Cephalopoda (Nautilids, Octopods, Cuttlefishes, Squids, and Relatives)

(invertebrate zoology) Exclusively marine animals constituting the most advanced class of the Mollusca, including squids, octopuses, and Nautilus.

(Nautilids, octopods, cuttlefishes, squids, and relatives)

Phylum: Mollusca

Class: Cephalopoda

Number of families: About 45

Thumbnail description
Mollusks bearing a radula (toothed tongue) and well-developed heads; mouth characterized by a dorsoventral pair of horny jaws known as beaks and encircled by the bases of 8–ca. 60 grasping appendages; single pair of lateral, image-forming eyes; well-developed brain and peripheral nervous system

Evolution and systematics

Cephalopod shells are very well represented in the fossil record as far back as the Upper Cambrian period, about 505 million years ago. Soft-tissue fossils, however, are extremely rare. Two very different subclasses of cephalopods live in modern seas: (1) the nautilids (subclass Nautiloidea), represented by two genera and about six species that have two pairs of gills and external shells into which they can withdraw; and (2) the neocoleoids (a division of subclass Coleoidea), sometimes called dibranchiates because they have a single pair of gills, comprising all other living species of cephalopods (fewer than a thousand). The neocoleoids include the familiar squids, cuttlefishes, and octopods.

There are about 44 families of extant neocoleoids, plus one family of nautilids. Only eight families have more than 20 species; many families consist of only one species or one genus. Although the familial relationships of living cephalopods are fairly stable, researchers have not completely resolved many relationships at higher and lower levels of classification (orders and genera). Eight distinctive groups of neocoleoids can be defined, however: Incirrata (common octopods); Cirrata (finned octopods); Vampyromorpha (the vampire squid, one species); Sepiida (cuttlefishes), Spirulida (the ram's horn squid, one species); Sepiolida (bobtail and bottle squids); Myopsida (inshore squids), and Oegopsida (oceanic squids). There is no consensus regarding the taxonomic level of these groups, and some families cannot be assigned with confidence to any of them. The first three groups listed comprise the Octopodiformes; the remaining five are grouped together as the Decapodiformes, commonly called decapods.

Physical characteristics

With regard to general structure, cephalopods have three easily distinguished regions. From front to rear, these regions are: (1) the brachial crown (arms and tentacles) surrounding the mouth; (2) the head, with prominent lateral eyes; and (3) the mantle, which may have a pair of fins on the sides. This overall three-part structure is less distinct in nautilids but is still recognizable. Nautilids have approximately 60 arms (sometimes called tentacles or cirri) arranged in two rings around the mouth. Neocoleoids have a single ring of either eight or ten appendages surrounding the mouth. On those that have ten, two appendages are modified into either ventrolateral tentacles (decapods) or dorsolateral velar filaments (vampires). Thus, all living cephalopods other than the nautilids have eight arms, and some have either two additional tentacles or two filaments. Although some nonspecialists refer to all cephalopod appendages as tentacles, one should avoid this usage because it confuses the differentiation of the specialized appendages in decapods and vampire squids.

Cephalopods range in size from the giant squids, Architeuthis spp., which are commonly longer than 6.56 ft (2 m) in mantle length (ML) and reportedly reach 16.4 ft (5 m) in ML, 59 ft (18 m) in total length, and 661.3 pounds (300 kg) in weight, to tiny species like Idiosepius spp., decapods which mature at an ML of about 0.23–0.31 in (6–8 mm); or the sexually dimorphic pelagic octopods, Argonauta spp., in which mature males are only 0.39 in (1 cm) in ML. Several squid species and at least two species of octopods reach sizes larger than an adult human. They include the true giant squid as well as the muscular (and dangerous) ommastrephids, the weakly muscled cranchiids and chiroteuthids, and the giant Pacific octopus or "devilfish."

