The octopus is a soft-bodied, eight-limbed mollusc of the order Octopoda. Around 300 species are recognised, and the order is grouped within the class Cephalopoda with squids, cuttlefish, and nautiloids. Like other cephalopods, the octopus is bilaterally symmetric with two eyes and a beak, with its mouth at the center point of the eight limbs. The soft body can rapidly alter its shape, enabling octopuses to squeeze through small gaps. They trail their eight appendages behind them as they swim. The siphon is used both for respiration and for locomotion, by expelling a jet of water. Octopuses have a complex nervous system and excellent sight, and are among the most intelligent and behaviourally diverse of all invertebrates.
Squid are cephalopods in the superorder Decapodiformes with elongated bodies, large eyes, eight arms and two tentacles. Like all other cephalopods, squid have a distinct head, bilateral symmetry, and a mantle. They are mainly soft-bodied, like octopuses, but have a small internal skeleton in the form of a rod-like gladius or pen, made of chitin.
The radula is an anatomical structure used by mollusks for feeding, sometimes compared to a tongue. It is a minutely toothed, chitinous ribbon, which is typically used for scraping or cutting food before the food enters the oesophagus. The radula is unique to the molluscs, and is found in every class of mollusc except the bivalves, which instead use cilia, waving filaments that bring minute organisms to the mouth.
The family Cranchiidae comprises the approximately 60 species of glass squid, also known as cockatoo squid, cranchiid, cranch squid, or bathyscaphoid squid. Cranchiid squid occur in surface and midwater depths of open oceans around the world. They range in mantle length from 10 cm (3.9 in) to over 3 m (9.8 ft), in the case of the colossal squid. The common name, glass squid, derives from the transparent nature of most species. Cranchiid squid spend much of their lives in partially sunlit shallow waters, where their transparency provides camouflage. They are characterised by a swollen body and short arms, which bear two rows of suckers or hooks. The third arm pair is often enlarged. Many species are bioluminescent organisms and possess light organs on the undersides of their eyes, used to cancel their shadows. Eye morphology varies widely, ranging from large and circular to telescopic and stalked. A large, fluid-filled chamber containing ammonia solution is used to aid buoyancy. This buoyancy system is unique to the family and is the source of their common name "bathyscaphoid squid", after their resemblance to a bathyscaphe. Often the only organ that is visible through the transparent tissues is a cigar-shaped digestive gland, which is the cephalopod equivalent of a mammalian liver. This is usually held in a vertical position to reduce its silhouette and a light organ is sometimes present on the lower tip to further minimise its appearance in the water.
Amphitretidae is a family of mesopelagic octopods which contains three subfamilies, formerly classified as families in their own right. It is classified in the superfamily Octopodoidea. Species in the family Amphipetridae are characterised by having a single row of suckers on each arm, a gelatinous body and non hemispherical eyes.
The Bolitaeninae are a subfamily, in the family Amphitretidae, of small, common pelagic octopuses found in all tropical and temperate oceans of the world. The taxonomy of this taxon is not entirely certain; recent research suggests just two genera exist, Bolitaena and Japetella, both of which are thought to be monotypic by some authorities and under this view, the family would represent two very similar species: Bolitaena pygmaea and Japetella diaphana. However, currently a second species of Bolitaena, B. massyae is also recognised.
Vitreledonella richardi, also known as the glass octopus, is an incirrate octopus. It is in the genus Vitreledonella and of the family Amphitretidae.
Idioteuthis cordiformis is a species of whip-lash squid found in tropical regions of the west Pacific Ocean. The species is commonly known as the 'love-heart squid' because the species name cordiformis is Latin for 'heart shaped'. Recently, this species has been found to consume small birdbeak dogfish.
All cephalopods possess flexible limbs extending from their heads and surrounding their beaks. These appendages, which function as muscular hydrostats, have been variously termed arms, legs or tentacles.
