The siphuncle is a strand of
tissue passing longitudinally through 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
** Thin-shell structure
Science Biology
* Seashell, a hard o ...
of a
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 ...
. Only cephalopods with chambered shells have siphuncles, such as the extinct
ammonite
Ammonoids are a group of extinct marine mollusc animals in the subclass Ammonoidea of the class Cephalopoda. These molluscs, commonly referred to as ammonites, are more closely related to living coleoids (i.e., octopuses, squid and cuttlefish) ...
s and
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 the living
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 ...
es,
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 ''
Spirula
''Spirula spirula'' is a species of deep-water squid-like cephalopod mollusk. It is the only extant member of the genus ''Spirula'', the family Spirulidae, and the order Spirulida. Because of the shape of its internal shell, it is commonly known ...
''. In the case of the cuttlefish, the siphuncle is indistinct and connects all the small chambers of that animal's highly modified shell; in the other cephalopods it is thread-like and passes through small openings in the
septa
The Southeastern Pennsylvania Transportation Authority (SEPTA) is a regional public transportation authority that operates bus, rapid transit, commuter rail, light rail, and electric trolleybus services for nearly 4 million people in five coun ...
(walls) dividing the
camerae (chambers). Some older studies have used the term siphon for the siphuncle, though this naming convention is uncommon in modern studies to prevent confusion with a
mollusc organ of the same name.
Function
![Siphuncle](https://upload.wikimedia.org/wikipedia/commons/4/4a/Siphuncle.svg)
The siphuncle is used primarily in emptying water from new chambers as the shell grows.
To perform this task, the cephalopod increases the saltiness of the
blood
Blood is a body fluid in the circulatory system of humans and other vertebrates that delivers necessary substances such as nutrients and oxygen to the cells, and transports metabolic waste products away from those same cells. Blood in the c ...
in the siphuncle, and the water moves from the more dilute chamber into the blood through
osmosis
Osmosis (, ) is the spontaneous net movement or diffusion of solvent molecules through a selectively-permeable membrane from a region of high water potential (region of lower solute concentration) to a region of low water potential (region of ...
. At the same time gasses, mostly
nitrogen
Nitrogen is the chemical element with the symbol N and atomic number 7. Nitrogen is a nonmetal and the lightest member of group 15 of the periodic table, often called the pnictogens. It is a common element in the universe, estimated at se ...
,
oxygen
Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as wel ...
, and
carbon dioxide
Carbon dioxide (chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is transpar ...
, diffuse from the blood in the siphuncle into the emptying chamber. This is not a form of active pumping: the gas moving into the chamber is a passive process. Most energy is expended through the absorption of water from the chamber.
Removing water from the chambers of the shell reduces the overall
density
Density (volumetric mass density or specific mass) is the substance's mass per unit of volume. The symbol most often used for density is ''ρ'' (the lower case Greek letter rho), although the Latin letter ''D'' can also be used. Mathematical ...
of the shell, and thus the shell behaves as a flotation device comparable to the
swim bladder
The swim bladder, gas bladder, fish maw, or air bladder is an internal gas-filled Organ (anatomy), organ that contributes to the ability of many bony fish (but not cartilaginous fish) to control their buoyancy, and thus to stay at their curren ...
in bony
fish
Fish are aquatic, craniate, gill-bearing animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish as well as various extinct related groups. Approximately 95% of li ...
. Typically, cephalopods maintain a density close to that of
sea water
Seawater, or salt water, is water from a sea or ocean. On average, seawater in the world's oceans has a salinity of about 3.5% (35 g/L, 35 ppt, 600 mM). This means that every kilogram (roughly one liter by volume) of seawater has approx ...
, allowing them to keep a stable buoyancy with minimal effort. In the geologic past, many cephalopods grew to an
enormous size (perhaps approaching ten meters in length) thanks to this.
Generally, the siphuncle is unable to provide a way to change the density of shell rapidly and thus cause the animal to rise or sink at will; rather, the animal must swim up or down as required.
