Octopus

A backup second stage on a longer hose, usually bright yellow, that lets a diver share gas with an out-of-air buddy.

The octopus is your spare mouthpiece, a second demand valve connected to the same first stage as your primary. Its whole purpose is gas sharing: if your buddy runs out of air or has a regulator failure, they breathe from your octopus while you both make a controlled ascent. It hangs on a slightly longer hose, typically around 90 to 100 cm, so two divers can swim side by side while sharing.

Octopus second stages are usually high visibility yellow and clipped or tucked into a marked spot in the triangle between your chin and the corners of your rib cage, where either diver can reach it in seconds. Locating your buddy's alternate is a fixed step of the pre-dive buddy check, and air sharing drills with it are practiced in every entry level course.

Some divers replace the traditional octopus with an inflator-integrated alternate, and many technical and guided divers instead donate the primary on a long hose and switch to a necklaced backup. Whatever the configuration, both divers must know exactly where the spare gas source lives before the dive starts.