The only squid known to live in a permanent pair, and one of the few fished commercially with no sign of stock collapse.
Named for its fins, which run the whole length of the mantle and meet at the tail to make a rhomb. An adult reaches a metre of mantle and perhaps thirty kilograms. What a blackwater diver sees is almost always a juvenile of a few centimetres, and at that size the fins are already the giveaway.
Two things make the species unusual. It lives in pairs — a male and a female that stay together for life, which is not known for certain in any other squid — and it lays its eggs in a floating gelatinous tube one to two metres long, drifting near the surface, with the eggs arranged in a spiral through the wall. Divers occasionally swim into one of these tubes and take it for a salp chain.
It is fished commercially off Japan, where it is caught with vertical longlines set in the open ocean, and unlike most large squid fisheries this one has not collapsed — probably because the animal is fast-growing, short-lived and spread thinly across the whole tropical and warm-temperate ocean rather than concentrated on a shelf.
Ratings are per location: 1 means found on most dives that look for it, 5 means a handful of records. Where enough dives have been logged the figure is calculated from them; otherwise it is an editorial estimate, and says so. We publish habitat and destination, not the dive site.
