yes
people and other octopuses
Usually octopuses eat crabs, clams and some other fishes too.
Octopuses eat shrimp, lobsters, crabs, snails and sometimes fish and other octopuses. Most octopuses hunt at night when their pray is sleeping.
no
What do octopuses eat is an English equivalent of 'Cosa mangiano i polpi'. The feminine noun 'cosa' literally means 'thing'. But it loosely translates as 'what'. The verb 'mangiano' means '[they] are eating, do eat, eat'. The masculine definite article 'i' means 'the'. The masculine noun 'polpi' means 'octopuses'. All together, they're pronounced 'KOH-sah MAHN-jah-noh ee POHL-pee'.
yes
they dont
"Necton" is a category name for Things That Swim, as "plankton" is one for Things That Float and "benthon" for Things That Crawl Around On The Bottom, so any thing, mostly a fish-thing, but also some mammals and reptiles that swim which is big enough to eat an octopus is likely to do so.
the like to eat clams
Sure, they can.
Octopuses eat pretty much whatever they like. They may not target clownfish but they will eat them if they want.