delle Chiaje, 1830
Mantle semiovoid or subtriangular in juveniles and males, ovoid in adult females, fused with head and connected with funnel by connective apparatus. Body firm, muscular, mantle surface smooth. Head wide, eyes large. Mantle opening wide, its upper edges reaching level of upper eye margin. Funnel long, its anterior end reaching anterior eye margin. Funnel free. Two pairs of skin pores (water pores) on the head, dorsally between bases of 1st and 2nd arms and ventrally beside funnel opening. Arms in larvae short, 1st longest, 2nd and 4th shorter, 3rd very short. Web very short in larvae, deep between 1st and 2nd arms in juveniles, these two arm pairs bordered by a film narrowing toward the arm ends. In immature females ends of 1st arms tapering into thin whip, their film widened into wide ragged film-like tissue on 1st arms, in adults gradually torn off, becoming shorter than on 2nd arms. Web vestigial between 3rd and 4th arms. Three primary suckers in one row, enlarging from 1st to 3rd, then 2 rows of small suckers on all arms, but on ends of 1st arms suckers arranged in 1 row and very far apart. Ink sac present, photophores present on 1st arm film in one subspecies (T. v. gracilis). Males dwarfed, their 3rd right arm wholly modified and enclosed in sac under skin, thus appearing that immature males (even newborns) have 7 arms. During maturation hectocotylus everts and becomes longer than specimen itself, detaches during mating. Ends of 1st and 2nd arms in males thin, without film. 13-16 lamellae per demibranch in females, 9-11 in males. Eggs (3.0-3.6 by 1.8-2.1 mm) connected with stalks, egg cluster (>100,000) born by female on its 1st arms. Color of females bright-blue to wine-red from above, silver tinged with rose or light gold from below. Females of the Atlantic subspecies Tremoctopus violaceus violaceus with row of ocellar dark spots along ventral (outer) side of film of 1st arms; females of the Indo-Pacific T. v. gracilis with these spots in 2 rows (or in a double row) along ventral (outer) side of 1st arm film, and in 1 row along dorsal (inner) side of 2nd arms; one row of elongate yellowish (in daylight) bodies along margin of 1st arm film is luminescent. Larvae, juveniles, and males semitransparent with dispersed small chromatophores. Males and juveniles (to ML approx. 15 mm) usually with pieces of burning tentacles of Physalia physalis, the Portuguese Man O'War in the 1st-2nd arm suckers, as an additional method of defense. Hatchlings ML <2 mm. Larvae without jelly coating but head and arms enveloped by a cuff-shaped membrane. Some suckers on the middle of 1st arms enlarged (last ones in one row), suckers on 1st arms larger than those on 2nd-4th. Web on 1st arms begins to form at ML approx. 10 mm. One row of chromatophores along outer side of 1st arms in larvae, 2 in juveniles. Larvae distinguishable by long 1st arms. ML of females up to 30-45 cm, of males to approx. 1.5 cm.
Two subspecies, one in the Atlantic (Tremoctopus violaceus violaceus). Other, Tremoctopus violaceus gracilis (Souleyet, 1852), in the Indian and Pacific oceans. Juveniles rise to the surface at night; adult males and females live constantly near the surface.