A sliver of space between two grains of sand. The intestines of a whale. The gill of a fish. A rock worn from centuries of pounding surf. An underwater eelgrass meadow. A salt marsh. An estuary. Thousands of miles of open ocean. Each of these places is a habitat—it's all a matter of scale.

For nematodes (tiny threadlike animals that are often less than a millimeter in length), the scale can be as small as the space between sand and/or mud particles. Nematodes are the most abundant type of animals in coastal and marine sediments, with several million nematodes commonly living within a square yard of mud or sand. In fact, if you took away all of the land and water of the earth, you would still see the outline of all continents and seas in the form of the nematodes left behind.

For whale habitat, the scale can cover thousands of miles of open ocean and coastal waters. Northern right whales (Eubalaena glacialis), for example, are known to give birth off the coast of Florida and migrate to areas off of Cape Cod in the late winter/early spring, later making their way up to Canada's Bay of Fundy in search of food. This species is known to roam further still. Amazingly, one right whale known as "Porter" was photographed off Cape Cod in May of 1999—and by September of that year he had wandered more than 8,500 miles to the waters of northern Norway!

Animals and plants themselves also serve as habitats. In fact, certain species of nematodes are whale parasites, which makes whales (or more specifically, whale internal organs) nematode habitat. Right whales are known habitat for barnacles, whale lice (small, parasitic crustaceans), and algae that take up residence on the distinctive white callosities that grow on whales' heads as they age.

Finally, habitat scale for certain species changes throughout their life cycle. For example, juvenile lobsters live for years in a burrow in a section of seafloor real estate that covers about a square foot. As they mature, these creatures can and do travel hundreds of miles to migrate, dramatically expanding their habitat scale in only two years' time.

No matter the scale, these habitats all provide the essential ingredients for the survival of the plants or animals that call them home.