Many species of squids, cuttlefishes, and octopods can radically change their appearance within a fraction of a second. This ability to transform themselves so rapidly is the reason that cephalopods have been called masters of disguise. These remarkable transformations result from interactions among brown, red and yellow pigment-filled chromatophore (colorproducing) organs under the control of the animal's nervous system; reflective iridophores (cells that produce a silvery or iridescent pigment); fixed white leucophores (pigment cells containing guanine); fixed tubercles (small nodules); and erectile muscular flaps and papillae in the animal's skin. Furthermore, some cephalopods have light-producing organs called photophores. Cephalopod photophores come in two fundamentally different types. In the first type, known as intrinsic photophores, the light is produced biochemically by the squid or octopod. The second type, called bacterial photophores, makes use of symbiotic photogenic bacteria that grow in special chambers associated with the ink sac in the host. The light produced by both types of photophores is usually blue-green in color; the color can be altered, however, by structures associated with the photophore.

Other features include an ink gland and ink sac associated with the intestine that is characteristic of neocoleoids. Complex, highly developed nervous and sensory systems are also typical of living cephalopods, although less so for the nautilids than for the others. Especially noteworthy are the image-forming eyes, with lenses in the neocoleoids; and the complex brain developed from the nerve ring surrounding the esophagus. Other noteworthy peculiarities, including a muscular hydrostatic skeleton and a pattern of muscle fiber alignment known as oblique striation, also are characteristics of cephalopods.

Distribution

All cephalopods are limited to marine environments. Only a few species can tolerate the low levels of salt in the waters of estuaries and fjords, the minimum being 17.5 PSU (practical salinity units), or about half the saltiness of full-strength seawater. No cephalopods can live in freshwater. Some octopods are known to crawl from one tidal pool to another but cannot remain out of the water for long.

Habitat

All cephalopods are mobile throughout their lives, except for the egg stages that in many species are attached to various substrates. As far as we know, all cephalopods are either carnivorous (the neocoleoids) or scavengers (the nautilids) throughout their life cycles after hatching. Some researchers have proposed that some cephalopod paralarvae feed on phytoplankton, but the evidence in support of this hypothesis currently is not very strong.

The life cycles of most neocoleoid cephalopods are very different from that of Nautilus. Whereas the latter is long-lived—it may live for 20 years or more—the lifestyles of other cephalopods have been characterized as "live fast and die young." Their life spans seem to range from a few months for small species to a few years for larger species. Many of the generalizations that have been made for neocoleoid cephalopods, however, have been based on observations of a few coastal species whose habitats are convenient for research.

Some cephalopod species, especially squids, can be very abundant; they are occasionally among the dominant organisms in their ecosystems. Because they are important food for larger animals, in addition to being voracious predators, such species are key members of some marine food webs. There are, however, significant gaps in current information about the life history and ecology of these organisms. Furthermore, many generalizations about cephalopods based on one or a few species are turning out to be either questionable or wrong. For example, because the common European octopus, Octopus vulgaris, and the California market squid, Loligo opalescens, spawn once and then die, their reproductive cycle has been widely regarded as the general pattern for cephalopods. Researchers are accumulating evidence, however, that many species of squids and octopods spawn many times and continue to live and feed after laying their eggs. Squids have been thought not to care for their eggs because the coastal species that have been most closely observed, as well as a few oceanic species, seem to release their egg masses and then depart either by moving away or by dying. Recently, though, some gonatid squids have been found to carry their egg masses around after releasing them. We know so little about most cephalopod species, especially oceanic and deep-sea species, that many such surprises likely await discovery. In short, many generalizations about the group may need to be modified.

Cephalopods live in a range of water depths from intertidal levels to over 16,400 ft (5,000 m). Different marine ecosystems have very different cephalopod faunas. For example, no cuttlefishes are found in American waters. Many octopod species have recently been described from Antarctic Ocean waters, whereas the Arctic appears to have few octopod species. The deep sea is home to both primitive vampire squids and morphologically similar finned octopods as well as some strange oegopsid squids. Oegopsid squids tend to be dominant in epipelagic and mesopelagic oceanic waters (the upper layer of water that admits enough light for photosynthesis to occur, and the "twilight zone" just below it); whereas myopsid squids share the continental shelves with incirrate octopods and cuttlefishes. Some incirrate octopods are found in deep-sea habitats on the ocean bottom whereas others are entirely pelagic. Because cephalopods are so widespread throughout marine habitats, their patterns of habitat utilization also vary widely.

