intertidal zone vs pelagic

intertidal zone

noun
  • The part of a shore between high water and low water, especially the beach exposed at maximum ebb spring tides. 

pelagic

noun
  • Any organism that lives in the open sea rather than in coastal or inland waters. 

adj
  • Living in the open sea rather than in coastal or inland waters. 

  • Of or pertaining to oceans. 

How often have the words intertidal zone and pelagic occurred in a corpus of books? (source: Google Ngram Viewer )