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Article by: Roper, Clyde F. E. Division of Molluscs, Smithsonian Institution, U.S. National Museum, Washington, DC. Last updated: 2014 DOI: https://doi.org/10.1036/1097-8542.465400 (https://doi.org/10.1036/1097-8542.465400)

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Incirrata Other features Additional Readings Cirrata Links to Primary Literature

An order of the class Cephalopoda (subclass ), characterized by eight appendages that encircle the mouth, a saclike body, and an internal shell that is much modified or reduced from that of its ancestors. The origins of the Octopoda can be traced back to the Mesozoic period, and the Recent forms comprise the two suborders of octopods —Cirrata and . Cirrate have paddle-shaped fins on the body and tendrillike cirri on the arms, whereas incirrates lack fins or cirri. The octopods are a speciose (species-rich) group, with well over 450 known species; in addition, many others are recognized but have yet to be scientifically described and named. The living octopods are characterized by having the following features: eight appendages (arms); radially symmetrical suckers without horny (chitinous) rings; suckers without muscular stalks; head fused to the body (mantle) dorsally; fins present in some forms; an internal shell, in the form of a cartilage-like fin support (cirrates), or a pair of stylets (octopodids/octopuses senso stricto) [but can be absent]; and a crop usually present. The octopods are exclusively marine , with species occupying virtually every marine habitat in the world's oceans and seas, from the North to the South Pole, and from the surface shallows to depths of more than 5000 m (16,400 ft). A few species of intertidal octopuses may tolerate lower or higher salinities for short periods.

Two main groups of octopuses occur: the incirrate octopuses, which are the familiar inhabitants of shallow-water habitats; and the cirrate octopuses, which are inhabitants of the deep sea, generally deeper than 600 m (1970 ft). Octopods are a large and important group as top-level predators in all the marine ecosystems where they occur. Many species of incirrate octopuses are valuable as fishery resources, both in large commercial fisheries as well as in artisanal fisheries, with a value of $1 billion (U.S. dollars) in annual global trade. See also: Cephalopoda (/content/cephalopoda/120800); Coleoidea (/content /coleoidea/148100); Deep-sea fauna (/content/deep-sea-fauna/183200); Marine ecology (/content/marine-ecology /405800)

Incirrata

The incirrate octopuses are shallow-water creatures that are found commonly in benthic habitats, including coral and rocky reefs, sea grass meadows, sponge gardens, and soft, muddy bottoms. Many of these species are highly valued fishery products for human consumption. However, a few species occupy the open oceans and some even extend into the deep sea. The highest diversity of species occurs in the tropical Indo-West Pacific region, especially in the Indo-Malayan Archipelago, where upwards of 300 species may be recognized. Species of the Indo-West Pacific genus Hapalochlaena (Figs. 1 and 2) have toxic saliva that has caused death in humans.

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Fig. 1 Hapalochlaena maculosa, the lesser blue-ringed , displaying vivid, small blue rings on its body and arms, highlighted with black outlines. Location: Mrs. Watson's Bay, Lizard Island, Great Barrier Reef, Australia. (Photo copyright © Clyde F. E. Roper)

Fig. 2 Hapalochlaena fasciata, the blue-lined octopus, displaying vivid blue lines on its body and rings on its arms, highlighted with black outlines. Location: Long Reef, New South Wales, Australia. (Photo copyright © Clyde F. E. Roper)

Although most incirrate species are benthic forms that spend their entire adult life on the bottom, two modes of reproduction and larval development occur. All species lay their eggs on the bottom, attached in strings to rocks, coral, coconut shells, mollusk shells, and even discarded beverage bottles. Once the female has laid her eggs, she remains with them, carefully attending them to keep them well oxygenated and free from sediments and detritus. The female dies after the young have hatched. Two types of larval dispersal and growth occur after hatching of the paralarvae, as and octopus larvae are termed. Some species produce eggs that are very small, and the hatchlings consequently also are small. These hatchlings are released directly into the seawater and become free-swimming planktonic paralarvae during their juvenile developmental stages. They eventually settle back onto the bottom in their particular habitat, where their cycle of life continues with the next generation. The other type of reproductive strategy involves large eggs that yield large paralarvae, which are able to survive directly on the bottom as crawl-away young.