Behavior

Cephalopods are renowned for their large brains, well-developed eyes, and complex behavior. Their social organization varies from the solitary life of octopods through small schools of cuttlefish to very large shoals of oceanic oegopsids. Some species of cuttlefishes and squids are known to form seasonal aggregations or gatherings for mating and spawning.

The sexes are separate in cephalopods. Courtship patterns vary from simple male grasping of the female followed by the implanting of spermatophores to complex courtship rituals in which both individuals display elaborate forms of touching prior to mating. Several species produce small "sneaker males" that resemble females; these sneaker males have been shown to be an important alternative to large, dominant mate-guarding males. The mating patterns of most oceanic cephalopods are largely unknown.

In addition to social organization and mating behavior, the use of visual communication among cephalopods has been a subject of debate. Whereas most observers who have watched cephalopods will agree that they perform complex visual signaling, these researchers debate whether such signaling can be considered a language. Cephalopod visual signaling involves a variety of behaviors, some of which are regularly directed toward conspecifics (e.g., mates or potential rivals), and others toward members of other species (e.g., prey or potential predators). The use of ink or other chemicals as an alarm signal to warn conspecifics has been demonstrated but not thoroughly investigated.

As with so many subjects about cephalopods, display patterns have been studied in detail for only a few easily accessible species. In general, these patterns include both acute patterns, lasting for a few seconds to a few minutes; and chronic patterns, lasting several minutes to several hours. The display patterns comprise chromatic (both dark and light), textural, postural, and movement changes. Commonly recorded displays include crypsis (hiding), in addition to what Hanlon and Messenger (1996) refer to as deimatic (threatening, startling, frightening, or bluffing) behaviors and protean (unpredictable or erratic) escape maneuvers.

Although some nearshore benthic octopods are known to occupy and defend territories for limited time periods, cephalopods generally are not territorial.

The primary variable in the activity patterns of cephalopods is the 24-hour diel cycle. Some neritic (coastal zone) species forage primarily by daylight, but most are nocturnal. Some species are crepuscular, which means that they are most active at dawn and dusk. Diel activities of oceanic species primarily involve vertical migration. The activity patterns of most deep-sea benthic species are unknown.

Many species of oceanic cephalopods undergo diel vertical migrations, wherein they live at depths of about 1,310–3,280 ft (400–1,000 m) during the day and then ascend into the uppermost 656 ft (200 m) of water during the night. Shifts in distribution over the course of a cephalopod's life history are common. Some oegopsid squids are believed to undertake life-cycle migrations over large geographic distances covering hundreds of miles.

Feeding ecology and diet

Neocoleoid cephalopods are active predators that feed upon shrimps, crabs, fishes, other cephalopods, planktonic crustaceans, and, in the case of octopods, on other mollusks. Nautilids are scavengers.

Foraging behavior varies widely among species in various taxonomic groups. Some examples include rapid raptorial attacks (oegopsid and myopsid squids); raptorial attacks after stalking (cuttlefishes); drifting with dangling tentacles (some oegopsid squids); ambush from a hiding place (some bobtail squids, cuttlefishes, and benthic incirrate octopods); and perhaps even the use of mucus to entangle small prey (a cirrate octopod). Special foraging adaptations also vary widely among cephalopod species.

Cephalopods are major food items in the diets of many marine vertebrates, including toothed whales; seals; pelagic birds (penguins, petrels, albatrosses, etc.); and both benthic and pelagic fishes (e.g., sea basses, lancetfishes, tunas, billfishes).

