A siphon is an anatomical structure which is part of the body of aquatic

mollusc Mollusca is a phylum of protostome, protostomic invertebrate animals, whose members are known as molluscs or mollusks (). Around 76,000 extant taxon, extant species of molluscs are recognized, making it the second-largest animal phylum ...

s in three

class Class, Classes, or The Class may refer to: Common uses not otherwise categorized * Class (biology), a taxonomic rank * Class (knowledge representation), a collection of individuals or objects * Class (philosophy), an analytical concept used d ...

es:

Gastropoda Gastropods (), commonly known as slugs and snails, belong to a large Taxonomy (biology), taxonomic class of invertebrates within the phylum Mollusca called Gastropoda (). This class comprises snails and slugs from saltwater, freshwater, and fro ...

Bivalvia Bivalvia () or bivalves, in previous centuries referred to as the Lamellibranchiata and Pelecypoda, is a class (biology), class of aquatic animal, aquatic molluscs (marine and freshwater) that have laterally compressed soft bodies enclosed b ...

and

Cephalopoda 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, ...

(members of these classes include saltwater and

freshwater snail Freshwater snails are gastropod mollusks that live in fresh water. There are many different families. They are found throughout the world in various habitats, ranging from ephemeral pools to the largest lakes, and from small seeps and springs t ...

clams Clam is a common name for several kinds of bivalve mollusc. The word is often applied only to those that are deemed edible and live as infauna, spending most of their lives halfway buried in the sand of the sea floor or riverbeds. Clams h ...

octopus An octopus (: octopuses or octopodes) is a soft-bodied, eight-limbed mollusc of the order Octopoda (, ). The order consists of some 300 species and is grouped within the class Cephalopoda with squids, cuttlefish, and nautiloids. Like oth ...

squid A squid (: squid) is a mollusc with an elongated soft body, large eyes, eight cephalopod limb, arms, and two tentacles in the orders Myopsida, Oegopsida, and Bathyteuthida (though many other molluscs within the broader Neocoleoidea are also ...

and relatives). Siphons in molluscs are tube-like structures in which water (or, more rarely, air) flows. The water flow is used for one or more purposes such as locomotion,

feeding Eating (also known as consuming) is the ingestion of food. In biology, this is typically done to provide a heterotrophic organism with energy and nutrients and to allow for growth. Animals and other heterotrophs must eat in order to survive – ...

, respiration, and

reproduction Reproduction (or procreation or breeding) is the biological process by which new individual organisms – "offspring" – are produced from their "parent" or parents. There are two forms of reproduction: Asexual reproduction, asexual and Sexual ...

. The siphon is part of the mantle of the mollusc, and the water flow is directed to (or from) the mantle cavity. A single siphon occurs in some gastropods. In those bivalves which have siphons, the siphons are paired. In

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

, there is a single siphon or funnel which is known as a hyponome.

In gastropods

In some (but not all)

sea snail Sea snails are slow-moving marine (ocean), marine gastropod Mollusca, molluscs, usually with visible external shells, such as whelk or abalone. They share the Taxonomic classification, taxonomic class Gastropoda with slugs, which are distinguishe ...

s, marine gastropod molluscs, the animal has an anterior extension of the mantle called a siphon, or inhalant siphon, through which water is drawn into the mantle cavity and over the gill for respiration.Örstan A. 13 April 2007
''Melongena's siphon''
Snail's Tales. This siphon is a soft fleshy tube-like structure equipped with

chemoreceptors A chemoreceptor, also known as chemosensor, is a specialized sensory receptor which transduces a chemical substance ( endogenous or induced) to generate a biological signal. This signal may be in the form of an action potential, if the chemorece ...

which "smell" or "taste" the water, in order to hunt for food. Marine gastropods that have a siphon are either

predator Predation is a biological interaction in which one organism, the predator, kills and eats another organism, its prey. It is one of a family of common List of feeding behaviours, feeding behaviours that includes parasitism and micropredation ...

s or

scavengers Scavengers are animals that consume dead organisms that have died from causes other than predation or have been killed by other predators. While scavenging generally refers to carnivores feeding on carrion, it is also a herbivorous feeding ...

