Editing Cephalopod (section)

=== Locomotion and buoyancy ===
[[Image:Octopus3.jpg|thumb|right|Octopuses swim headfirst, with arms trailing behind]]
While most cephalopods can move by [[jet (gas)|jet]] propulsion, this is a very energy-consuming way to travel compared to the tail propulsion used by fish.<ref name="mollusca12-11"/> The efficiency of a [[propeller]]-driven [[Jet (fluid)|waterjet]] (i.e. [[Propulsive efficiency#Propeller engines|Froude efficiency]]) is greater than a [[rocket]].<ref name="pmid10952883">{{cite journal|last1=Anderson |first1=E. |last2=Demont |first2=M.|title=The mechanics of locomotion in the squid ''Loligo pealei'': Locomotory function and unsteady hydrodynamics of the jet and intramantle pressure |journal=Journal of Experimental Biology|volume=203|issue=18|pages=2851–2863 |year=2000 |doi=10.1242/jeb.203.18.2851 |pmid=10952883 |bibcode=2000JExpB.203.2851A |url=http://jeb.biologists.org/content/203/18/2851}}</ref> The relative efficiency of [[jet propulsion]] decreases further as animal size increases; [[paralarva]]e are far more efficient than juvenile and adult individuals.<ref name="Bartol2008">{{cite journal|last1=Bartol |first1=I. K. |last2=Krueger |first2=P. S. |last3=Thompson |first3=J. T.|last4=Stewart|first4=W.J.|title=Swimming dynamics and propulsive efficiency of squids throughout ontogeny |doi=10.1093/icb/icn043 |journal=Integrative and Comparative Biology|volume=48|issue=6 |pages=720–733 |year=2008|pmid=21669828|doi-access=free }}</ref> Since the [[Paleozoic era]], as competition with [[fish]] produced an environment where efficient motion was crucial to survival, jet propulsion has taken a back role, with [[Cephalopod fin|fins]] and [[cephalopod limb|tentacles]] used to maintain a steady velocity.<ref name="mollusca12"/> Whilst jet propulsion is never the sole mode of locomotion,<ref name="mollusca12"/>{{rp|208}} the stop-start motion provided by the jets continues to be useful for providing bursts of high speed – not least when capturing [[prey]] or avoiding [[predator]]s.<ref name="mollusca12"/> Indeed, it makes cephalopods the fastest marine invertebrates,<ref name=Cephalopods>{{cite book |isbn=978-0-19-852761-9 |first1=Marion |last1=Nixon |first2=J.Z.|last2=Young |year=2003 |publisher=Oxford University Press |location=New York |title=The Brains and Lives of Cephalopods}}</ref>{{Rp|Preface}} and they can out-accelerate most fish.<ref name=Gilbert1990>{{cite book|title=Squid as Experimental Animals |first1=Daniel L. |last1=Gilbert |first2=William J. |last2=Adelman |first3=John M.|last3=Arnold|edition=illustrated |publisher=Springer |year=1990 |isbn=978-0-306-43513-3 |url=https://archive.org/details/squidasexperimen00gilb}}</ref> The jet is supplemented with fin motion; in the squid, the fins flap each time that a jet is released, amplifying the thrust; they are then extended between jets (presumably to avoid sinking).<ref name="Bartol2008"/> Oxygenated water is taken into the [[Mantle (mollusc)|mantle cavity]] to the [[gill]]s and through muscular contraction of this cavity, the spent water is expelled through the [[hyponome]], created by a fold in the mantle. The size difference between the posterior and anterior ends of this organ control the speed of the jet the organism can produce.<ref name="Shea2002">{{cite journal|doi=10.1007/s00227-001-0772-7|title=Quantification of ontogenetic discontinuities in three species of oegopsid squids using model II piecewise linear regression |year=2002 |last1=Shea |first1=E. |last2=Vecchione|first2=M. |s2cid=84822175|journal=Marine Biology|volume=140|issue=5 |pages=971–979 |bibcode=2002MarBi.140..971E }}</ref> The velocity of the organism can be accurately predicted for a given mass and morphology of animal.<ref>{{cite journal|title=A study in jet propulsion: an analysis of the motion of the squid, ''Loligo vulgaris'' |url=http://jeb.biologists.org/content/56/1/155|journal=Journal of Experimental Biology|volume=56|pages=155–165 |issue=1972|first1=W.|last1=Johnson |first2=P. D. |last2=Soden |first3=E. R. |last3=Trueman |date=February 1972|doi=10.1242/jeb.56.1.155 |bibcode=1972JExpB..56..155J }}</ref> Motion of the cephalopods is usually backward as water is forced out anteriorly through the hyponome, but direction can be controlled somewhat by pointing it in different directions.{{sfnp|Campbell|Reece|Mitchell|1999|p=612}} Some cephalopods accompany this expulsion of water with a gunshot-like popping noise, thought to function to frighten away potential predators.<ref name="Guerra2007">{{cite journal|doi=10.1017/S0025315407058225 |title=A new noise detected in the ocean |year=2007|last1=Guerra |first1=A.|last2=Martinell|first2=X. |last3=González |first3=A. F. |last4=Vecchione |first4=M. |last5=Gracia|first5=J.|last6=Martinell |first6=J. |journal=Journal of the Marine Biological Association of the United Kingdom|volume=87|issue=5|pages=1255–1256 |bibcode=2007JMBUK..87.1255G |hdl=10261/27009 |s2cid=85770435 |hdl-access=free}}</ref>

