Tonna perdix

Tonna perdix, common name the partridge tun, is a species of very large sea snail, a marine gastropod mollusc in the family Tonnidae, the tun shells.

Description
The pattern and brownish colour of the shell is reminiscent of a partridge's plumage hence the common name. The size of an adult shell varies between 70 mm and 220 mm.

The ovate-oblong shell is ventricose and pretty thin. It is of a reddish-brown color, pleasingly varied with white spots in transverse series, for the most part semi-lunar, and more or less distant. The conical spire is slightly projecting and, pointed. It is composed of from five to six whorls which are furnished with numerous ribs, often widened, feebly convex, and separated by furrows hardly apparent. The suture is very distinct and is slightly channeled towards the body whorl. The aperture is large, subovate, marked by transverse and slightly projecting bands, which correspond to the furrows of the exterior. The interior of this cavity is of a fawn color. The outer lip is thin, everted, a little undulated, and adorned with a white band the whole length of its interior, with the exception of the edge, which is of a deep brown. The inner lip is spread out over the body of the shell. It is very thin, transparent, and terminated below by a projecting plate which covers the umbilicus, and seems also to form a part of it. The columella is smooth, polished, and forms at the left of the umbilicus a thick rib, marked by transverse striae, which terminates at the emargination of the base.

The ground color of the shell is a bluish white, covered with irregular spots of a red brown, bordering upon the violet. The foot is colored in the same manner beneath, but above, it is furrowed with wide triangular radiating bands. The tentacles are doubly ringed with brown.

Distribution
This species occurs in the Red Sea and in the Indian Ocean off Aldabra, Chagos, Madagascar, the Mascarene Basin, Mauritius and Tanzania, and in the Pacific Ocean off New Zealand, Australia (Northern Territory, Queensland, Western Australia) and the Galapagos Islands.

The mollusk of this shell inhabits rapid waters, and appears pretty quick in its movements.

Biology and diet
This gastropod is a highly specialized carnivorous predator, preying on sea cucumbers.