the funnel where the eggs are met and fertilized with spermatozoa from seminal receptacles that were attached to the female's buccal membrane.

To capture the diamond squid in the Sea of Japan, inshore trap nets and free-floating angling gear called "taru-nagashi" are used.

[4] "Taru-nagashi" is gear that consists of a vertical weighted long line with two or three artificial lures and with two or three rows of stainless steel hooks.