THE JET SET: THE ANATOMY OF SWIMMING IN CEPHALOPODS

This topic submitted by Carl Howard ( howardca@miamioh.edu) at 4:18 PM on 7/31/03.

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THE JET SET: THE ANATOMY OF SWIMMING IN CEPHALOPODS
by Carl Howard


INTRODUCTION

Cephalopods include squid, octopus, cuttlefish, nautilus, spirula, and the extinct ammonites and belemnites. Cephalopods first appear in the fossil record nearly 500 million ago during the Ordovician Period. They are all mollusks, members of the same Phylum Mollusca as oysters, conches, and the common garden snail. The Class Cephalopoda is divided into two Orders: Tetra branchia, containing the nautilus and Dibranchia, containing the squid, octopus and cuttlefish.

Cephalopods are the most mobile of all mollusks. The gastropods (snails, whelks, conchs, etc.) and nudibranchs are quite mobile and crawl along on their large foot. Some bivalves (clams) can even jet surprising distances by pumping water through their siphons with rapid opening and closing movement of their mantles. Nearly all mollusks are mobile when they first emerge from eggs. Most are simply planktonic, spending a brief period in the water column drifting at the mercy of currents before finding a suitable surface to settle on or burrow into. Many mollusks spend their adult life in a highly sedentary state, attaching themselves permanently to a hard surface with bissus threads, such as mussels, or borrowing into the substrate, such as gribbles, shipworms and many clams. Other species of mollusks are more mobile. They crawl around on their foot, but rarely travel any great distance throughout their lives.

Cephalopods are the jet set. With the exception of the octopus, most spend much of their lives swimming above the bottom. Some squid will even migrate enormous distances in the open oceans. Some large squid can even rival fish, reaching speeds of nearly 20 miles per hour over short periods. Some can even propel themselves out of the water and through the air for considerable distances, reminiscent of flying fish.


ANATOMY OF JET-PROPELLED SWIMMING

Cephalopod swimming is quite different from that of fish. Cephalopods use jet propulsion, pumping water in over their gills and out through a tube called the siphon or funnel. This siphon is a muscular and mobile organ that the animal can use to direct the water jet in almost anydirection to steer itself. When observing cephalopods, look for the siphon and watch how the animal moves it.

Jetting isn’t the only control mechanism in cephalopod swimming, but it is the main engine. Body form, mantle movements, and positioning of tentacles are also important for steering and reducing drag. To begin to appreciate all that is involved in cephalopod swimming, we need to take a look at the basic cephalopod body plan and some of the variations among different cephalopod families.

Cephalopoda means ‘head-footed’. This refers to the the fact that that the body plan is organized with
the tentacles (feet) encircling the mouth parts, with the eyes typically directly behind and above the ring of tentacles . The tentacles and siphon are evolutionary modifications of the foot, an extension of the mantle, found in gastropods (snails, etc. [gastropoda means ‘stomach footed’]).

The mouth parts of cephalopods are a hard, beak-like structure shaped like an inverted parrot beak. This beak is very sharp, used to grasp and tear food. Be careful when handling a cephalopod because it can potentially deliver a nasty bite. They have poison glands to inject toxins. The tiny blue-ringed octopus of Australia has symbiotic bacteria that produce a deadly poison which is injected with the bite. The paralyzing toxin from a blue-ringed octopus bite can kill an adult human in minutes. While the venom of Atlantic and Caribbean octopi is not highly toxic to most people, any wound could become seriously infected by bacteria in seawater and pose health risks.

Behind the beak is a tongue-like structure called the radula. The radula is rasp-like, a conveyor belt of tiny teeth that the animal uses to grind up the food it bites off. The end of the radula is constantly being worn down and replaced by new teeth pushing forward from behind. The beak and radula of an octopus is used like a circle-cutting drill bit to bore holes through the shells of clams or other mollusks it feeds on. While there are no fossil remains of octopuses, these very characteristic drill-holes in the shells of fossil clams give geologic evidence that the octopus is a very ancient family of cephalopods. Gastropods are the only other group of mollusks to have a radula, but they do not have beaks.


The eyes are located just above and behind the tentacles. They are highly mobile and spaced widely to allow the animal to see behind itself for danger. The eye of a cephalopod is strikingly large for the size of the animal, and in the case of the giant squid represents the largest eye in the animal world, nearly the size of a dinner plate. The octopus eye feels almost uncanny and seems to express complex motions and observant intelligence. The eyes of cephalopods, especially Loligo squid, are much studied by scientists to learn more about vision. The visual nerve, called the giant axion, is large and easy to work with in studying how nerves and visual receptors throughout the animal kingdom function.