Reproductive biology

The sexes are separate in all cephalopods. The unpaired gonads, either ovary or testis, are always located in the posterior region of the mantle cavity. Females may have one or two oviducts depending on their major taxonomic group. Each oviduct has an oviducal gland that secretes a primary coating around the egg. Many female cephalopods have an additional nidimental gland that adds extra layers of protective coating to the egg. This gland is found beyond the end of the oviduct.

In males, the sperm passes through the spermatophoric gland complex, which packages the sperm into a packet known as a spermatophore. The hectocotylus is a modified arm on the males of many types of cephalopods that is used to transfer spermatophores to the female. The modifications of the arm take many forms, some quite bizarre. The males of some species also exhibit modifications of other arms in addition to the hectocotylus. Females of some species also develop such modified structures as arm-tip photophores when mature. Other forms of sexual dimorphism vary among species within families. These forms include such characters as greatly elongated arm pairs or tails; enlarged suckers; and special photophores. Although researchers have speculated about the functions of these features, little is known about them as of 2003.

Spermatophores are implanted in specific locations on the females by the male hectocotylus in species possessing one, or by an elongated penis (the terminal organ of the sperm duct) in nonhectocotylized species. The time required for the process ranges from seconds in some squids to hours in some octopods. The spermatophores may be implanted in the female's oviducal gland; inside the mantle cavity; around the mantle opening on the neck; in a pocket under the eye; or around the mouth. The mode of reproduction and egg-laying for many cephalopods is unknown, especially oceanic and deep-sea species.

All cephalopod eggs have substantial amounts of yolk. Embryonic development is unlike that of any other mollusk. Nautilid eggs are as much as 1.14 in (2.9 cm) in length. Neocoleoid eggs vary in size from about 1.6 in (4.2 cm) in some Graneledone and Megaledone species (among the largest of any invertebrate) to 0.03 in (0.8 mm) long in Argonauta. The eggs have one or more layers of protective coatings and generally are laid as egg masses. Egg masses may be benthic (laid on the ocean bottom) or pelagic, varying among major taxonomic groups. The time span of embryonic development also varies widely from a few days to many months, depending on the species and temperature conditions. Hatching may occur synchronously from a single clutch or extend over a period of 2–3 weeks.

Except for nautilids, which reproduce repeatedly over a period of years, extant cephalopods apparently mate only once. Their reproductive period, however, may comprise the brief terminus of their life span, may extend for a considerable period, or may be intermediate between these extremes. Different species, even within a single family, are found at different points along this continuum but generally cluster at the two extremes. Similarly, the number of eggs produced by a female range from a few dozen to hundreds of thousands. Hatchlings from benthic eggs may either be benthic or temporarily planktonic, eventually settling back to the adult benthic habitat. Pelagic hatchlings are planktonic.

The development of cephalopod embryos is direct, which means that the embryos do not have true metamorphic stages. The hatchlings of species with large eggs look like miniatures of the adult, whereas hatchlings of species with small eggs undergo changes in body proportion during development. The young of some species differ conspicuously in the proportion of their body parts and development of specialized structures (e.g., photophores; modification of suckers into hooks) from the adults. Thus, researchers have coined the term "paralarva" for early stages of cephalopods that differ morphologically and ecologically from later stages.

Most incirrate octopods and, as has recently been discovered, some squids care for their egg masses until the eggs hatch. Pelagic species that exhibit such behavior swim while holding the egg mass in their arms. Benthic octopods use such structures as the interiors of mollusk shells, rock crevices, or discarded bottles as dens. They manipulate the eggs to keep them free of detritus and blow water across them, presumably to aerate them.

Reproduction, at least among coastal species, is seasonal, although there may be either one or two peak periods of seasonal reproduction. Because of their short life spans, neocoleoid cephalopods tend to exhibit strong seasonality in the occurrence of their different life-history stages.

Conservation status

Distribution and conservation status vary among cephalopod species. Some coastal species are quite common whereas others, especially among the oceanic fauna, are rarely encountered. Small-scale endemism (confinement to a specific locality) is uncommon, although endemism is established among cephalopods within ocean basins.

