| Spirula spirula Temporal range: Miocene–Recent[1] |
|
|---|---|
| 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 | |
|
|
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.
Contents |
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 The arms and tentacles can all be withdrawn completely into the mantle.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]
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]
Spirula are capable of emitting a green light from a photophore at the tip of their mantle.[5]
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]
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.