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.

Description In the scientific literature, a cephalopod arm is often treated as distinct from a tentacle, though the terms are sometimes used interchangeably, often with the latter acting as an umbrella term for cephalopod limbs. Generally, arms have suckers along most of their length, as opposed to tentacles, which have suckers only near their ends.Young, R.E., M. Vecchione & K.M. Mangold 1999. Cephalopoda Glossary. Tree of Life web project. Barring a few exceptions, octopuses have eight arms and no tentacles, while squid and cuttlefish have eight arms (or two "legs" and six "arms") and two tentacles.Norman, M. 2000. Cephalopods: A World Guide. ConchBooks, Hackenheim. p. 15. "There is some confusion around the terms arms versus tentacles. The numerous limbs of nautiluses are called tentacles. The ring of eight limbs around the mouth in cuttlefish, squids and octopuses are called arms. Cuttlefish and squid also have a pair of specialised limbs attached between the bases of the third and fourth arm pairs [...]. These are known as feeding tentacles and are used to shoot out and grab prey." The limbs of nautiluses, which number around 90 and lack suckers altogether, are called cirri.

The tentacles of Decapodiformes are thought to be derived from the fourth arm pair of the ancestral coleoid, but the term arms IV is used to refer to the subsequent, ventral arm pair in modern animals (which is evolutionarily the fifth arm pair).

The males of most cephalopods develop a specialised arm for sperm delivery, the hectocotylus.

Cephalopod limbs function using a core of transverse muscle mass with a ring of longitudinally-oriented muscles embedded near its edge. The transverse muscle is wrapped by circular muscle fibres. The circular muscles are in turn wrapped by a pair of helical muscles with opposing handedness. Since the volume of each limb remains relatively constant, a decrease in one dimension causes an increase in another. For example, when the transverse and circular muscles contract, the cross-secional area of the limb is reduced. The limb's volume must remain constant so its length increases as a result. Likewise when the longitudinal muscles contract, the limb shortens and so the transverse muscle extends to increase the cross-sectional area.

Suckers Cephalopod limbs bear numerous suckers along their ventral surface as in octopus, squid and cuttlefish arms and in clusters at the ends of the tentacles (if present), as in squid and cuttlefish. Each sucker is usually circular and bowl-like and has two distinct parts: an outer shallow cavity called an infundibulum and a central hollow cavity called an acetabulum. Both of these structures are thick muscles, and are covered with a chitinous cuticle to make a protective surface. Suckers are used for grasping substratum, catching prey and for locomotion. When a sucker attaches itself to an object, the infundibulum mainly provides adhesion while the central acetabulum is free. Sequential muscle contraction of the infundibulum and acetabulum causes attachment and detachment.

Keel The keel, more specifically known as the swimming membrane or swimming keel, is a muscular extension, membrane, or vane on the aboral surface of coleoid limbs. These keels are composed of a core of non-fibrous connective tissue interlaced with transverse muscle bundles, which are encased in a layer of longitudinal muscle fibres which extend towards the keel's edge. The keels presumably streamline and improve the hydrodynamic characteristics of cephalopod arms, though this property has not been verified.

Abnormalities Many octopus arm anomalies have been recorded, including a 6-armed octopus (nicknamed Henry the Hexapus), a 7-armed octopus, a 10-armed Octopus briareus,Minor birth defect resulting in 10-armed juvenile, all arms fully present and functional. CephBase. one with a forked arm tip,Minor birth defect showing bifurcated arm tip. Both tips were fully functional. CephBase. octopuses with double or bilateral hectocotylization,Palacio, F.J. 1973. The Nautilus 87: 99–102. and specimens with up to 96 arm branches.Monster octopi with scores of extra tentacles. Pink Tentacle, July 18, 2008.

Branched arms and other limb abnormalities have also been recorded in cuttlefish, squid, and bobtail squid.

Variability Cephalopod limbs and the suckers they bear are shaped in many distinctive ways, and vary considerably between species. Some examples are shown below.

Arms For hectocotylized arms see hectocotylus variability. Shape of arm Species Family Todarodes pacificus Ommastrephidae

Tentacular clubs

Shape of tentacular club Species Family Abraliopsis morisi Enoploteuthidae Ancistroteuthis lichtensteini Onychoteuthidae Architeuthis sp. Architeuthidae Austrorossia mastigophora Sepiolidae Berryteuthis magister Gonatidae Idioteuthis cordiformis Mastigoteuthidae Iridoteuthis iris Sepiolidae Mastigoteuthis glaukopis Mastigoteuthidae Onykia ingens Onychoteuthidae Semirossia tenera Sepiolidae Spirula spirula Spirulidae Todarodes pacificus Ommastrephidae

Suckers

Shape of sucker Species Family Idioteuthis cordiformis Mastigoteuthidae Idioteuthis latipinna Mastigoteuthidae Magnapinna talismani Magnapinnidae Mastigoteuthis agassizii Mastigoteuthidae Mastigoteuthis atlantica Mastigoteuthidae Mastigoteuthis dentata Mastigoteuthidae Mastigoteuthis grimaldii Mastigoteuthidae Mastigoteuthis magna Mastigoteuthidae

Notes

References