| Spirula spirula | ||||||||||||||||||||
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| Spirula spirula (Linnaeus, 1758) |
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Spirula spirula is a species of deepwater squid-like cephalopod. It is the only extant member of the genus Spirula, the family Spirulidae, and the order Spirulida. It is commonly known as the ram's horn squid[1] or little post horn 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 quite strong; 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.
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Spirula have a squid-like body between 35 mm and 45 mm long. They are decapods, with 8 arms and 2 longer tentacles, all with suckers. The arms and tentacles can all be withdrawn completely into the mantle.
The most distinctive feature of this species is its buoyancy organ, an internal shell, itself chambered, in the shape of an open planispiral (a flat spiral wherein the coils do not touch each other). The gas-filled chambers keep the spirula in a vertical, head-down attitude. The posterior also contains a light-emitting organ that can glow for hours at a time.
 Two shells of Spirula spirula from Mission Beach, Queensland, Australia, 2002 |
 Another view of two shells of Spirula spirula, Mission Beach, Queensland, Australia, 2002 |
Spirula inhabit continental slopes with water depths of 1,000 to 2,000 m. By day these cephalopods dwell at a depth of 550 to 700 m, sometimes as deep as 1,000 m. At night, they rise to a depth of 100 to 300 m.
Most sources cite this species as tropical, and they are observed to be plentiful in the seas around the Canary Islands. However, significant quantities of shells from dead Spirula are washed ashore even in temperate regions, such as the western coasts of South Africa and New Zealand. These have possibly been carried long distances by ocean currents.
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.