Tagging a squid is not too difficult, as long as you keep away from its arms! But with the electronic tags that are available today, you need a big squid. That's why we have only tagged Humboldt squid so far. There are other squid that would be big enough, but they are really rare and nobody knows how to catch them. There is a new kind of tag that sends acoustic (sound) signals to receivers on the bottom of the sea that let you see where the tagged squid goes, and these have been used with the much smaller California market squid.

Lots of different types of fish have been tagged, and they just need to be large enough so that the tag does not disturb them too much. Same for an octopus -- tagging a giant octopus would be great. Do they really only hang out in their caves waiting...?