Stauroteuthis syrtensis, also known as the glowing sucker octopus or bioluminescent octopus, is a species of small pelagic octopus found at great depths in the north Atlantic Ocean. It is one of a very small number of octopuses to exhibit bioluminescence.
Sepia trygonina, the trident cuttlefish, is a species of cuttlefish in the genus Sepia from the Red Sea and the western Indian Ocean. Cuttlefish are a specific type of cephalopod that is a highly evolved branch of the Mollusca phylum. They are characterized by having a calcareous shell that is covered by a membrane with free fin lobes that are laterally placed on both sides of their head. They have a centered beak that is used for feeding which is surrounded by 10 appendages. The trident cuttlefish are carnivores that prey on fish, crustaceans, and shellfish. They are also a major source of food for larger marine life like dolphins, seals, and even birds.
Macrochlaena winckworthi, Winckworth's octopus, is a little known species of octopus, it is the only species in the monotypic genus Macrochlaena, in the family Octopodidae. It was described by the British malacologist Guy Coburn Robson in 1926, the type specimens having been collected in the Gulf of Mannar, off Thoothukudi in Tamil Nadu, southeastern India.
Cryptoteuthis brevibracchiata, the short-arm flapjack octopod, is a deepwater species of octopod. It is the only species in the monotypic genus Cryptoteuthis one of the cirrate octopuses of the family Opisthoteuthidae, the umbrella octopuses. It is known from a single specimen which was collected in the northeastern Atlantic Ocean. It has characteristics which are shared with two other genera in the Opisthoteuthidae, Opisthoteuthis and Grimpoteuthis, but is sufficiently distinctive from either of these to warrant the erection of a new genus.
Luteuthis is a small genus of cirrate octopuses from the family Opisthoteuthidae. There are two species classified in this genus on from waters west of New Zealand and the other from the South China Sea. This genus is characterised by having the head elongated with the mantle and funnel completely joined to the head. They are rather gelatinous and lack areolar spots, have a simple web with no nodules and long dorsal arms, much longer than the ventral arms. The arms have crenulated suckers along their whole length and each sucker has two cirra which are about half as long as the width of the sucker. The gills resemble a "half-orange" and have seven lamellae. They have a W-shaped shell and lateral wings which have inrolled margins and which taper to acute points. They have a radula and large teeth on the palate but they do not have an ink sac or salivary glands.
Taonius belone is a glass squid belonging to the genus Taonius from the family Cranchiidae. It occurs in the northern subtropical and in the tropical or equatorial waters of the Pacific and Indian Oceans.
Gonatopsis okutanii is a species of squid from the family Gonatidae from the northern Pacific Ocean. It is of uncertain taxonomic status, the presence of remnant tentacles on spent females indicate that this species does not belong in the genus Gonatopsis and the differences between this species and Gonatus makodai have led to some authorities stating that G. okutanii is a junior synonym of Eogonatus tinro. However the World Register of Marine Species still recognises Gonatopsis okutanii as the valid name for this taxon.
Grimpoteuthis meangensis is known from either one or two specimens, though the second may be of a completely different species. The first was damaged. Both were found by William Evans Hoyle within one year of each other, and no other animals of the species have been identified since 1886.
Wunderpus photogenicus, the wunderpus octopus is a small-bodied species of octopus with distinct white and rusty brown coloration. 'Wunderpus' from German “wunder” meaning ‘marvel or wonder’. Due to the appearance and behavior of the wunderpus, it is frequently confused with its close relative, the mimic octopus. The wunderpus octopus was not discovered until the 1980s and was only officially described in detail in 2006. The wunderpus octopus is important commercially to the underwater photography, dive and tourism communities, especially throughout Indonesia. The wunderpus is also valued as an expensive ornamental marine species for the home aquarium.
Opisthoteuthis agassizii is a lesser-known, deep-sea octopus first described in 1883 by Addison E. Verrill.
Opisthoteuthis grimaldii is an octopus found near the Azores.
