Tentacles vs arms

Solving (partly) an old conundrum

You might have come across confusing statements like "octopuses have arms, squids have tentacles" and wondered what's the difference. I did and here's the (unsatisfactory) answer.

Cephalopods (squid, octopus, nautilus and a number of other aquatic creatures, which are a class of mollusc) have a number of muscular "limbs". Traditionally experts make the distinction between arms and tentacles. For example:

  • Octopuses have eight arms and no tentacles
  • Squid and cuttlefish have eight arms and two tentacles
  • Nautiluses have around 90 (suckerless) tentacles

What's the difference?

  • Arms occur around the mouth
  • Arms have suckers all the way along, while tentacles have them only at the tips.
  • Arms have finer control
  • Tentacles have an elongated shape and are longer than arms
  • Tentacles are mainly used to catch prey with the arms assisting and grasping
  • The internal structure of tentacles and arms differ.

Easy? Let's confuse the issue:

  • Variation and anomalies are common within cephalopods, including missing or extra arms, forking limbs, etc.
  • The tentacles of squid and cuttlefish appear to be evolutionarily derived from arms
  • It's not clear (to me anyway) that what's is called a tentacle in a nautilus is anything to do with a tentacle on a squid.
  • It's also unclear that a cephalopod "arm" is in any way related to the arms or limbs found on better known animals like mammals.
  • Biologists are prone to getting lazy and using the words interchangeably.