Spirula
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| Spirula spirula (Linnaeus, 1758) |
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The Ram's Horn Squid (Spirula spirula) is a cephalopod, the only extant member of the family Spirulidae, and order Spirulida. Live specimens are very rarely seen because they are deep ocean dwellers. It is also known as the Little Post Horn Squid or simply by its genus Spirula.
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[edit] Description
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 The arms and tentacles can all be withdrawn completely into the mantle.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 is the 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.
[edit] Habitat
Spirula inhabit continental slopes with water depths of 1,000 to 2,000 m. By day they dwell at a depth of 550 to 700 m and sometimes as deep as 1,000 m. At night, they rise to a depth of 100 to 300 m.
[edit] Distribution
Most sources cite them 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 in temperate regions, such as the western coasts of South Africa and New Zealand. These have possibly been carried long distances by ocean currents.
[edit] Taxonomy
The order Spirulida also contains two extinct suborders: Groenlandibelina (including extinct families Groenlandibelidae and Adygeyidae), and Belopterina (including extinct families Belemnoseidae and Belopteridae).
[edit] Evolutionary relationships
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.
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