. Although in gastropods the siphon functions perfectly well as a tube, it is not in fact a hollow organ, it is simply a flap of the mantle that is rolled into the shape of a tube. In many marine gastropods where the siphon is particularly long, the structure of the

shell Shell may refer to: Architecture and design * Shell (structure), a thin structure ** Concrete shell, a thin shell of concrete, usually with no interior columns or exterior buttresses Science Biology * Seashell, a hard outer layer of a marine ani ...

has been modified in order to house and protect the soft tissue of the siphon. This shell modification is known as the siphonal canal. For a gastropod whose

has an exceptionally long siphonal canal, see Venus comb murex. In the case of some other marine gastropod shells, such as the volute and the ''Nassarius'' pictured to the right, the shell has a simple "siphonal notch" at the anterior edge of the

aperture In optics, the aperture of an optical system (including a system consisting of a single lens) is the hole or opening that primarily limits light propagated through the system. More specifically, the entrance pupil as the front side image o ...

instead of a long siphonal canal. The ''Aplysia'' gill and siphon withdrawal reflex is a defensive

reflex In biology, a reflex, or reflex action, is an involuntary, unplanned sequence or action and nearly instantaneous response to a stimulus. Reflexes are found with varying levels of complexity in organisms with a nervous system. A reflex occurs ...

which is found in sea hares of the genus '' Aplysia''; this reflex has been much studied in

neuroscience Neuroscience is the scientific study of the nervous system (the brain, spinal cord, and peripheral nervous system), its functions, and its disorders. It is a multidisciplinary science that combines physiology, anatomy, molecular biology, ...

Siphon as a snorkel

Freshwater apple snails in the genera '' Pomacea'' and '' Pila'' have an extensible siphon made from a flap of the left mantle cavity. They use this siphon in order to breathe air while they are submerged in water which has a low oxygen content so they cannot effectively use their gill. Apple snails use the siphon in a way that is reminiscent of a human swimmer using a snorkel, except that the apple snail's siphon can be retracted completely, or extended to various lengths as needed.''Respiratory system''
The apple snail website, http://www.applesnail.net, accessed 26 February 2009. For these freshwater snails, the siphon is an anti-predator adaptation. It reduces their vulnerability to being attacked and eaten by birds because it enables the apple snails to breathe without having to come all the way up to the surface, where they are easily visible to predators. The shells of these freshwater snails have simple round apertures; there is no special notch for the siphon.

Paired siphons of bivalves

Those bivalves that have siphons, have two of them. Not all bivalves have siphons however: those that live on or above the substrate, as is the case in

scallop Scallop () is a common name that encompasses various species of marine bivalve molluscs in the taxonomic family Pectinidae, the scallops. However, the common name "scallop" is also sometimes applied to species in other closely related famili ...

oyster Oyster is the common name for a number of different families of salt-water bivalve molluscs that live in marine or brackish habitats. In some species, the valves are highly calcified, and many are somewhat irregular in shape. Many, but no ...

s, etc., do not need them. Only those bivalves that burrow in

sediment Sediment is a solid material that is transported to a new location where it is deposited. It occurs naturally and, through the processes of weathering and erosion, is broken down and subsequently sediment transport, transported by the action of ...

, and live buried in the sediment, need to use these tube-like structures. The function of these siphons is to reach up to the surface of the sediment, so that the animal is able to respire, feed, and excrete, and also to reproduce. The deeper a bivalve species lives in the sediment, the longer its siphons are. Bivalves which have extremely long siphons, like the geoducks pictured here, live very deeply buried, and are hard to dig up when clamming. Valve-InternalView

Many bivalves that have siphons can withdraw them completely into the shell when needed, but this is not true of all species. Bivalves that ''can'' withdraw the siphons into the shell have a "pallial sinus", a sort of pocket, into which the siphons can fit when they are withdrawn, so that the two shell valves can close properly. The existence of this pocket shows even in an empty shell, as a visible indentation in the pallial line, a line which runs along parallel to the ventral margin of the shell. The bivalve's two siphons are situated at the posterior edge of the mantle cavity. There is an inhalant or incurrent siphon, and an exhalant or excurrent siphon. The water is circulated by the action of the

gill A gill () is a respiration organ, respiratory organ that many aquatic ecosystem, aquatic organisms use to extract dissolved oxygen from water and to excrete carbon dioxide. The gills of some species, such as hermit crabs, have adapted to allow r ...

