''Spirula spirula'' is a

species In biology, a species is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of the appropriat ...

of deep-water

squid True squid are molluscs with an elongated soft body, large eyes, eight arms, and two tentacles in the superorder Decapodiformes, though many other molluscs within the broader Neocoleoidea are also called squid despite not strictly fittin ...

-like

cephalopod A cephalopod is any member of the molluscan class Cephalopoda ( Greek plural , ; "head-feet") such as a squid, octopus, cuttlefish, or nautilus. These exclusively marine animals are characterized by bilateral body symmetry, a prominent head ...

mollusk Mollusca is the second-largest phylum of invertebrate animals after the Arthropoda, the members of which are known as molluscs or mollusks (). Around 85,000 extant species of molluscs are recognized. The number of fossil species is e ...

. It is the only

extant Extant is the opposite of the word extinct. It may refer to: * Extant hereditary titles * Extant literature, surviving literature, such as ''Beowulf'', the oldest extant manuscript written in English * Extant taxon, a taxon which is not extinct, ...

member of the

genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification, genus comes above species and below family. In binomial nom ...

''Spirula'', the

family Family (from la, familia) is a group of people related either by consanguinity (by recognized birth) or affinity (by marriage or other relationship). The purpose of the family is to maintain the well-being of its members and of society. Idea ...

Spirulidae, and the

order Order, ORDER or Orders may refer to: * Categorization, the process in which ideas and objects are recognized, differentiated, and understood * Heterarchy, a system of organization wherein the elements have the potential to be ranked a number of ...

Spirulida. Because of the shape of its internal shell, it is commonly known as the ram's horn squid or the little post horn squid. Because the live animal has a light-emitting organ, it is also sometimes known as the 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

beachcomber A beachcomber is a person who practices beachcombing. Beachcomber or Beachcombers may also refer to: Arts and entertainment * ''The Beachcomber'' (1915 film), an American drama * ''The Beachcomber'' (1938 film), starring Charles Laughton and a ...

s. The shell of ''Spirula'' is extremely light in weight, very buoyant, and surprisingly durable; it very commonly floats ashore onto tropical beaches (and sometimes even temperate beaches) all over the world. This

seashell A seashell or sea shell, also known simply as a shell, is a hard, protective outer layer usually created by an animal or organism that lives in the sea. The shell is part of the body of the animal. Empty seashells are often found washe ...

is known to shell collectors as the ram's horn shell or simply as ''Spirula''.

Description

''S. spirula'' has a

-like body between 35 mm and 45 mm long. It is a decapod, with eight arms and two longer

tentacle In zoology, a tentacle is a flexible, mobile, and elongated organ present in some species of animals, most of them invertebrates. In animal anatomy, tentacles usually occur in one or more pairs. Anatomically, the tentacles of animals work main ...

s, all with suckers. The arms and tentacles can all be withdrawn completely into the

mantle A mantle is a piece of clothing, a type of cloak. Several other meanings are derived from that. Mantle may refer to: *Mantle (clothing), a cloak-like garment worn mainly by women as fashionable outerwear **Mantle (vesture), an Eastern Orthodox ve ...

. The species lacks a radula (or, at most, has a vestigial radula).

Shell

The most distinctive feature of this species is its buoyancy organ, an internal, chambered, endogastrically coiled 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. The shell functions to osmotically control

buoyancy Buoyancy (), or upthrust, is an upward force exerted by a fluid that opposes the weight of a partially or fully immersed object. In a column of fluid, pressure increases with depth as a result of the weight of the overlying fluid. Thus the ...

. Another trait is that it is mineralized, a feature only seen in

cuttlefish Cuttlefish or cuttles are marine molluscs of the order Sepiida. They belong to the class Cephalopoda which also includes squid, octopuses, and nautiluses. Cuttlefish have a unique internal shell, the cuttlebone, which is used for control of ...

and the

nautilus The nautilus (, ) is a pelagic marine mollusc of the cephalopod family Nautilidae. The nautilus is the sole extant family of the superfamily Nautilaceae and of its smaller but near equal suborder, Nautilina. It comprises six living species in ...

