Diamond squid,, Diamondback squid, Rhomboid squid
Thysanoteuthis rhombus, also known as the diamond squid, diamondback squid, or rhomboid squid, is a large species of squid from the family Thysanoteuthidae which is found worldwide, throughout tropical and subtropical waters. T. rhombus is given its name for the appearance of the fins that run the length of the mantle. They are a fast growing species with a lifespan of approximately 1 year. The diamond squid is the only cephalopod species known to be monogamous. T. rhombus often preys on fish and other small cephalopods at varying water depths. This species is commercially fished in Japan, specifically in the Sea of Japan and Okinawa.
Thysanoteuthis rhombus are distinguishable by the presence of arms with two series of suckers, whereas the tentacular clubs have four. It lacks photophores. T. rhombus is named for its fins, which run in equal length along the mantle, giving the appearance of a rhombus. The species is able to grow up to 100 cm (3.3 ft) in mantle length and a maximum weight of 30 kg, although it averages around 20 kg. 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.
The lifespan of T. rhombus is 1 year. Males mature at a mantle length between 400 to 550mm (170 to 200 days of age) while females mature at a mantle length between 550 to 650mm (230 to 250 days).
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. The species feeds during the daytime at deep water levels and during the night at shallow water levels. T. rhombus are often found in pairs, but groups up to 20 have been recorded.
T. rhombus typically inhabits open ocean waters of the subtropical and tropical locations with temperatures of >20 °C. 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.
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 400 to 650m.
The pairing of males and females occurs at an immature stage where mantle length is less than 100mm and pairs remain monogamous. 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. Spawning is year round in tropical waters and lasts for 2–3 months, but in temperate regions spawning is more 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. 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. This swollen secretion will then be molded by the female into a cylinder. Female's 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 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. The fertilized egg cord is then wound onto the cylinder. A female can produce 8 to 12 masses if properly utilizing its vitelline oocytes.