No cephalopods appear on the IUCN Red List. In addition, no cephalopods appear on any United States regional listings as of 2003, although there has been discussion of proposing nautilids for listing under CITES. Most large declines in cephalopod abundance are probably cyclic in nature, although overfishing has been implicated in the declines of commercially important species. The decline in abundance of nautilids probably results from human harvesting of them for both shells and meat.

The abundance of cephalopods varies (depending on group, habitat, and season) from isolated territorial individuals (primarily benthic octopods and sepioids) through small schools with a few dozen individuals to huge schools with millions of individuals.

Significance to humans

Cephalopods have figured in the art and literature of many human cultures. Stories of large or intelligent cephalopods are well known from many ocean-oriented cultures, including Japanese, Polynesian, and Western European. Octopods are depicted on classical Greco-Roman pottery, frescoes, and similar artifacts. Medieval Germanic legends about "Kraken" or sea monsters probably refer to giant squids. Even at the present time, large squids and octopods continue to be subjects of much public interest. For example, the giant squid was described in a special issue of Time magazine on ocean exploration as "the last bona fide sea monster." This interest has been sustained by a series of popular novels and movies.

Many species of squid, cuttlefish, and octopod are very good food when prepared properly. Many people around the world enjoy dishes made from certain species of cephalopods. Fishermen also often equate cephalopods with "good bait" because other marine species like to eat them as well. These culinary considerations, both human and otherwise, reflect the importance of cephalopods in commercial fisheries and marine food webs. Although worldwide fisheries for squids, cuttlefishes, and octopods are dwarfed in comparison with those for shrimps, bivalves, and many fishes, catching and marketing cephalopods can dominate such local economies as that of the Falkland Islands. The total cephalopod catch officially reported for the year 2000 was about 4.0 million tons (3.6 million metric tons); this figure represents about 4.2% of the world total marine catch for the same year. Some statistics indicate that as humans reduce the populations of competing and predatory fishes and mammals, regional populations of cephalopods are increasing.

Octopods occasionally are considered to be pests in trap fisheries for mollusks (e.g., whelk) and crustaceans (e.g., lobsters) because the octopods enter the traps and eat the catch.

The rediscovery of squid giant axons by J. Z. Young in the 1930s allowed experiments that demonstrated much of what is known about the basic functioning of nerves. The giant axons of squid nerves have been widely used as one of the primary bases of neurobiology. Cephalopods are also being used increasingly as models in other biomedical fields, including sensory biology, information processing, and biochemistry. They are even being used as a source of information regarding the detoxification of nerve gas. The results of an electronic search of scientific literature for the word "squid" may be dominated by biomedical studies based on inshore squid as a convenient and interesting model.

Cephalopod bites, especially by octopods, can be painful at the least. Some are poisonous because of the injection of salivary secretions, and may even be lethal on rare occasions. Poisonous bites from the small blue-ringed octopus, Hapalochlaena spp., which secretes a substance known as cephalotoxin, have resulted in documented human deaths. Furthermore, schools of the large Humboldt squid, Dosidicus gigas, are reported to attack scuba divers and fishermen who have fallen into the water.

Many marine animals of particular interest to people, like whales, specialize in eating cephalopods. Among the animals with large brains that people consider intelligent, the cephalopods are unique because they are not vertebrates. The cephalopod brains and their remarkably familiar eyes have developed from an early precursor completely unlike the forerunners of dogs, dolphins, parrots, lizards, and fishes. This evolutionary history makes cephalopods the closest thing to an alien intelligence that humans have ever encountered.

Because cephalopods are important to biomedical researchers and fisheries, cephalopod biology is comparatively well-known from a few inshore species that can be caught close to such major marine biological laboratories as those in the northeastern United States; Plymouth, England; Naples, Italy; and Hakodate, Japan.

Species accounts

Resources

Abbott, J., R. Williamson, and L. Maddock, eds. Cephalopod Neurobiology. New York: Oxford University Press, 1995.

