From the deep oceans to the shallow coastlines, the world's seas are full of strange and wonderful creatures. The tiger shark glides quietly through the waters seeking a meal while Antarctic krill float under the ice near Antarctica. Sea otters play in shallow water along the coastline; meanwhile the ochre sea star sucks its meal from an unsuspecting mussel. All are keystone species. Each species is so important to its habitat that its absence might cause the whole habitat to change or even disappear. The balance of nature that keeps our environments healthy depends on all keystone species.