common spirula

<em>Spirula spirula</em>, commonly known as the common spirula or ram's horn squid, is a small deep-sea cephalopod mollusk in the family Spirulidae, representing the sole extant species of the order Spirulida. This species typically inhabits mesopelagic and bathypelagic depths in tropical and subtropical oceanic waters worldwide, typically found between approximately 200 and 1,000 metres depth during the day, undertaking diel vertical migrations toward the surface at night to feed. Its geographic range is circumglobal in warm oceanic waters, with its distinctive coiled chambered internal shell commonly washing ashore on beaches in tropical and subtropical regions worldwide. The species has not been formally evaluated under IUCN Red List criteria. <em>Spirula spirula</em> is unique among living cephalopods for possessing an internal coiled aragonite shell that functions as a buoyancy device, similar in principle to the shell of <em>Nautilus</em>. The animal typically reaches a mantle length of approximately 4–5 cm and is carnivorous, feeding typically on small planktonic crustaceans and other invertebrates. Biological traits such as average lifespan in years and precise body weight measurements remain poorly documented due to the difficulty of observing live individuals in their deep-sea habitat. Bioluminescent organs at the posterior end of the mantle are used for counterillumination.