Shrimplike marine crustacean; the principal food of whalebone whales (5)

I believe the answer is:

krill

(Other definitions for krill that I've seen before include "Very small crustaceans" , "Tiny crustaceans" , "Whales' staple diet" , "Crustaceans, diet of whales" , "What baleen whales eat" .)

I've seen this clue in The Mirror.

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