Boyle, P. R., ed. Cephalopod Life Cycles. Vol. I, Species Accounts. London: Academic Press, 1983. ——. Cephalopod Life Cycles. Vol. II, Comparative Reviews. London: Academic Press, 1987.

Budelman, B. U., R. Schipp, and S. von Boltezky. "Cephalopoda." In Microscopic Anatomy of Invertebrates. Vol. 6A, Mollusca II, edited by F. W. Harrison and A. J. Kohn. New York: Wiley-Liss, Inc., 1997.

Clarke, M. R., ed. A Handbook for the Identification of Cephalopod Beaks. Oxford, U.K.: The Clarendon Press, 1986.

Gilbert, D. L., W. J. Adelman, and J. M. Arnold, eds. Squid as Experimental Animals. New York: Plenum Press, 1990.

Hanlon, R. T., and J. B. Messenger. Cephalopod Behaviour. Cambridge, U.K.: Cambridge University Press, 1996.

Kinne, O., ed. Diseases of Marine Animals, Vol. III, Cephalopoda through Urochordata. Hamburg, Germany: Biologische Anstalt Helgoland, 1990.

Mangold, K. Traité de Zoologie. Anatomie, Systématique, Biologie. Tome 5, fascicule 4, Céphalopodes. Paris: Masson et Cie, 1989.

Nesis, K. N. Cephalopods of the World: Squids, Cuttlefishes, Octopuses and Allies. English translation. Neptune City, NJ:T. F. H. Publications, 1987.

Okutani, T. Cuttlefish and Squids of the World in Color. Tokyo: National Cooperative Association of Squid Processors, 1995.

Saunders, W. B., and N. H. Landman, eds. Nautilus. The Biology and Paleobiology of a Living Fossil. New York: Plenum Press, 1987.

Ward, P. D. Natural History of Nautilus. London: Allen and Unwin, 1987.

Clarke, M. R., ed. "The Role of Cephalopods in the World's Oceans." Philosophical Transactions of the Royal Society, London 351, no. 1343 (1996): 977–1112.

Roper, C. F. E., and M. Vecchione, eds. "The Gilbert L. Voss Memoral Issue: Systematics, Fisheries and Biology of Cephalopods." Bulletin of Marine Science 49, nos. 1–2 (1991): 1–670.

Sweeney, M. J., et al., eds. "'Larval' and Juvenile Cephalopods: A Manual for Their Identification." Smithsonian Contributions to Zoology 513 (1992): 1–282.

Voss, N. A., et al., eds. "Systematics and Biogeography of Cephalopods." Smithsonian Contributions to Zoology 586 (1998): 1–599.

Cephalopod International Advisory Council (CIAC). Dr. Michael Vecchione, President. NOAA/NMFS National Systematics Laboratory, National Museum of Natural History, MRC-153, Washington, DC 20013-7012.

"Cephalopoda Cuvier, 1797." Tree of Life Web Project. 1996 (21 July 2003). "Cephalopods at the National Museum of Natural History." Smithsonian National Museum of Natural History. 28 Jan. 2002 (21 July 2003). .

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Fossil Coleoidea Page. 1998 (21 July 2003). .

[Article by: Michael Vecchione, PhD]

The most highly evolved class of the phylum Mollusca. It consists of squids, cuttlefishes, octopuses, and the chambered nautiluses. The earliest known cephalopods are small, shelled fossils from the Upper Cambrian rocks of northeast China that are 500 million years old. Cephalopods always have been marine, never fresh-water or land, animals. Most fossil cephalopods, among them the subclasses Nautiloidea and Ammonoidea, had external shells and generally were shallow-living, slow-moving animals. Of the thousands of species of such shelled cephalopods that evolved, all are extinct except for four species of the only surviving genus, Nautilus. All other recent cephalopods belong to four orders of the subclass Coleoidea, which also contains five extinct orders.