Two groups, the ctenoglossans and the argonautoids, are pelagic in the upper 600 m (1970 ft) of the water column for their entire lives. The ctenoglossans are characterized by gelatinous, transparent bodies, with small organs placed so as to minimize their silhouettes when viewed from below by potential predators in the twilight (dysphotic) zone. Argonautoids (Fig. 3), or paper , are muscular, near-surface, open-ocean dwellers that show extreme sexual dimorphism, with dwarf

2 of 5 2/1/2017 9:11 AM Octopoda - AccessScience from McGraw-Hill Education http://www.accessscience.com/content/octopoda/465400 males that reside in the shell-like egg case carried by the female. Incirrate octopus species can vary greatly in size, with arm spans ranging from a few millimeters to at least 3–4 m (10–13 ft) in total length. See also: Sexual dimorphism (/content /sexual-dimorphism/617800)

Fig. 3 Lateral view of a female Argonauta (family ).

Cirrata

The cirrate octopods live in the deep sea, generally below 600 m (1970 ft). Little is known about their biology and behavior. Although most species live in the water column, some species live just above the bottom and are able to settle on the substrate, perhaps to feed. These octopods have flaplike, long to rounded fins on their semigelatinous bodies, which are supported by a cartilaginous axial “shell.” They possess a single row of suckers on the arms, bordered on each side by a row of papilla-like cirri; the suckers are small and weakly developed, without chitinous rings, and are embedded in the gelatinous flesh. Cirrates lack an ink sac. These octopods are unsuitable for human consumption.

Other features

Most octopods have extremely well-developed eyes, and the incirrate forms (for example, the vulgaris) have an acuity of vision that rivals or exceeds that of fishes. Octopuses have highly developed and enlarged brains and a complex nervous system to support their superior visual and tactile senses. Experiments have shown that octopuses learn complicated tasks and have long-term memories. Behavioral, neurological, and anatomical studies of the octopus brain have contributed immeasurably to understanding the human brain. See also: Nervous system (invertebrate) (/content/nervous- system-invertebrate/449210)

Clyde F. E. Roper

Links to Primary Literature

G. Bello, Exaptations in (Cephalopoda, Coleoidea, Octopoda), N. Jb. Geol. Paläont. Abh., 266(1):85–92, 2012 DOI: https://doi.org/10.1127/0077-7749/2012/0290 (https://doi.org/10.1127/0077-7749/2012/0290)

Additional Readings

P. Jereb and C. F. E. Roper, of the World: Annotated and Illustrated Catalogue of Species Known to Date, FAO species catalogue no. 4, vol. 2, FAO, Rome, 2010

3 of 5 2/1/2017 9:11 AM Octopoda - AccessScience from McGraw-Hill Education http://www.accessscience.com/content/octopoda/465400 K. N. Nesis, Cephalopods of the World, T. F. H. Publications, Neptune City, NJ, 1987

M. Nixon and J. B. Messenger, The Biology of Cephalopods, Academic Press, London, 1977

M. Norman, Cephalopods: A World Guide, Conch Books, Hackenheim, Germany, 2000

M. Norman and F. G. Hochberg, Octopods, in Cephalopods of the World: Annotated and Illustrated Catalogue of Cephalopod Species Known to Date, FAO species catalogue no. 4, vol. 3, FAO, Rome, 2010

C. F. E. Roper, M. J. Sweeney, and C. E. Nauen, Cephalopods of the World: An Annotated and Illustrated Catalogue of Species of Interest to Fisheries, FAO species catalogue no. 125, vol. 3, FAO, Rome, 1984

M. J. Wells, Octopus: Physiology and Behavior of an Advanced Invertebrate, Chapman and Hall, London, 1978

S. B. Hedges and S. Kumar, The Timetree of Life, Oxford University Press, Oxford, UK, 2009

Dr. James B. Wood: The Cephalod Page: Order Octopoda (http://www.thecephalopodpage.org/octopoda.php)

TONMO: The Octopus News Magazine Online (http://www.tonmo.com/)

Tree of Life Web Project: Octopoda (http://tolweb.org/Octopoda/20083)

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