Ram's Horn Squid
Encyclopedia
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, 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 beachcomber
s. 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.
-like body between 35 mm and 45 mm long. They are decapods
, with 8 arms
and 2 longer tentacle
s, all with suckers. The arms and tentacles can all be withdrawn completely into the mantle
.
The species lacks a radula (or, at most, has a vestigial radula).
control buoyancy
. 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 gas-filled chambers keep the spirula in a vertical, head-down attitude.
at the tip of their mantle.
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.
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.
Species
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...
of deepwater squid
Squid
Squid are cephalopods of the order Teuthida, which comprises around 300 species. Like all other cephalopods, squid have a distinct head, bilateral symmetry, a mantle, and arms. Squid, like cuttlefish, have eight arms arranged in pairs and two, usually longer, tentacles...
-like cephalopod
Cephalopod
A cephalopod is any member of the molluscan class Cephalopoda . These exclusively marine animals are characterized by bilateral body symmetry, a prominent head, and a set of arms or tentacles modified from the primitive molluscan foot...
mollusk. It is the only extant member of the genus
Genus
In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...
Spirula, the family
Family (biology)
In biological classification, family is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, class, order, genus, and species, with family fitting between order and genus. As for the other well-known ranks, there is the option of an immediately lower rank, indicated by the...
Spirulidae, and the order
Order (biology)
In scientific classification used in biology, the order is# a taxonomic rank used in the classification of organisms. Other well-known ranks are life, domain, kingdom, phylum, class, family, genus, and species, with order fitting in between class and family...
Spirulida
Spirulida
Spirulida is an order of cephalopods comprising one extant species and several extinct taxa.-Classification:*Order Spirulida**?Family Shimanskyidae**Suborder †Groenlandibelina***Family †Groenlandibelidae...
. It is commonly known as the ram's horn squid, 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 beachcomber
Beachcomber
Beachcomber may refer to:* Beachcomber * Beachcomber , a nom de plume used by several humor columnists* Beachcomber , the name of multiple characters in the Transformers universe...
s. 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
Seashell
A seashell or sea shell, also known simply as a shell, is a hard, protective outer layer created by an animal that lives in the sea. The shell is part of the body of the animal. Empty seashells are often found washed up on beaches by beachcombers...
is known to shell collectors
Conchology
Conchology is the scientific or amateur study of mollusc shells. Conchology is one aspect of malacology, the study of molluscs, however malacology studies molluscs as whole organisms, not just their shells. Conchology pre-dated malacology as a field of study. It includes the study of land and...
as the ram's horn shell or simply as Spirula.
Description
Spirula have a squidSquid
Squid are cephalopods of the order Teuthida, which comprises around 300 species. Like all other cephalopods, squid have a distinct head, bilateral symmetry, a mantle, and arms. Squid, like cuttlefish, have eight arms arranged in pairs and two, usually longer, tentacles...
-like body between 35 mm and 45 mm long. They are decapods
Decapodiformes
Decapodiformes is a superorder of Cephalopoda, which includes all species with ten limbs; the name derives from the Greek word meaning ten feet. The ten limbs are divided into 8 short arms and 2 long tentacles. It is presumed that an ancestral coleoid had five identical pairs of limbs, and that one...
, with 8 arms
Cephalopod arm
A cephalopod arm is distinct from a tentacle, though the terms are often used interchangeably.Generally, cephalopod arms have suckers along most of their length, as opposed to tentacles, which have suckers only near their ends. Octopuses have eight arms and no tentacles, while squid and cuttlefish...
and 2 longer tentacle
Tentacle
A tentacle or bothrium is one of usually two or more elongated flexible organs present in animals, especially invertebrates. The term may also refer to the hairs of the leaves of some insectivorous plants. Usually, tentacles are used for feeding, feeling and grasping. Anatomically, they work like...
s, all with suckers. The arms and tentacles can all be withdrawn completely into the mantle
Mantle (mollusc)
The mantle is a significant part of the anatomy of molluscs: it is the dorsal body wall which covers the visceral mass and usually protrudes in the form of flaps well beyond the visceral mass itself.In many, but by no means all, species of molluscs, the epidermis of the mantle secretes...
.
The species lacks a radula (or, at most, has a vestigial radula).
Shell
The shell functions to osmoticallyOsmosis
Osmosis is the movement of solvent molecules through a selectively permeable membrane into a region of higher solute concentration, aiming to equalize the solute concentrations on the two sides...
control buoyancy
Buoyancy
In physics, buoyancy is a force exerted by a fluid that opposes an object's weight. In a column of fluid, pressure increases with depth as a result of the weight of the overlying fluid. Thus a column of fluid, or an object submerged in the fluid, experiences greater pressure at the bottom of the...
. 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 gas-filled chambers keep the spirula in a vertical, head-down attitude.
Behaviour
Spirula are capable of emitting a green light from a photophorePhotophore
A photophore is a light-emitting organ which 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...
at the tip of their mantle.
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. Their preferred temperature is around 10°C, and they tend to live around oceanic islands, near the continental shelf.Most sources cite this species as tropical, and they are observed to be plentiful in the seas around the Canary Islands
Canary Islands
The Canary Islands , also known as the Canaries , is a Spanish archipelago located just off the northwest coast of mainland Africa, 100 km west of the border between Morocco and the Western Sahara. The Canaries are a Spanish autonomous community and an outermost region of the European Union...
. However, significant quantities of shells from dead Spirula are washed ashore even in temperate regions, such as the western coasts of South Africa
South Africa
The Republic of South Africa is a country in southern Africa. Located at the southern tip of Africa, it is divided into nine provinces, with of coastline on the Atlantic and Indian oceans...
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.
Evolutionary relationships
Spirulida
Spirulida is an order of cephalopods comprising one extant species and several extinct taxa.-Classification:*Order Spirulida**?Family Shimanskyidae**Suborder †Groenlandibelina***Family †Groenlandibelidae...
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
Cuttlefish
Cuttlefish are marine animals of the order Sepiida. They belong to the class Cephalopoda . Despite their name, cuttlefish are not fish but molluscs....
, as well as to the true squids.