Spirula spirula

Spirula spirula is a species of deep-water squid-like cephalopod mollusk. It is the only extant member of the genus Spirula, the family Spirulidae, and the order Spirulida. Because of the shape of its internal shell, it is commonly known as the ram's horn squid 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 (and sometimes even temperate beaches) all over the world. This seashell is known to shell collectors as the ram's horn shell or simply as Spirula. 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 The arms and tentacles can all be withdrawn completely into the mantle.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:110:26 (or, at most, has a vestigial radula). The most distinctive feature of this species is its buoyancy organ, an internal, chambered, endogastrically coiled shell in the shape of an open planispiral (a flat spiral wherein the coils do not touch each other), 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. S. spirula is capable of emitting a green light from a photophore located at the tip of its mantle, between the ear-shaped fins. By day, Spirula lives in the deep oceans, reaching depths of 1,000 m. At night, it rises to 100 to 300 m. Its preferred temperature is around 10 °C, and it tends to live around oceanic islands, near the continental shelf.