Living cephalopods are bilaterally symmetrical mollusks with a conspicuously developed head that has a crown of 8–10 appendages (8 arms and 2 tentacles) around the mouth. These appendages are lined with one to several rows of suckers or hooks. Nautilus is exceptional in having many simple arms. The mouth contains a pair of hard chitinous jaws that resemble a parrot's beak and a tonguelike, toothed radula (a uniquely molluscan organ). Eyes are lateral on the head; they are large and well developed. The “cranium” contains the highly developed brain, the center of the extensive, proliferated nervous system. The shell of ancestral cephalopods has become, in living forms, internal, highly modified, reduced, or absent; and is contained in the sac- or tubelike, soft muscular body, the mantle. A pair of fins may occur on the mantle as an aid to locomotion, but primary movement is achieved through jet propulsion in which water is drawn into the mantle cavity and then forcibly expelled through the nozzlelike funnel. Fewer than 1000 species of living cephalopods inhabit all oceans and seas.

The classification given here concentrates on the living groups and lists only the major fossil groups; see separate articles on each subclass and order.

Class Cephalopoda

     Subclass Nautiloidea

     Subclass Ammonoidea

     Subclass Coleoidea

          Order Belemnoidea

          Order Sepioidea

          Order Teuthoidea

               Suborder Myopsida

               Suborder Oegopsida

          Order Vampyromorpha

          Order Octopoda

               Suborder Cirrata

               Suborder Incirrata

Species of cephalopods inhabit most marine habitats. Cephalopods inhabit tide pools, rocky patches, sandy bottoms, coral reefs, grass beds, mangrove swamps, coastal waters, and the open ocean from the surface through the water column to depths on the abyssal bottom at over 16, 000 ft (5000 m). See also Nervous system (invertebrate).

Cephalopods are high-level, active predators that feed on a variety of invertebrates, fishes, and even other cephalopods. The relatively sluggish nautiluses feed primarily on slow-moving prey such as reed shrimps, and even are scavengers of the cast-off shells of molted spiny lobsters. Cuttlefishes prey on shrimps, crabs, and small fishes, while squids eat fishes, pelagic crustaceans, and other cephalopods. Benthic octopuses prey mostly on clams, snails, and crabs. Salivary glands secrete toxins that subdue the prey and, in octopuses, begin digestion.

To protect themselves from predators cephalopods would rather hide than fight. To this end they have become masters of camouflage and escape. Benthic forms especially (for example, Sepia and Octopus) have evolved an intricate, complex system of rapid changes in color and patterns via thousands of individually innervated chromatophores (pigment cells) that allow precise matching to the color and pattern of the background. In addition, they regulate the texture of their skins by erecting papillae, flaps, and knobs that simulate the texture of the background. Many midwater oceanic squids camouflage against predation from below by turning on photophores (light organs) that match the light intensity from the surface and eliminate their silhouettes. See also Chromatophore; Photophore gland; Protective coloration.

Cephalopods have perfected jet propulsion for many modes of locomotion, from hovering motionless, to normal cruising, to extremely rapid escape swimming. Water enters the mantle cavity through an opening around the neck when the muscular mantle (body) expands. The mantle opening seals shut as the mantle contracts and jets the water out through the hoselike funnel, driving the cephalopod tail-first through the water.

The sexes are separate in cephalopods, and many species display complex courtship, mating, spawning, and parental care behavior. At mating, the male of most species transfers the spermatophores to the female with a specially modified arm, the hectocotylus. The spermatophores are implanted into the female's mantle cavity, around the neck, under the eyes, or around the mouth, depending on the species. Incubation takes a few weeks to a few months depending on the species.

Cephalopods are extremely important in the diets of toothed whales (sperm whales, dolphins), pinnipeds (seals, sea lions), pelagic birds (petrels, albatrosses), and predatory fishes (tunas, billfishes, groupers). For example, pilot whales in the North Atlantic feed almost exclusively on one species of squid, Illex illecebrosus, that aggregates for spawning in the summer. See also Mollusca.