Spirula

Swedish botanist Carl Linnaeus described Nautilus spirula Linnaeus, 1758 in his book Systema Naturae. In 1799, French naturalist Jean-Baptiste Lamarck described the genus Spirula and transferred this species to it, and Spirula spirula is the name still used today for the ram's horn squid. S. spirula is the only species in the monotypic genus Spirula. A morphometric study published in 2010 showed that shell characteristics of S. spirula vary with geography, but no subspecies or additional species were proposed. == 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 The arms and tentacles can all be withdrawn completely into the mantle.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 (or, at most, has a vestigial radula). Unlike the nautilus, which exchange air and liquid only in the three most adoral chambers (the remaining chambers always being gas-filled), spirula can bring cameral fluid into all of their chambers. Another trait is that it is mineralized, a feature only seen in cuttlefish and the nautilus amongst extant species of cephalopods. The siphuncle is marginal, on the inner surface of the spiral. == Behaviour ==

File:Spirula spirula illustration.jpg|thumb|Dated illustration of a live animal, but shown upside down; the photophore on a live animal points downward. == Habitat and distribution ==

By day, Spirula lives in the deep oceans, reaching depths of 1,000 m. At night, it rises to 100–300 m. 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 species was observed for the first time in its natural habitat in 2020, when an ROV of the Schmidt Ocean Institute recorded it in the depths near the northern Great Barrier Reef. ==Evolutionary relationships==

The order Spirulida also contains two extinct suborders: Groenlandibelina (including extinct families Groenlandibelidae and Adygeyidae), and Belopterina (including extinct families Belemnoseidae and Belopteridae). 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. ==See also==