s. Usually water enters the mantle cavity through the inhalant siphon, moves over the gills, and leaves through the exhalant siphon. The water current is utilized for respiration, but also for

filter feeding Filter feeders are aquatic animals that acquire nutrients by feeding on organic matters, food particles or smaller organisms (bacteria, microalgae and zooplanktons) suspended in water, typically by having the water pass over or through a spe ...

excretion Excretion is elimination of metabolic waste, which is an essential process in all organisms. In vertebrates, this is primarily carried out by the lungs, Kidney (vertebrates), kidneys, and skin. This is in contrast with secretion, where the substa ...

, and reproduction.

Feeding

Depending on the species and family concerned, some bivalves utilize their inhalant siphon like the hose of a vacuum cleaner, and actively suck up food particles from the marine substrate. Most other bivalves ingest microscopic phytoplankton as food from the general water supply, which enters via the inhalant siphon and reaches the mouth after passing over the gill. Please also see pseudofeces.

Hyponome of cephalopods

The hyponome or siphon is the organ used by

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

s to expel water, a function that produces a locomotive force. The hyponome developed from the foot of the molluscan ancestor.''Class Cephalopoda: the Head-Feet''
Accessed 21 November 2008. Water enters the mantle cavity around the sides of the funnel, and subsequent contraction of the hyponome expands and then contracts, expelling a jet of water. In most cephalopods, such as

, and

cuttlefish Cuttlefish, or cuttles, are Marine (ocean), marine Mollusca, molluscs of the order (biology), suborder Sepiina. They belong to the class (biology), class Cephalopoda which also includes squid, octopuses, and nautiluses. Cuttlefish have a unique ...

, the hyponome is a muscular tube. The hyponome of the

nautilus A nautilus (; ) is any of the various species within the cephalopod family Nautilidae. This is the sole extant family of the superfamily Nautilaceae and the suborder Nautilina. It comprises nine living species in two genera, the type genus, ty ...

differs however, in that it is a one-piece flap that is folded over. Whether

ammonite Ammonoids are extinct, (typically) coiled-shelled cephalopods comprising the subclass Ammonoidea. They are more closely related to living octopuses, squid, and cuttlefish (which comprise the clade Coleoidea) than they are to nautiluses (family N ...

s possessed a hyponome and if so what form it may have taken, is as yet not known.''Discussion''
http://palaeo-electronica.org/ Accessed 21 November 2008.

Gallery

File:Nassarius tiarula.jpg, The sea snail '' Nassarius fossatus'' is a scavenger. Siphon on the left File:Pomacea canaliculata siphonout.jpg, '' Pomacea canaliculata'', seen through glass, has reached its siphon up to the water surface to breathe air File:Clam with its siphon out in Boothbay Maine.jpg, Clam with its siphon out File:Cymbiola magnifica.jpg, The siphon of a large herbivore marine

volute A volute is a spiral, scroll-like ornament that forms the basis of the Ionic order, found in the capital of the Ionic column. It was later incorporated into Corinthian order and Composite column capitals. Four are normally to be found on an ...

, '' Cymbiola magnifica'' Image:Nautilus belauensis front.jpg, '' Nautilus belauensis'' seen from the front, showing the opening of the hyponome. File:Pomacea paludosa drawing.jpg, Engraving of Florida freshwater applesnail '' Pomacea paludosa''; siphon on lower right File:GooeyduckSeafood.jpg, Four specimens of '' Panopea generosa'' in a seafood tank; the paired siphons (or "necks") of this species can be one meter long

References

External links

Glossary

{{Use dmy dates, date=May 2017 Bivalve anatomy Cephalopod zootomy Gastropod anatomy Invertebrate respiratory system Articles containing video clips