Cephalopod arm

Cephalopod arm
Arm of Illex illecebrosus with two rows of suckers along its length
Tentacle of Illex illecebrosus with a distal tentacular club (right)
Octopus arm with two rows of suckers

A cephalopod arm is distinct from a tentacle, though the terms are often used interchangeably.

Generally, cephalopod arms have suckers along most of their length, as opposed to tentacles, which have suckers only near their ends.[1] Octopuses have eight arms and no tentacles, while squid and cuttlefish have eight arms and two tentacles.[2] The limbs of nautiluses, which number around 90 and lack suckers altogether, are called tentacles.[2][3][4]

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).[1]

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


Many octopus arm anomalies have been recorded,[5][6] including a 6-armed octopus nicknamed Henry the Hexapus, a 7-armed octopus,[7] a 10-armed Octopus briareus,[8] one with a forked arm tip,[9] an octopus with double hectocotylization,[10] bilateral hectocotylization,[11] and specimens with up to 96 tentacle branches.[12][13][14]

Branched arms have also been recorded in cuttlefish.[15]


  1. ^ a b Young, R.E., M. Vecchione & K.M. Mangold 1999. Cephalopoda Glossary. Tree of Life web project.
  2. ^ a b 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."
  3. ^ Fukuda, Y. 1987. Histology of the long digital tentacles. In: W.B. Saunders & N.H. Landman (eds.) Nautilus: The Biology and Paleobiology of a Living Fossil. Springer Netherlands. pp. 249–256. doi:10.1007/978-90-481-3299-7_17
  4. ^ Kier, W.M. 1987. The functional morphology of the tentacle musculature of Nautilus pompilius.PDF In: W.B. Saunders & N.H. Landman (eds.) Nautilus: The Biology and Paleobiology of a Living Fossil. Springer Netherlands. pp. 257–269. doi:10.1007/978-90-481-3299-7_18
  5. ^ Kumph, H.E. 1960. Arm abnormality in octopus. Nature 185(4709): 334-335. doi:10.1038/185334a0
  6. ^ Toll, R.B. & L.C. Binger 1991. Arm anomalies: cases of supernumerary development and bilateral agenesis of arm pairs in Octopoda (Mollusca, Cephalopoda) Zoomorphology 110(6): 313–316.doi:10.1007/BF01668021
  7. ^ Gleadall, I.G. 1989. An octopus with only seven arms: anatomical details. Journal of Molluscan Studies 55: 479–487.
  8. ^ Minor birth defect resulting in 10-armed juvenile, all arms fully present and functional. CephBase.
  9. ^ Minor birth defect showing bifurcated arm tip. Both tips were fully functional. CephBase.
  10. ^ Palacio, F.J. 1973. On the double hectocotylization of octopods.PDF The Nautilus 87: 99–102.
  11. ^ Robson, G.C. 1929. On a case of bilateral hectocotylization in Octopus rugosus. Journal of Zoology 99(1): 95–97. doi:10.1111/j.1469-7998.1929.tb07690.x
  12. ^ Okada, Y.K. 1965. On Japanese octopuses with branched arms, with special reference to their captures from 1884 to 1964. Proceedings of the Japan Academy 41(7): 618–623.
  13. ^ Okada, Y.K. 1965. Rules of arm-branching in Japanese octopuses with branched arms. Proceedings of the Japan Academy 41(7): 624–629.
  14. ^ Monster octopi with scores of extra tentacles. Pink Tentacle, July 18, 2008.
  15. ^ Okada, Y.K. 1937. An occurrence of branched arms in the decapod cephalopod, Sepia esculenta Hoyle. Annotated Zoology of Japan 17: 93–94.