Don't know. The spell affects a 20' radius spread. The tentacles do not have reach outside that area.You can cast it on them. They are then automatically within the area of the spell.
Must be within the area, as the tentacles have no reach.
1) When you cast the spell.
2) On the enemies' turns, assuming they attempt grapple checks to get ungrappled, or when the enemy enters the area
3) On your turn to do damage, as well as to "reacquire" targets
Note that the tentacles have a BAB of 1/2 your caster level. This may entitle them to multiple grapple checks per round (when attempting to do damage).
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