Chambered nautilus

A cephalopod in a coiled external shell to about 20 cm, cream with red-brown flame markings over the outer whorl and mother-of-pearl inside. The shell is divided into gas-filled chambers, and a strand of living tissue called the siphuncle runs through them and controls the balance of gas and fluid, which is how the animal holds its depth. The body occupies only the outermost chamber. There are up to about ninety tentacles, none of them with suckers; they grip by ridges and mucus. The eye is a pinhole with no lens, open to the sea, and vision is correspondingly poor.

It is a scavenger and an opportunist, taking carrion, moults and slow prey found largely by smell. During the day it stays deep, several hundred metres down the slope, and it moves up at night to feed, which is the only reason a diver ever meets one. Growth is slow, maturity takes years and few eggs are laid, so populations recover from fishing badly; the genus is listed on CITES Appendix II largely because of the shell trade.

Encounters are confined to steep slopes and drop-offs with deep water beneath them, and in this region they are reported from Papua New Guinea rather than from the muck sites further west. They are night events, at the deep end of the dive, and irregular enough that no useful frequency can be given. Animals brought up quickly and held in warm surface water do badly, which is worth knowing wherever traps are used to arrange a sighting.