Cephalopods employ a similar method of propulsion despite their increasing size (as they grow) changing the dynamics of the water in which they find themselves. Thus their paralarvae do not extensively use their fins (which are less efficient at low [[Reynolds number]]s) and primarily use their jets to propel themselves upwards, whereas large adult cephalopods tend to swim less efficiently and with more reliance on their fins.<ref name="Bartol2008"/>

[[Image:Nautilus belauensis front.jpg|thumb|left|''[[Nautilus belauensis]]'' seen from the front, showing the opening of the hyponome]]

Early cephalopods are thought to have produced jets by drawing their body into their shells, as ''Nautilus'' does today.<ref name=Wells1991>{{cite journal|title=Jet Propulsion and the Evolution of the Cephalopods|journal=Bulletin of Marine Science|volume=49|issue=1 |date=July 1991 |pages=419–432(14) |last1=Wells |first1=Martin J. |last2=O'Dor |first2=R. K. }}</ref> ''Nautilus'' is also capable of creating a jet by undulations of its funnel; this slower flow of water is more suited to the extraction of oxygen from the water.<ref name=Wells1991/> When motionless, ''Nautilus'' can only extract 20% of oxygen from the water.<ref name="ingentaconnect.com"/> The jet velocity in ''Nautilus'' is much slower than in [[coleoid]]s, but less musculature and energy is involved in its production.<ref name="CHAMBERLAINJR1993">{{cite journal|doi=10.1016/S0016-6995(06)80360-8 |title=Locomotion in ancient seas: Constraint and opportunity in cephalopod adaptive design |year=1993 |last1=Chamberlain |first1=J. Jr. |journal=Geobios|volume=26|issue=Suppl. 1 |pages=49–61|bibcode=1993Geobi..26...49C }}</ref> Jet thrust in cephalopods is controlled primarily by the maximum diameter of the funnel orifice (or, perhaps, the average diameter of the funnel)<ref name=oDor1988/>{{Rp|440}} and the diameter of the mantle cavity.<ref name=oDor2000>{{cite journal|last1=O'Dor |first1=R. K. |last2=Hoar |first2=J. A. |year=2000 |title=Does geometry limit squid growth? |journal=ICES Journal of Marine Science|volume=57|issue=1 |pages=8–14 |doi=10.1006/jmsc.1999.0502|doi-access=free |bibcode=2000ICJMS..57....8O }}</ref> Changes in the size of the orifice are used most at intermediate velocities.<ref name=oDor1988/> The absolute velocity achieved is limited by the cephalopod's requirement to inhale water for expulsion; this intake limits the maximum velocity to eight body-lengths per second, a speed which most cephalopods can attain after two funnel-blows.<ref name=oDor1988>{{cite journal|last1=O'Dor |first1=R. K. |title=The forces acting on swimming squid |url=http://jeb.biologists.org/content/137/1/421 |journal=Journal of Experimental Biology|volume=137|pages=421–442 |year=1988 |issue=1 |doi=10.1242/jeb.137.1.421 |bibcode=1988JExpB.137..421O }}</ref> Water refills the cavity by entering not only through the orifices, but also through the funnel.<ref name=oDor1988/> Squid can expel up to 94% of the fluid within their cavity in a single jet thrust.<ref name="pmid10952883"/> To accommodate the rapid changes in water intake and expulsion, the orifices are highly flexible and can change their size by a factor of 20; the funnel radius, conversely, changes only by a factor of around 1.5.<ref name=oDor1988/>

