It is a crab. One of the creatures from down in the water.

[She might not be saying that once she's tasted it, but Rei's already scooping some out into a shard of shell and offering it...along with a cannister of salt from her exorcism supplies.]

A friend of mine is a talented hunter, and he brought it down. And you may want to add some salt to mask the taste. It is...seriously unpleasant.