Sharktocrab
{2}{G}{U}: Adapt 1. (If this creature has no +1/+1 counters on it, put a +1/+1 counter on it.) Whenever one or more +1/+1 counters are put on this creature, tap target creature an opponent controls. That creature doesn't untap during its controller's next untap step.
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Official rulings (4) · Scryfall
Sharktocrab's triggered ability can target a creature that's already tapped. That creature won't untap during its controller's next untap step.
An ability that triggers when counters are put on a permanent will trigger if that permanent somehow enters the battlefield with those counters.
You can always activate an ability that will cause a creature to adapt. As that ability resolves, if the creature has a +1/+1 counter on it for any reason, you simply won't put any +1/+1 counters on it.
If a creature somehow loses all of its +1/+1 counters, it can adapt again and get more +1/+1 counters.