Thysanoteuthis rhombus, also known as the diamond squid or diamondback squid, is a large species of squid growing to 100 cm in mantle length and a maximum weight of 30 kg, although it averages around 20 kg. The species occurs worldwide, throughout tropical and subtropical waters. Arms have 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 commercially fished in Japan, specifically in the Sea of Japan and Okinawa. The species is one of the only cephalopods observed exhibiting pair-like mating relationships. If one member of a male-female mating pair is caught by squid fishermen the other will likely remain in the area until it is also caught.
Sepioteuthis major, Cirrobranchium danae, Cirrobranchium filiferum, Thysanoteuthis danae, Thysanoteuthis elegans, Thysanoteuthis nuchalis
Thysanoteuthis rhombus has been assessed as Least Concern, as it is an oceanic species with a wide geographic distribution, making it less susceptible to human impact. It is targeted by commercial fisheries in some regions of its range (e.g. Japan) at certain times of the year. Catches appear to be consistent, suggesting that current levels are sustainable. However, further research is recommended in order to determine the precise distribution, population dynamics, life history and ecology, and potential threat processes affecting this species.