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. Description 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). Behavior 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. 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 400