The tentacles of cephalopods are perhaps their most notable feature. The number and characteristics of different types of tentacles are key to identifying different species. The Octopus (Octopoda) has eight tentacles (often called legs or arms). Octopus means ‘eight-footed’. Octopus are typically benthic dwellers (living on the bottom) and crawl along the bottom on their tentacles, rarely propelling themselves above the bottom into the water column . They hide in holes much of the time. When hunting they use their tentacles to prod among sand and rocks and pry prey out of holes.

Squid and cuttlefish (Decapoda) have eight similarly sized tentacles, plus two additional arms. The arms are longer and more elastic than regular tentacles, with wide grasping pads on the end. The squid typically holds its tentacles straight out in front of itself, maintaining a streamlined torpedo shape (1). The cuttlefish has comparatively shorter and thicker tentacles than a squid, and tends to hold them curled close to its head and hanging slightly down, like a keel .

Squid and cuttlefish use these extensible arms to catch prey. They maneuver near their quarry then suddenly shoot the arms forward to grab their victim tightly with the suckers of the grasping pads. They then pull close to deliver the fatal bite with their beaks. Some squid even practice ‘ambush feeding’, hanging nearly motionless in the water column until an unsuspecting prey ventures near. The squid suddenly whips its long arms forward to snatch its next meal. Some scientists believe that the little known giant squid of the deep oceans, which can reach over 60 feet in length, is an ambush predator. Its great size would demand conservation of energy and its exceptionally long arms suggest that it is designed to reach out considerable distances to catch prey.

The nautilus has up to 95 small tentacles in several rows encircling the beak. There is only moderate differentiation in form according to position of the tentacles. The nautilus tentacles lack suckers and they do not have extensible grabbing arms as squid do. Instead, nautilus tentacles have sticky, velcro-like ridges that allow the nautilus to cling to the prey or other objects. Several longer tentacles above and below the beak are used to pull food toward the beak, while the tentacles on either side are simply used to hang on to the food or to a surface. Several outer tentacles are not used for grasping, but rather for steering and feeling out to the sides like cat whiskers. It lives much of its life in deep, dark waters. It has poor vision and must feel its way around. The nautilus spends much of its time close to the bottom, using its tentacles to pull itself along and to poke for food.

In most cephalopod species, one tentacle of the male is modified into a sex organ called the hectocotylus, used to implant a fertilizing sperm sac into the mantle of the female during mating.

The body of most cephalopods is covered by the exposed mantle, the skin-like surface. The mantle is perhaps the most complex organ of cephalopod design. In most species of octopus, squid, and cuttlefish the mantle contains chromatophores that allow the animal to rapidly change coloration. Coloration modification behavior in cephalopods has been much studied. In addition to providing camouflage, it is believed that color changing behavior is a method of communication between animals, signaling emotions, sexual interest, warnings, etc. Many species also have special glands in the skin similar to the chromatophores which house symbiotic bioluminescent (light-producing) bacteria. These special bioluminescence glands allow the cephalopod to flash brightly at night to signal each other and attract prey and to obscure the light-producing cells to hide from predators.

The skin-like mantle is a tough, elastic integument, muscular and flexible. It is filled with highly sensitive nerve cells. The mantle plays a particularly important function in swimming in most cephalopods. In squid and cuttlefish, the body ends in a broad, tail-like structure that aids in steering and maneuverability. Cuttlefish have a skirt of skin along both sides, which undulates constantly to hold their position in the water column. In some deep sea octopuses the mantle forms a broad connective web between each tentacles, allowing these normally bottom crawling animals to launch up into the water column and pump along by bellowing with their webbed legs.

Among shelled mollusks, the shell is formed by excretions from the mantle. The mantle covers the lip of the shell. As the mollusk grows, the mantle continues to excrete calcite deposits, steadily enlarging the shell. This is true of nautilus, the only living externally shelled cephalopod. Part of the mantle forms a tough, elongated, knobby-surfaced hood over the nautilus’s head region. As with many other types of mollusks, new shell is laid down around the lip of the shell. Unlike other shelled mollusks, the shell of the nautilus grows as hollow chambers, connected by a small tube called the siphuncle . This siphuncle allows the animal to regulate levels of air and liquid in the hollow chambers as part of the nautilus buoyancy control system.

The nautilus’s internal organs are housed only in the largest outer chamber and move forward when a new septa is formed to create a new chamber as the animal grows. The animal is able to withdraw its tentacles and outer parts entirely within this chamber and cover the opening with the exposed hood, similar to the way a gastropod covers its shell opening with the shell-like disk in its foot, called the operculum. While tough and leathery, the nautilus’s hood is not sufficient protection against predators like parrotfish and octopus.

