Covering more than 71 per cent of our planet, the oceans are home to more than 230,000 species of fish, mollusks, crustaceans, and marine mammals. The marine ecosystems vary greatly; coastal areas, open ocean, deep ocean, shallow waters, and intertidal zones are all vitally important habitats to the organisms that live, breed, and hunt there. Unlike terrestrial habitats the marine environment is constantly changing and the organisms that reside in these complex habitats need to adapt to the changes in temperature, salinity, sunlight, depth, turbulence of waves and ocean currents, and the nutrients available in order to survive.

The populations of many species are declining rapidly. Pollution, overfishing, urban developments, tourism, shipping, and drilling for oil and gas all have a major effect on the sensitive aquatic ecosystems. In addition to these temperature, precipitation, climate change, and naturally occurring tectonic activity all have an impact on the marine habitat and the creatures that live there.

This book explores the anatomical characteristics, various habitats and diet, as well as the reproductive, defensive, and social behavior of a variety of creatures that inhabit the vast oceans, from Antarctic krill, whose dense populations are a vital food source for a number of marine animals, to the mighty blue whale, the largest animal to have ever lived on earth.