The zoanthus polyps in both videos are giving a typical feeding responce. In this case it looks to be a combination of tiny organic matter, as well as baby pods swimming freely. Perhaps there was just a hatch, and the newborns unsuspectingly swam right into the colony.
As things brush the tentacles of cnidarians, it causes their stinging cells, nematocysts, to project out into the prey. This paralizes the prey, and then the food can be moved into the gut for digestion. This is the why larger cnidarians such as anemones appear to have "sticky" tentacles, they are actually nematocysts grabbing on. Most often, the stinging cells of smaller cnidarians cannot penetrate the skin, however certain people are more sensitive than others. On the other hand, larger species such as jellyfish and carpet anemones have a potent dose of stinging cells to capture prey, and can easily penetrate our skin causing a sting.
Allthough we cannot see the nematocysts projected into the prey in these videos, as they are microscopic in size, we do however see the the zoanthus polyps partially and full retract. This is because the polyps have captured prey, and the closing of the polyps helps push the prey into the oral slit.
