Tentacles vs arms
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.