marine ecosystems can be divided into many zones depending upon water depth and shoreline features. the oceanic zone is the vast open part of the ocean where animals such as whales, sharks, and tuna live. the benthic zone consists of substrates below water where many invertebrates live. the intertidal zone is the area between high and low tides; in this figure it is termed the littoral zone. other near-shore (neritic) zones can include estuaries, salt marshes, coral reefs, lagoons and mangrove swamps. in the deep water, hydrothermal vents may occur where chemosynthetic sulfur bacteria form the base of the food web.