Some octopus species are also able to walk along the seabed. Squids and cuttlefish can move short distances in any direction by rippling of a flap of [[muscle]] around the mantle.

While most cephalopods float (i.e. are [[neutral buoyancy|neutrally buoyant]] or nearly so; in fact most cephalopods are about 2–3% denser than seawater<ref name=Packard1972/>), they achieve this in different ways.<ref name="mollusca12-11">{{The Mollusca|chapter=11: Evolution of Buoyancy and Locomotion in recent cephalopods|volume=12}}</ref>
Some, such as ''[[Nautilus]]'', allow gas to diffuse into the gap between the mantle and the shell; others allow purer water to ooze from their kidneys, forcing out denser salt water from the body cavity;<ref name="mollusca12-11"/> others, like some fish, accumulate oils in the liver;<ref name="mollusca12-11"/> and some octopuses have a gelatinous body with lighter [[chloride]] [[ion]]s replacing [[sulfate]] in the body chemistry.<ref name="mollusca12-11"/>

Squids are the primary sufferers of negative buoyancy in cephalopods. The negative buoyancy means that some squids, especially those whose habitat depths are rather shallow, have to actively regulate their vertical positions. This means that they must expend energy, often through jetting or undulations, in order to maintain the same depth. As such, the cost of transport of many squids are quite high. That being said, squid and other cephalopod that dwell in deep waters tend to be more neutrally buoyant which removes the need to regulate depth and increases their locomotory efficiency.<ref>{{Cite journal|last1=Seibel |first1=B. A. |last2=Thuesen|first2=E. V. |last3=Childress |first3=J. J. |last4=Gorodezky |first4=L. A. |date=April 1997 |title=Decline in Pelagic Cephalopod Metabolism With Habitat Depth Reflects Differences in Locomotory Efficiency |url=https://pubmed.ncbi.nlm.nih.gov/28581868/|journal=The Biological Bulletin|volume=192|issue=2 |pages=262–278 |doi=10.2307/1542720 |jstor=1542720 |issn=1939-8697 |pmid=28581868}}</ref><ref name="ingentaconnect.com"/>

The ''Macrotritopus defilippi'', or the sand-dwelling octopus, was seen mimicking both the coloration and the swimming movements of the sand-dwelling flounder ''Bothus lunatus'' to avoid predators. The octopuses were able to flatten their bodies and put their arms back to appear the same as the flounders as well as move with the same speed and movements.<ref>{{cite journal|last1=Hanlon |first1=Roger T. |last2=Watson |first2=Anya C.|last3=Barbosa |first3=Alexandra |date=2010-02-01 |title=A 'Mimic Octopus' in the Atlantic: Flatfish Mimicry and Camouflage by ''Macrotritopus defilippi'' |journal=The Biological Bulletin|volume=218|issue=1 |pages=15–24|doi=10.1086/BBLv218n1p15 |pmid=20203250 |issn=0006-3185 |hdl=1912/4811 |s2cid=12935620 |hdl-access=free}}</ref>