The nautilus lives in deep, cold waters in the southwestern Pacific close to reef walls. It is largely a scavenger and eater of small crustacean and worms, so it hunts close to the bottom and along the reef wall. At night, when most of its predators are asleep, it ascends upward along the reef wall in search of food, using its internal buoyancy system. Nitrogen gas is pumped into chambers through the siphuncle to increase buoyancy and rise toward the surface. The gas is forced out and replaced by a fluid to descend. It returns to deep, dark waters out of reach of the predators before dawn. The hollow chambers of its shell limit the depths to which the nautilus can descend. Much like a naval submarine, it is in danger of imploding under the increased pressure of depths. It is rarely found below about 700 meters (about 2300 feet). Most live at depths between 1000 and 1500 feet, where temperatures range around 40OF.

Nautilus are captured in baited wire traps by fishermen who sell the meat in Philippine markets and the shell to the shell trade. Some dive tour operators also capture them to give their clients a chance to dive with nautilus. While this latter practice is less lethal than the fishing industry on nautilus, it still puts individual animals at high risk. A nautilus pulled to the surface too fast, before it can adjust its internal shell pressure to changes in depths may suffer rupturing that permanently damages the bouyancy system. Nautiluses are cold blooded animals who do not tolerate temperatures above about 70OF. They rarely even ascend above 250 feet depths along the reef walls because of temperature levels. Being brought to the surface in warm tropic waters is highly stressful, if not outright lethal. The dive tours provide the animals to their clients for swimming with and photographing in shallow diving and snorkeling depths. The animals are released in the process, after being much handled and posed for photo shots by the dive participants. Those that have survived the stresses and have not had their siphuncle ruptured, are still exposed to overly warm water and daytime predators before they can descend to safer depths.
Nautilus are the only living cephalopods having an external shell. Another cephalopod, however, is also called a nautilus--the paper nautilus , which is actually a type of octopus, the paper argonaut. The beautiful keeled shell-like structure associated with it is not a shell at all, but an egg-case. This ornate, curved white egg case is occasionally found floating on the sea surface, sometimes with the female argonaut still clinging to it. It is not made of calcite, but of chitin.

Octopuses are entirely soft body without even vestigial shell. Squid have a thin chitin pen that acts like a rudimentary backbone to give their body internal support. One species of the squid family, the spirula, forms a small, chambered spiral shell internally near its anterior (tail) region. Very little is known about this deep sea dweller and specimens have only rarely been captured in trawl nets. The spirula shell, however, frequently washes up along Atlantic and Caribbean beaches. This thin, white curled shell is about an inch across and commonly called a ramshorn because of its shape. Unlike a gastropod shell, the spirula shell is chambered, much like nautiloid shells. From its form it is believed that this internal shell is use for buoyancy regulation by the spirula.

The best known cephalopod internal shell structure is that of the cuttlefish. This very squid-like cephalopod has a substantial internal bonelike structure, porous and made of calcium carbonate. This is the cuttlebone sold by pet shops for parakeets to chew on. The cuttlefish uses this cuttlebone for both structural support and buoyancy control. By displacing the fluids that fill the porous cavities in the cuttlebone with nitrogen gases, the cuttlefish is able to maintain neutral buoyancy and hover in the water column over one location, much as a scuba diver would regulate buoyancy by controlling the air in his/her BC (buoyancy control vest).

In the typical cephalopod, the body mantle forms a lip over the head region and a cavity under the ventral side from which the siphon or funnel protrudes. The brachial tissue, called the ctenidium, or gills, lies near the back of the mantle cavity behind the intake opening of the funnel. The ink sac lies within the body cavity just over the gills, with the ink sac tube protruding into the funnel. The mantle cavity is the engine room of the cephalopod.

Water is constantly pumped into the mantle cavity and over the gills by a pumping action of the mantle lip, providing for efficient respiration. By closing the mantle lip, the animal can force the water out through the siphon to create a jet of water to propel itself. The highly flexible opening of the siphon mouth can be angled to aim the jet and control the direction of movement. Squid and cuttlefish that are simply resting in the water column constantly move their funnel about to redirect small jets of water and are able to keep themselves suspended over a single spot for long periods. This is impressive to watch, especially several animals together in a group holding their position, typically accompanied with shimmering changes of their coloration, possibly to communicate with each other.

Octopus do not normally swim unless threatened. It’s bag-like body lacks the streamlining and internal support structure of squid and cuttlefish, and prefers to crawl along the bottom on its bent tentacles or hide in crevices. If forced to swims it propels itself backward in typical cephalopod fashion, forcing water through its funnel and extending its arms behind. It’s soft body lacks adequate steering mechanisms and most species can sustain such jet swimming only for short sprints.