amongst extant species. The siphuncle is marginal, on the inner surface of the spiral. left, 475px, Illustration showing the position of the shell inside the mantle center, 240px, End of mantle showing the photophore

Behaviour

''S. spirula'' is capable of emitting a green light from a

photophore A photophore is a glandular organ that appears as luminous spots on various marine animals, including fish and cephalopods. The organ can be simple, or as complex as the human eye; equipped with lenses, shutters, color filters and reflectors, ...

located at the tip of its mantle, between the ear-shaped fins. Evidently this seems as a

counter-illumination Counter-illumination is a method of active camouflage seen in marine animals such as firefly squid and midshipman fish, and in military prototypes, producing light to match their backgrounds in both brightness and wavelength. Marine animals ...

strategy, as ''in situ'' observations have captured footage of animals in a vertical stance, with photophore pointing downward and head up.

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. Its preferred temperature is around 10 °C, and it tends to live around oceanic islands, near the continental shelf. Most sources cite this species as

tropical The tropics are the regions of Earth surrounding the Equator. They are defined in latitude by the Tropic of Cancer in the Northern Hemisphere at N and the Tropic of Capricorn in the Southern Hemisphere at S. The tropics are also referred to ...

, and they are observed to be plentiful in the

subtropical The subtropical zones or subtropics are geographical and climate zones to the north and south of the tropics. Geographically part of the temperate zones of both hemispheres, they cover the middle latitudes from to approximately 35° north a ...

seas around the

Canary Islands The Canary Islands (; es, :es:Canarias, Canarias, ), also known informally as the Canaries, are a Spanish Autonomous communities of Spain, autonomous community and archipelago in the Atlantic Ocean, in Macaronesia. At their closest point to ...

. Shells are regularly found along the western coasts of

South Africa South Africa, officially the Republic of South Africa (RSA), is the southernmost country in Africa. It is bounded to the south by of coastline that stretch along the South Atlantic and Indian Oceans; to the north by the neighbouring coun ...

. However, significant quantities of shells from dead spirula are washed ashore even in temperate regions, such as coasts of

New Zealand New Zealand ( mi, Aotearoa ) is an island country in the southwestern Pacific Ocean. It consists of two main landmasses—the North Island () and the South Island ()—and over 700 smaller islands. It is the sixth-largest island coun ...

. 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 Schmidt Ocean Institute (SOI) is a 501(c)(3) non-profit operating foundation established in March 2009 by Eric Schmidt and Wendy Schmidt. The Institute's goal is to advance innovative oceanographic research and discovery through technological adv ...

recorded it in the depths near the northern

Great Barrier Reef The Great Barrier Reef is the world's largest coral reef system composed of over 2,900 individual reefs and 900 islands stretching for over over an area of approximately . The reef is located in the Coral Sea, off the coast of Queensland, A ...

Evolutionary relationships

The order Spirulida also contains two extinct suborders: Groenlandibelina (including extinct families Groenlandibelidae and

Adygeyidae ''Adygeya'' is a genus of cephalopods assigned to the Spirulida.Doguzhaeva, L. A., Mapes, R. H., & Mutvei, H. (1999). A Late Carboniferous spirulid coleoid from the southern mid-continent (USA): shell wall ultrastructure and evolutionary implic ...

), and

Belopterina Spirulida is an order of cephalopods comprising one extant species (''Spirula spirula'') and several extinct taxa. Fossil record * Oldest representative: Carboniferous, though contested: see ''Shimanskya'' * Oldest uncontested representative ...

(including extinct families Belemnoseidae and Belopteridae). ''Spirula'' is likely the closest living relative of the extinct

belemnite Belemnitida (or the belemnite) is an extinct order of squid-like cephalopods that existed from the Late Triassic to Late Cretaceous. Unlike squid, belemnites had an internal skeleton that made up the cone. The parts are, from the arms-most ...

s and aulacocerids. These three groups as a unit are closely related to the

, as well as to the true squids.

References

External links

* * {{Taxonbar, from=Q1509955 Cephalopod genera Monotypic mollusc genera Cenozoic cephalopods Miocene animals Pliocene animals Extant Miocene first appearances Taxa named by Jean-Baptiste Lamarck Molluscs described in 1758 Taxa named by Carl Linnaeus Bioluminescent molluscs