Opal Crab

Opal Crab is a catchable aquatic creature found in the pond in The Encroachment. It is one of the fishable creatures tied to that area, and it can also be obtained through trading rather than only by fishing.

For practical use, Opal Crab has a trade use with The Blacksmith, who exchanges it for a Cement Bag. If you are farming it from the pond, it is worth optimizing your fishing setup there, since the source notes that wearing a specific item increases the chance of catching it.