On beach, Solomon Islands, 24.5mm.
The « Ram's Horn Squid » is a very unique cephalopod with a loosely coiled chambered shell. It is a not a true squid (order Teuthida) but the only surving species of the order Spirulida; its shell is a true shell and is actually internalised like a true squid but is visible from the outside even in a live animal. It is considered to be the closest extant relative of the extinct belemnites (order Belemnitida), and is also closely related to extant cuttlefishes and squids. Like its distant cousin the nautiluses, it uses the chambered shell (around 25-37 chambers) as a buoyancy organ for vertical movement by controlling the gas / liquid ratio in the chambers using osmosis. A deep-sea carnivorous cephalopod distributed throughout the world's tropical oceans, it lives in water column around continental shelf down to great depths of -1000m and is most often seen around -200~700m although very occasionally also ventures into shallow water less than -20m deep. It exhibits diurnal vertical migration, rising to shallower depths for feeding at night and spends the day in the deep. Like many deep-sea cephalopods it is capable of bioluminescence from a photophore at the very posterior end between the fins, which emits a green light. Its radula is extremely reduced. The animal is very rarely seen but the shell is common and often washes up to beaches and shores, because after the animal dies the gas-filled shell floats to the sea surface and becomes pelagic. Typical shell length around 25mm., very large specimens may exceed 35mm. The animal is typically around 35mm. in mantle length and 70mm. in entire length.