Narwhals!

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Across
  1. 3. Narwhals live in this ocean
  2. 6. Mythical horse with one horn
  3. 7. Narwhal lifespan
Down
  1. 1. Narwhals communicate with whistles and these
  2. 2. Cross between Narwhals and Belugas
  3. 4. The length of the longest narwhal tusk
  4. 5. Narwhals eat these