Spirula spirula is a species of deep-water squid-like cephalopodmollusk. It is the only extant member of the genusSpirula, the familySpirulidae, and the orderSpirulida. Because of the shape of its internal shell, it is commonly known as the ram's hornsquid or the little post horn squid. Because the live animal has a light-emitting organ, it is also sometimes known as the tail-light squid. Live specimens of this cephalopod are very rarely seen because it is a deep-ocean dweller. The small internal shell of the species is, however, quite a familiar object to many beachcombers. The shell of Spirula is extremely light in weight, very buoyant, and surprisingly durable; it very commonly floats ashore onto tropical beaches all over the world. This seashell is known to shell collectors as the ram's horn shell or simply as Spirula.
Description
S. spirula has a squid-like body between 35 mm and 45 mm long. It is a decapod, with eight arms and two longer tentacles, all with suckers. The arms and tentacles can all be withdrawn completely into the mantle. The species lacks a radula.
Shell
The most distinctive feature of this species is its buoyancy organ, an internal, chambered, endogastrically coiled shell in the shape of an open planispiral, and consisting of two prismatic layers. The shell functions to osmotically control buoyancy; the gas-filled chambers keep the spirula in a vertical, head-down attitude. Another trait is that it is mineralized, a feature only seen in cuttlefish and the nautilus amongst extant species. The siphuncle is marginal, on the inner surface of the spiral.
By day, Spirula lives in the deep oceans, reaching depths of 1,000 m. At night, it rises to 100–300 m. Its preferred temperature is around 10 °C, and it tends to live around oceanic islands, near the continental shelf. Most sources cite this species as tropical, and they are observed to be plentiful in the subtropical seas around the Canary Islands. Shells are regularly found along the western coasts of South Africa. However, significant quantities of shells from dead spirula are washed ashore even in temperate regions, such as coasts of New Zealand. Because of the great buoyancy of the shells, these may possibly have been carried long distances by ocean currents. Much of the organism's life history has not been observed; for instance, they are thought to spawn in winter in deeper water, yet no spawnlings have been directly seen. They must occasionally venture into the upper 10 m of the sea, for they are sometimes found in albatross guts.
The order Spirulida also contains two extinct suborders: Groenlandibelina, and Belopterina. Spirula is likely the closest living relative of the extinct belemnites and aulacocerids. These three groups as a unit are closely related to the cuttlefish, as well as to the true squids.
