Cephalopod
A cephalopod is any member of the molluscan class Cephalopoda (Greek plural κεφαλόποδες, kephalópodes; "head-feet")[3] such as a squid, octopus, cuttlefish, or nautilus. These exclusively marine animals are characterized by bilateral body symmetry, a prominent head, and a set of arms or tentacles (muscular hydrostats) modified from the primitive molluscan foot. Fishers sometimes call cephalopods "inkfish", referring to their common ability to squirt ink. The study of cephalopods is a branch of malacology known as teuthology.
Cephalopods became dominant during the Ordovician period, represented by primitive nautiloids. The class now contains two, only distantly related, extant subclasses: Coleoidea, which includes octopuses, squid, and cuttlefish; and Nautiloidea, represented by Nautilus and Allonautilus. In the Coleoidea, the molluscan shell has been internalized or is absent, whereas in the Nautiloidea, the external shell remains. About 800 living species of cephalopods have been identified. Two important extinct taxa are the Ammonoidea (ammonites) and Belemnoidea (belemnites). Extant cephalopods range in size from the 10 mm (0.3 in) Idiosepius thailandicus to the 14 m (45.1 ft) colossal squid, the largest extant invertebrate.
There are over 800 extant species of cephalopod,[4] although new species continue to be described. An estimated 11,000 extinct taxa have been described, although the soft-bodied nature of cephalopods means they are not easily fossilised.[5]
Cephalopods are found in all the oceans of Earth. None of them can tolerate fresh water, but the brief squid, Lolliguncula brevis, found in Chesapeake Bay, is a notable partial exception in that it tolerates brackish water.[6] Cephalopods are thought to be unable to live in fresh water due to multiple biochemical constraints, and in their >400 million year existence have never ventured into fully freshwater habitats.[7]
Cephalopods occupy most of the depth of the ocean, from the abyssal plain to the sea surface. Their diversity is greatest near the equator (~40 species retrieved in nets at 11°N by a diversity study) and decreases towards the poles (~5 species captured at 60°N).[8]
Cephalopods are widely regarded as the most intelligent of the invertebrates, and have well developed senses and large brains (larger than those of gastropods).[9] The nervous system of cephalopods is the most complex of the invertebrates[10][11] and their brain-to-body-mass ratio falls between that of endothermic and ectothermic vertebrates.[8]: 14 Captive cephalopods have also been known to climb out of their aquaria, maneuver a distance of the lab floor, enter another aquarium to feed on the crabs, and return to their own aquarium.[12]
Left: A pair of cuttlefish (Sepia officinalis) in shallow water
Right: An octopus (Benthoctopus sp.) on the Davidson Seamount at 2,422 m depth
Left: An octopus opening a container with a screw cap
Right: Hawaiian bobtail squid, Euprymna scolopes, burying itself in the sand, leaving only the eyes exposed
The primitive nautilus eye functions similarly to a pinhole camera.
The W-shaped pupil of the cuttlefish expanding when the lights are turned off
A cuttlefish with W-shaped pupils which may help them discriminate colors.
This broadclub cuttlefish (
Sepia latimanus) can change from camouflage tans and browns (top) to yellow with dark highlights (bottom) in less than a second.
Viscera of Chtenopteryx sicula
Viscera of Ocythoe tuberculata
Octopuses swim headfirst, with arms trailing behind
Nautilus belauensis seen from the front, showing the opening of the hyponome
Cross section of Spirula spirula, showing the position of the shell inside the mantle
Cuttlebone of Sepia officinalis
Gladius of Sepioteuthis lessoniana
Left: A giant squid found in Logy Bay, Newfoundland, in 1873. The two long feeding tentacles are visible on the extreme left and right.
Right: Detail of the tentacular club of Abraliopsis morisi
The two-part beak of the giant squid,
Architeuthis sp.
Amphioctopus marginatus eating a crab
Female
Argonauta argo with eggcase and eggs
Detail of the hectocotylus of
Ocythoe tuberculata A dissected male specimen of
Onykia ingens, showing a non-erect penis (the white tubular structure located below most of the other organs)
A specimen of the same species exhibiting an elongation of the penis to 67 cm in length
Egg cases laid by a female squid
Chtenopteryx sicula paralarvae.
Left: Two very young paralarvae. The circular tentacular clubs bear approximately 20 irregularly arranged suckers. Two chromatophores are present on each side of the mantle.
Centre: Ventral, dorsal and side views of a more advanced paralarva. An equatorial circulet of seven large yellow-brown chromatophores is present on the mantle. Posteriorly the expanded vanes of the gladius are visible in the dorsal view.
Right: Ventral and dorsal views of a very advanced paralarva.
Left: Immature specimens of Chiroteuthis veranyi. In this paralarval form, known as the doratopsis stage, the pen is longer than the mantle and 'neck' combined
Right: A mature Chiroteuthis veranyi. This species has some of the longest tentacles in proportion to its size of any known cephalopod.
An ammonoid with the body chamber missing, showing the septal surface (especially at right) with its undulating lobes and saddles
Chambered nautilus (
Nautilus pompilius)
Common cuttlefish (
Sepia officinalis)
Atlantic bobtail (
Sepiola atlantica)
European squid (
Loligo vulgaris)
Common octopus (
Octopus vulgaris)
Gyronaedyceras eryx, a nautiloid from the Devonian of Wisconsin
Various species of ammonites
Holotype of
Ostenoteuthis siroi from family Ostenoteuthidae.
Pyritized fossil of
Vampyronassa rhodanica, a vampyromorphid from the Lower Callovian (
166.1 million years ago)
Pen and wash drawing of an imagined colossal octopus attacking a ship, by the malacologist Pierre de Montfort, 1801
The NROL-39 mission patch, depicting the National Reconnaissance Office as an octopus with a long reach