Mature specimens of Illex coindetii from different populations can vary significantly in both size and form.

[3] The upper part of the beak (rostrum) has a long and robust hood, smooth shoulder and short wings.

The funnel–mantle locking apparatus (cartilaginous structure connecting the funnel to the mantle) is T-shaped, a characteristic of all ommastrephid squids.

[8] The species was first described as Loligo coindetii by the French naturalist Jean Baptiste Vérany in 1837, from a specimen recovered from the Mediterranean Sea in Nice, France.

Populations can vary in form and size considerably, but two consistent morphotypes have been tentatively identified between specimens from the Mediterranean and the Atlantic.

[10][9] Illex coindetii inhabit muddy, sandy, and silty bottoms often in association with sea pens of the genus Funiculina.

[10] Larger and smaller individuals are more often found in deeper and shallower waters respectively, but otherwise the adults and juveniles share the same depth range.

[10][4] The dominant prey of Illex coindetii includes fish, crustaceans (particularly krill in some populations), and other cephalopods.

Like other members of the family Ommastrephidae, the paralarvae exhibit distinctive fused tentacles called the "proboscis".

The proboscis begins to divide into the pair of tentacles once the paralarvae reach a mantle length of 4 mm (0.16 in).

[6] In the Mediterranean, they constitute an important by-catch of the fishing industry in Sicily, along with the European flying squid (Todarodes sagittatus).