See 31-Across

FISH

This aquatic creature might be found swimming in a school.

This clue is a cross-reference, meaning the answer to 31-Across will help you solve this one. Since the answer is "FISH," it suggests that 31-Across likely contains a word or phrase that hints at "fish," such as a type of fish, a fishing term, or a related pun. Cross-references are common in crosswords to connect answers and add an extra layer of solving.