T. rhombus typically inhabits open ocean waters of the subtropical and tropical locations with temperatures of . The diamond squid was found to be largely inactive or even die to sudden drops in temperatures, which went below This species feeds during the daytime at deep water levels and during the night at shallow water levels (
Diel vertical migration).
Ecology Diet In subsurface water levels,
T. rhombus juveniles were found to feed on crustaceans, small cephalopods and
fishes. As adults, the stomach contents of the Diamond squid were found to consist mostly of nonactive fishes at water depths of .
Predation The predators that feed on
T. rhombus include various species of
ommastrephid squids,
giant squid (
Architeuthis dux),
dolphinfish (
Coryphaena hippurus),
lancetfish (
Alepisaurus sp.),
tuna,
swordfish (
Xiphias gladius),
snake mackerel (
Gempylus serpens), and
sharks. Other predators include
cetaceans;
oceanic dolphins such as
rough-toothed dolphins (
Steno bredanensis) and
false killer whales (
Pseudorca crassidens), along with
sperm whales (
Physeter macrocephalus), prey on diamond squid.
Reproduction T. rhombus has a relatively low population density, which has led to a unique situation among squids, where male and female couples of the same size remain together from their juvenile stage until death; this species is
monogamous. and
La Gomera. The pairing of males and females occurs at an immature stage where mantle length is less than and pairs remain monogamous. Mating occurs in a head-to-head position, in which the male uses his
hectocotylus to attach to the female's
buccal membrane to transfer his
spermatophores. Spawning is year round in tropical waters and lasts for 2–3 months, but in temperate regions spawning is restricted to summer or early autumn periods and warmer currents.
T. rhombus is known to be an intermittent spawner and is known to have
multiple spawning in succession. During spawning, females will produce secretions of gel-like substance from
nidamental glands, similar to the
Japanese flying squid, that will enter the water and swell; this engorged secretion will then be molded by the female into a cylinder. Her oviductal glands will then begin to form two
mucous threads, each with one row of eggs, which will fuse into a single cord containing a double row of eggs in the mantle cavity. The fused egg-cord or mass is expelled into the water through the
funnel where the eggs are fertilized with spermatozoa that were attached to the female's buccal membrane, in her
seminal receptacles. The fertilized egg cord is then wound onto the cylinder. A female can produce 8 to 12 egg-masses if properly utilizing its
vitelline oocytes. == Commercial value ==