Parasites are animals that live at the expense of other animals. They rob the host animal of food, and can make it become sick. However, in other forms of relationship different animals can help one another. Some hermit crabs place sea anemones on their shells, hiding under their protective stinging tentacles. At the same time, the sea anemone benefits because it shares the crab’s food. Similarly, a species of shrimp digs a burrow that it shares with the small goby fish. The fish benefits from being able to hide in the burrow, while acting as a lookout to warn the shrimp of approaching predators. This kind of relationship can only be revealed by the careful study of animal behaviour. Most true parasites are very simple animals, because they do not need complicated organs to digest their food. Some parasites are simply a mass of reproductive organs.
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