Cheapest priced copy (Commander Legends): $0.35 at Card Kingdom, near mint, their price list as of .
Sharktocrab 
Creature — Shark Octopus Crab · 4/4 · Uncommon · Commander Legends · #450
{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.
Legal inCommanderPioneerModernLegacyVintage
Not legal inBrawlStandardPauper
3 printings · $0.35 to $0.79 at Card Kingdom
| Set | Number | Rarity | Card Kingdom | Foil |
|---|---|---|---|---|
| Commander Legends | 450 | Uncommon | $0.35 | — |
| Ravnica Allegiance | 206 | Uncommon | $0.35 | $0.79 |
| Ravnica Remastered | 222 | Uncommon | $0.35 | $0.49 |
4 rulings from Wizards of the Coast
- 25 January 2019Sharktocrab's triggered ability can target a creature that's already tapped. That creature won't untap during its controller's next untap step.
- 25 January 2019An ability that triggers when counters are put on a permanent will trigger if that permanent somehow enters the battlefield with those counters.
- 25 January 2019You 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.
- 25 January 2019If a creature somehow loses all of its +1/+1 counters, it can adapt again and get more +1/+1 counters.