Females of two species, ''Ocythoe tuberculata'' and ''Haliphron atlanticus'', have evolved a true [[swim bladder]].<ref>{{Cite web|url=https://www.researchgate.net/publication/44613041|title=(PDF) The argonaut shell: Gas-mediated buoyancy control in a pelagic octopus}}</ref>

==== Octopus vs. squid locomotion ====
Two of the categories of cephalopods, octopus and squid, are vastly different in their movements despite being of the same class. Octopuses are generally not seen as active swimmers; they are often found scavenging the sea floor instead of swimming long distances through the water. Squid, on the other hand, can be found to travel vast distances, with some moving as much as 2,000&nbsp;km in 2.5&nbsp;months at an average pace of 0.9&nbsp;body lengths per second.<ref name=Gosline-deMont-1985>{{cite magazine|last1=Gosline |first1=John M. |last2=de Mont |first2=M. Edwin |date=1985 |title=Jet-propelled swimming in squids |url=https://www.jstor.org/stable/24967551 |magazine=Scientific American|volume=252|issue=1 |pages=96–103 |doi=10.1038/scientificamerican0185-96 |jstor=24967551 |issn=0036-8733}}</ref> There is a major reason for the difference in movement type and efficiency: anatomy.

Both octopuses and squids have mantles (referenced above) which function towards respiration and locomotion in the form of jetting. The composition of these mantles differs between the two families, however. In octopuses, the mantle is made up of three muscle types: longitudinal, radial, and circular. The longitudinal muscles run parallel to the length of the octopus and they are used in order to keep the mantle the same length throughout the jetting process. Given that they are muscles, it can be noted that this means the octopus must actively flex the longitudinal muscles during jetting in order to keep the mantle at a constant length. The radial muscles run perpendicular to the longitudinal muscles and are used to thicken and thin the wall of the mantle. Finally, the circular muscles are used as the main activators in jetting. They are muscle bands that surround the mantle and expand/contract the cavity. All three muscle types work in unison to produce a jet as a propulsion mechanism.<ref name=Gosline-deMont-1985/>

Squids do not have the longitudinal muscles that octopus do. Instead, they have a tunic.<ref name=Gosline-deMont-1985/> This tunic is made of layers of collagen and it surrounds the top and the bottom of the mantle. Because they are made of collagen and not muscle, the tunics are rigid bodies that are much stronger than the muscle counterparts. This provides the squids some advantages for jet propulsion swimming. The stiffness means that there is no necessary muscle flexing to keep the mantle the same size. In addition, tunics take up only 1% of the squid mantle's wall thickness, whereas the longitudinal muscle fibers take up to 20% of the mantle wall thickness in octopuses.<ref name=Gosline-deMont-1985/> Also because of the rigidity of the tunic, the radial muscles in squid can contract more forcefully.

The mantle is not the only place where squids have collagen. Collagen fibers are located throughout the other muscle fibers in the mantle. These collagen fibers act as elastics and are sometimes named "collagen springs".<ref name=Gosline-deMont-1985/> As the name implies, these fibers act as springs. When the radial and circular muscles in the mantle contract, they reach a point where the contraction is no longer efficient to the forward motion of the creature. In such cases, the excess contraction is stored in the collagen which then efficiently begins or aids in the expansion of the mantle at the end of the jet. In some tests, the collagen has been shown to be able to begin raising mantle pressure up to 50ms before muscle activity is initiated.<ref name=Gosline-deMont-1985/>

These anatomical differences between squid and octopuses can help explain why squid can be found swimming comparably to fish while octopuses usually rely on other forms of locomotion on the sea floor such as bipedal walking, crawling, and non-jetting swimming.<ref>{{Cite journal|last=Huffard |first=Christine L. |date=2006-10-01 |title=Locomotion by ''Abdopus aculeatus'' (Cephalopoda: Octopodidae): Walking the line between primary and secondary defenses|journal=Journal of Experimental Biology|volume=209|issue=19 |pages=3697–3707 |doi=10.1242/jeb.02435 |pmid=16985187 |s2cid=26862414 |issn=0022-0949|doi-access=free|bibcode=2006JExpB.209.3697H }}</ref>