Spirula

Spirula spirula Temporal range: Miocene–Recent[1] PreЄ Є O S D C P T J K Pg N
Dorsal view of female
Ventral view of female (chromatophores of mantle missing)
Scientific classification
Kingdom:	Animalia
Phylum:	Mollusca
Class:	Cephalopoda
Subclass:	Coleoidea
Superorder:	Decapodiformes
Order:	Spirulida
Suborder:	Spirulina
Family:	Spirulidae Owen, 1836
Genus:	Spirula Lamarck, 1799
Species:	S. spirula
Binomial name
Spirula spirula (Linnaeus, 1758)
Synonyms
Nautilus spirula Linnaeus, 1758

Spirula spirula is a species of deepwater squid-like cephalopod mollusk. 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,^[2] little post horn squid or 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 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.

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 The arms and tentacles can all be withdrawn completely into the mantle.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 species lacks a radula^{[3]:110[4]:26} (or, at most, has a vestigial radula).^[5]

Female with dissected mantle cavity (left) and immature specimens at various stages of development (centre and right)

Shell

The shell functions to osmotically control buoyancy.^[5] The most distinctive feature of this species is its buoyancy organ, an internal, chambered, endogastrically coiled^[5] 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.^[5] The gas-filled chambers keep the spirula in a vertical, head-down attitude.^[5]

Lateral (left) and dorsal (centre) views of a Spirula shell. In the latter, the siphuncle and the last septum of the phragmocone are visible. The position of the shell inside the mantle is shown in the illustration on the right.

Behaviour

End of mantle showing the photophore

Spirula are capable of emitting a green light from a photophore at the tip of their mantle.^[5]

Habitat and distribution

By day Spirula lives in the deep oceans, reaching depths of 1,000 m. At night, they rise to a depth of 100 to 300 m.^[6] Their preferred temperature is around 10°C, and they tend to live around oceanic islands, near the continental shelf.^[5]

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. 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 metres of the sea, for they are sometimes found in albatross guts.^[7]

Evolutionary relationships

Oral view of the left tentacular club

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

• Jellyella

References

1. ^ BW Hayward (1977). "Spirula (Sepioidea: Cephalopoda) from the Lower Miocene of Kaipara Harbour, New Zealand (note)". New Zealand Journal of Geology and Geophysics. http://www.kaiparaharbour.net.nz/publications/documents/Hayward1976Spirula%20from%20lower%20miocene%20of%20kaipara%20hrb%20NZ.pdf. 
2. ^ Norman, M. 2000. Cephalopods: A World Guide. Hackenheim, ConchBooks.
3. ^ Nixon, M (1985), "The buccal mass of fossil and recent Cephalopoda", in Wilbur, Karl M., The Mollusca, New York: Academic Press, ISBN 0-12-728702-7 
4. ^ Landman, Neil H; Tanabe, Kazushige; Davis, Richard Arnold (1996). Ammonoid paleobiology by Neil H. Landman, Kazushige Tanabe, Richard Arnold Davis. ISBN 9780306452222. http://books.google.ca/books?id=hKbkB4MzUIkC. 
5. ^ ^{a b c d e f g} Warnke, K.; Keupp, H. (2005). "Spirula—a window to the embryonic development of ammonoids? Morphological and molecular indications for a palaeontological hypothesis". Facies 51: 60. doi:10.1007/s10347-005-0054-9.  edit
6. ^ Clarke, M. R. (2009). "Cephalopoda collected on the SOND Cruise". Journal of the Marine Biological Association of the United Kingdom 49: 961. doi:10.1017/S0025315400038042.  edit
7. ^ Price, G. D.; Twitchett, R. J.; Smale, C.; Marks, V. (2009). "Isotopic Analysis of the Life History of the Enigmatic Squid Spirula Spirula, with Implications for Studies of Fossil Cephalopods". PALAIOS 24 (5): 273–279. doi:10.2110/palo.2008.p08-067r.  edit

External links

• CephBase: Spirula^{[dead link]}
• "Spirula Spirula". Tree of Life web project. http://tolweb.org/accessory/?acc_id=19989. 
• TONMO.com Spirulidae Forum