Hatching time varies with temperature, but generally takes about two weeks. After a period of growth and development, the larval squid becomes a juvenile about 6 mm in mantle length with adult features. By this time, the proboscis has split to form the two tentacles, and the other eight arms have grown larger. The juvenile Illex live in the Gulf Stream frontal zone and the Slope Water off the edge of the Continental Shelf until they reach about 10 cm in mantle length and move into the adult feeding areas on the Shelf. Little is known of juvenile growth rates, but the adults grow rapidly adding roughly 1.5 mm in mantle length per day, to reach a maximum size of 25 to 30 cm in mantle length in October or November.
Adults eat voraciously, consuming a variety of crustacea and fish.
Smaller squid tend to feed more heavily on small crustacea such as euphasids, turning more to fish and fellow squid as they mature. The extent of cannibalism among squid is unclear, but it would appear that the larger specimens are the most inclined to eat their own species. Squid are also a food source for a variety of fish, marine mammals and birds. Some of the better-known predators are pilot whales, dolphins, shearwaters, fulmars, gannets, gulls, tunas, swordfish, haddock, cod, pollock and sharks. Squid spend the daylight hours near the bottom of the ocean, seeming to prefer areas where the bottom temperature is 6 to 7 C or greater.