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This creature can't attack unless defending player controls an Island.
Whenever this creature blocks or becomes blocked by a creature that has been dealt damage this turn, this creature gets +2/+0 and gains trample until end of turn.
When you control no Islands, sacrifice this creature.
The bonus happens when a previously damaged creature becomes a blocker and the bonus is not removed if that creature is removed from the blocking situation by any means.
Giant Shark only gains its bonus if the blocking creature was damaged before blocking was declared. Damaging the creature later will not give the Shark the bonus.
Giant Shark gets the bonus once for each creature it blocks or is blocked by that meets the criteria.
If a creature enters a blocking situation with the Giant Shark due to being part of a banded group of attackers, the Shark will get the bonus if that creature is damaged at that time.