When a squid, cuttlefish or octopus is confronted with some potential danger or other aggravation, it can suddenly emit a cloud of black ink into the water and disappear behind it. The position of the ink gland opening well into the intake of the funnel protects the animal from getting ink over its gills.

The nautilus is the only group of cephalopods that lack an ink gland for protection. The nautilus is a slow and cumbersome animal by comparison with other cephalopods. It is capable of short distance burst of jetting speed, but not effectively enough to avoid most of its predators even if it did have an ink sac. It’s greatest defense is simply to dive deep, out of range of reef predators who cannot follow into the dark, cold depths below the reef wall.

The hydraulic pressure from the cephalopods siphon jet can serve more uses than simply moving through the water. Squid, cuttlefish and octopus also use their siphon and tentacles to dig rapidly into a sandy bottom disappearing completely in scant seconds.

Cephalopod swimming is more than just body shape and pumping water into the mantle cavity and out through the siphon. The positioning of the tentacles plays a major roll in directing the jet stream of water from the siphon. The tentacles can be positioned to effectively act as an extension to the funnel tube, increasing the force of the jet to move more quickly and direct it at an angle to quickly change directions (1). Tentacles also act to break up turbulence to increase speed.

The smooth, flexible body of the squid and cuttlefish improve the laminar flow of water over and around the animal to reduce the friction of drag. Since the animal essentially swims backward, the wide set eyes let it look backwards to see where it is going. Once the cephalopod has expended its jet and opens the mantle lip to take in fresh water, the shape of the head helps force the water down into the mantle cavity, reducing the energy needed to pump it in. The water coming in must flow over the gills before being expelled through the siphon, assuring that the animal gets a constant fresh supply of oxygen and body waste products released through the gills is swiftly flushed away.

Cephalopod swimming is a set of complex and elaborate maneuvers. The anatomical features of the mantle, mantle cavity, siphon and tentacles are evolutionary modifications of the mantle and foot of primitive mollusks that show us grand examples of how Nature morphs similar forms to different and remarkable functions. The next time you have a chance to watch a cephalopod swimming, whether on film, in an aquarium, or in the open ocean, watch the movement of its siphon, its tentacles and the pumping action of the lip of its mantle. You will discover for yourself how precise and efficient the process of cephalopod swimming really is.


REFERENCES

Cousteau, Jacques Yves and Philippe Diolé. 1973. Octopus and Squid, The Soft Intelligence. 1st Ed. Doubleday.Garden City, N.Y.:

Ellis, Richard. 1998. The Search for the Giant Squid . New York: The Lyons Press.

Hanlon, Roger T. and John B. Messenger. 1996. Cephalopod Behaviour. New York: Cambridge University Press.

Holloway, Marguerite. 2000. Cuttlefish Say It With Skin. American Museum of Natural History, N. Y. (Online at www.findarticles.com/cf_0/m1134/3_109/61524425/print.jhtml)

Howard, Carl. 1990. The Hood and Tentacles of Nautilus. Unpublished graduate paper, Harvard University, Cambridge, MA.

Lane, Frank. 1974. Kingdom of the Octopus. Sheridan House, N. Y.

Trueman, E. R. 1983. Locomotion in Molluscs. Chapter 4 in The Mollusca, Volume II. Form and Function. Edited by E.R. Trueman and M. R. Clarke. 1988. Academic Press, Inc. N.Y.

Vogel, Steven. 1987. Life in Moving Fluids. Princeton University Press, Princeton, N.J.


Web Resources

http://www.cephbase.utmb.edu/ (May 2003)
-CephBase: General clearinghouse of online articles on cephalopods

http://www.giantsquidcenter.com/ (May 2003)
-Giant Squid: The Last Sea Monster -

www.terrax.org/projects/australia/reef/reeftalk.aspx (May 2003)
-Descibes diving with nautilus in the Coral Sea from a dive cruise boat.

online.sfsu.edu/~biol240/Labs/lab_18molluscs/Pages/cephalopods.html (May 2003)
-Anatomy of cephalopods.

www.cox-internet.com/coop/cephalopoda.html (May 2003)
-Cephalopod evolution and classification.

members.lycos.co.uk/Mollusks/Kopffuesser/nautilus.html (May 2003)
-Natural history and pictures of nautilus

www.geocities.com/darthdusan/cephalopods.html (May 2003)
-General natural history of cephalopods and history of cephalopod research.

www.seasky.org/reeflife/sea2f2.html (May 2003)
-Pictures and general facts about cephalopods



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