Tentacle/Arm, Arm/Tentacle???

[Graeme]

Just wondered, as a direct result of my uni-related cephy exploits, what on earth's the difference between an arm and a tentacle!?

I know all about squid having arms with rows of suckers, and tentacles with a club filled with suckers, and the carpal having very sparse arrangements-to-no suckers etc, and the tentacles are specialised for (usually) catching prey but I start getting confused when octopuses don't have tentacles, they have arms, meanwhile nautiluses don't have arms they have TENTACLES! And they're devoid of suckers! Instead they have adhesive ridges or something... so what acutally IS the difference between an arm and a tentacle?

Would it have something ot do with the amount of motor control? Are arms more dextrous than tentacles? I'm kinda lost now....

Graeme

 

[Phil]

Hi Graeme, not exactly an answer for you but here goes anyway.

You may find the attached diagram of some interest as it shows the order of arms and tentacles in different cephalopods. As far as I understand it, a tentacle is a specialised feeding adaption of an arm capable of much greater expansion.

I know I've mentioned this before a few times, but the original state for coleoid cephalopods was to have ten equal arms, as is demonstrated in particularly well preserved belemnoid fossils. From a belemnoid-type ancestor, the decapods (squid, cuttlefish etc), modified Arm Pair IV into specialised feeding tentacles which the Tree of Life Glossary defines thus:

Tentacles - Modified 4th pair of appendages in decapodiforms that are capable of considerable extension and contraction. Although the tentacles are derived evolutionarily from the 4th pair of arms, the term, "arms IV" is reserved for the subsequent arm pair, the ventral arms, which are evolutionarily the 5th pair of arms. Each tentacle consist of a proximal stalk usually devoid of suckers, and a distal club armed with suckers and occasionally hooks.

The octopods did not modify or lose Arm Pair IV, instead they lost Arm Pair II entirely, though Vampyroteuthis which is neither squid nor octopus reduced the same Arm Pair II to unique feeding filaments. This probably implies there is a closer link between the Vampire Squid to the octopods than to the decapods.

I am not sure about Nautilus, though. The tentacles, up to 95-ish, are modified to a minor extent as some are used for steering, pulling food towards the mouth, grabbing on to prey etc. Why they are not referred to as arms I'll need to look up; maybe it's nothing more than traditional nomenclature that has become generally accepted and is without a functional explanation. Perhaps the specialised definition of tentacle only applies to coleoid cephalopods? Not sure, I'm afraid.

 

[Graeme]

Yeah, I've been looking at that pic on the Tree of Life site. Was there not some mention about ancestral cephs probably not having as much motor control in their appendages compared to modern ceph's? This is kinda why I thought it was a dexterity thing. Tentacles seem pretty much to be used to extend and contract only, whereas arms are used for everything else. Interesting that octopods didn't lose the most obvious-sounding pair! I take it that they lost the Pair II arms before the differentiation of the arms IV into tentacles then? Unless tentacles are the ancestral appendage, providing it IS a motor thing... AARGH!!

Right. I understand about hte specialisation of the Hectocotylus etc, and why it's got such a weird name.

Graeme