Tonna pennata is a large species of sea snail, a marine gastropod mollusc in the family Tonnidae, the tun shells.

[2] Commonly known as the Atlantic partridge tun,[3] the species is a predator of animals much larger than itself, particularly sea cucumbers.

This shell varies in colour from beige to dark brown, and the whitish markings are quite variable, which may account for the large number of synonyms that exist.

It attacks prey much larger than itself, such as the slow-moving sea cucumber Isostichopus badionotus, which can grow to a length of 50 cm (20 in).

It stiffens the area round the injury which then softens, and muscular contractions cause a patch of integument to autotomise (come away from its body).