"Fish congregate on the reef because that's where the resources are," said James Albert, curator of fishes at the Florida Museum of Natural History in Gainesville.
As the movie shows, reefs are also a great place to hide.
"If you're a big shark, it's hard to get a meal right over those reefs," Heithaus said. "Unless you catch something unaware, it'll just dart down into the coral, and you'll never get it out."
Fish lack the social organization found in insects, for example. Many fish species stick together in schools to protect themselves from predators, but they do not have one captain or commander who is in charge.
However, like most animals, fish regard their own species in a completely different context from everything else in nature. They may not necessarily "care" about their fellow fish, but they are certainly aware of them.
"The dominant form of social interaction [among fish] is trying to find a mate," Albert said. "So males will tend to the distribution of other males and females to other females, because they are competitors for mating."
Albert has studied the electric currents that some fish discharge to attract the opposite sex. He found that each species has its own courtship signal. Females seek males with strong signals, an indication of health and virility.
"They use these signals very much like birds use song," he said. "The big difference is birds turn their song on and off, while electrical fish continue discharging these signals their whole life, day and night."
While size is a determining factor among most fish species, the biggest male will not always win the fight for a female. Territoriality may play an even bigger part.
"The resident male will often win the battle for a female," Albert said. "Every visiting animal has to make a calculation before the fight: Is this going to be worth it?"
A study of African cichlids (a freshwater and brackish-water fish species), led by Russell Fernald, a professor of neuroscience at Stanford University in Palo Alto, California, showed remarkable physiological changes in male cichlids that foughtand beatother males.
Within seconds, the victorious cichlid would sport an eye stripe and bright yellow and blue coloration, controlled by cells like those in a chameleon. Over the next week, he would develop several more physical changes, including extra muscles. Once another fish defeated the dominant male, however, the changes would begin to reverse, turning the fish back to a muted, female-like appearance.
Many male damselfish, wrasses, and angelfish, among others, maintain harems. Some haremic species are also sex changers.
"Should [a male] cease his domination, one of the females will change sex and take over part of his harem," said George Losey, a zoology professor at the University of Hawaii in Honolulu. "The female can change her behavior to a male in a few hours and become a fully functional male in days to weeks."
Surprisingly little is known about the mating of the sea's top predatorssharks. When it comes to mating, scientists have only really been able to study nurse sharks, one of the few species that come into shallow waters to mate. Males have to grab the female's pectoral fin to be able to mate and will try to pull females into deeper waters.
One observation by Heithaus suggests that male sharks may sometimes work together in the mating ritual. Some sharks may not go for the pectoral fin, but instead block the female from escaping in order to help another male.
As top predators, sharks can change the marine habitat by eating lots of one prey or scaring other fish into changing their behavior.
"Not a lot of people get hit by cars while they're crossing the road, but we change our behavior when we're near the road," Heithaus said. "Similarly, in a marine environment, many animals may decide to not go into a spot where there are lots of sharks."
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