Cephalopods with a wider siphuncle have a higher rate of metabolic activity.
Morphology
![Nautilus pompilius - Fernbank Museum of Natural History - DSC00294](https://upload.wikimedia.org/wikipedia/commons/7/71/Nautilus_pompilius_-_Fernbank_Museum_of_Natural_History_-_DSC00294.JPG)
The siphuncle of
fossil
A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved ...
ised cephalopods is assumed to have worked in the same general way as in living
nautiluses. The siphuncle itself is only rarely preserved, but its shape can be inferred from hardened structures which lie around it. Many fossils show the holes where the siphuncle passes through each septum. Around these holes, the rim of the septum is bent into a stout
aragonitic
Aragonite is a carbonate mineral, one of the three most common naturally occurring crystal forms of calcium carbonate, (the other forms being the minerals calcite and vaterite). It is formed by biological and physical processes, including pre ...
tube known as a septal neck (or siphuncle notch).
![Orthoceras---orthocère](https://upload.wikimedia.org/wikipedia/commons/8/89/Orthoceras---orthoc%C3%A8re.jpg)
In each chamber of the shell, the siphuncle is encased by a tubular structure known as a connecting ring. In living nautiluses, the connecting ring is a simple, thin-walled cylinder, with organic or thinly
calcitic layers secreted from the tissues of the siphuncle. This fragile and poorly-mineralized form is known as a ''nautilosiphonate'' morphology. Many extinct cephalopods have a much more prominent connecting ring, with a very thick and porous inner
calcitic layer. This more strongly-mineralized form is known as a ''calciosiphonate'' connecting ring. Connecting rings are strongly variable in morphology, from narrow homogenous tubes to bulbous, segmented cavities. Some are infolded, sending lobes or blades of calcite into the siphuncle. Connecting rings are typically continuous with the septal necks, and are difficult to distinguish without close examination. However, their developmental origin is wholly separate from the shell and septa, and they utilize calcite rather than aragonite as a biomineralized reinforcement.
Biomineralized structures which develop within the siphuncle are known as endosiphuncular deposits (or simply siphonal deposits). These may include horizontal partitions (''diaphragms''), stacked conical structures (''endocones''), longitudinal rods, and various other concretions. Endosiphuncular deposits are typically thin structures which may be homologous to parts of the septae or connecting rings.
In most fossil nautiluses, the siphuncle runs more or less through the center of each chamber, but in ammonites and belemnites it usually runs along the ventral edge of the shell. In some fossil straight shelled nautiloids, cylindrical calcareous growths ("siphuncular deposits") around the siphuncle can be seen towards the apex of the shell. These were apparently counterweights for the soft body at the other end of the shell, and allowed the nautilus to swim in a horizontal position. Without these deposits, the apex of the buoyant shell would have pointed upwards and the heavier body downwards, making horizontal swimming difficult. The siphuncle of the
Endocerida
Endocerida is an extinct nautiloid order, a group of cephalopods from the Lower Paleozoic with cone-like deposits in their siphuncle. Endocerida was a diverse group of cephalopods that lived from the Early Ordovician possibly to the Late Silu ...
also contained much of the organisms' body organs.
See also
*
Phragmocone
The phragmocone is the chambered portion of the shell of a cephalopod. It is divided by septa into camerae.
In most nautiloids and ammonoids, the phragmocone is a long, straight, curved, or coiled structure, in which the camarae are linked by a ...
*
Orthoceras
''Orthoceras'' ("straight horn") is a genus of extinct nautiloid cephalopod restricted to Middle Ordovician-aged marine limestones of the Baltic States and Sweden. This genus is sometimes called ''Orthoceratites''. Note it is sometimes misspelle ...
* ''
Baculites
''Baculites'' ("walking stick rock") is an extinct genus of cephalopods with a nearly straight shell, included in the heteromorph ammonites. The genus, which lived worldwide throughout most of the Late Cretaceous, and which briefly survived the ...
''
References
{{Cephalopod anatomy
Cephalopod zootomy