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Coral Reef Notes
A Night in the Coral Reef
In the life sciences, certain patterns begin to emerge when we shift our focus from
studying individuals to studying collections of species and how they interact with and
within their physical environment. Through careful observation of these patterns, we can
enhance our understanding of the world around us.
We can observe several such patterns within the coral reef environment. For example,
fishes inhabit different vertical bands along the reef. Some fish live on or near the
sediment, others flit between the myriad coral formations, and still others swim in the
shallow waters above the reef and in the open sea surrounding it.
Not only do reef dwellers occupy distinct zones, but they also exhibit discernible feeding
habits. Some reef animals are diurnal, meaning they are active only during the day,
while others are nocturnal, or active primarily at night.
As light diminishes, daytime animals seek shelter in the reef's caves and crevices—
often the very same spaces vacated by their nocturnal counterparts. Included among
the diurnal species are the plant-eaters (herbivores), some flesh-eaters (carnivores),
and animals that feed on both plant and animal matter (omnivores).
By contrast, the animals most active on the reef at night are primarily carnivorous.
Shrimp make their way to the surface to feed on plankton. Thus exposed, they become
easy prey for larger predators. So too do crustaceans, mollusks, and starfish, which
comb the sea floor at night for food. The reef's top nocturnal predators, the tiger shark,
octopus, and eel, feed on these and other available prey.
Many species have evolutionary adaptations that function to enhance survival in a
particular environment. Nocturnal reef dwellers have evolved unique characteristics that
help them find their prey with little or no available light. For example, a type of nonvisual
sensory system allows certain nocturnal fish to detect even the slightest disturbance in
water when a potential prey swims past.
Energy Flow in a Coral Reef System
All living things require energy to carry out life functions such as growth, movement, and
reproduction. For nearly all ecosystems—diverse collections of species that interact with
each other and their physical environment—the major source of energy is the sun. The
flow of energy tends to follow the same basic pattern whether the ecosystem is a
tropical rainforest or a coral reef.
To portray the transfer of energy through an ecosystem, ecologists use simple models
called food chains. Organisms can be organized into different trophic levels, or positions
in a food chain; organisms at higher trophic levels feed on those at lower levels.
As producers, plants and some microorganisms are an ecosystem's foundation species.
Producers make their own food from energy that comes directly from the sun. When
sunlight enters a coral reef, for example, phytoplankton, algae, and plants such as sea
grasses convert the light energy into chemical energy through the process of
photosynthesis.
However, not all organisms can make their own food. Consumers are organisms that
obtain food and the energy stored within food by eating organisms. Consumers that
feed only on plant life are called herbivores. Consumers that feed only on animals are
called carnivores. Omnivores feed on both plants and other animals. Because
organisms use about 90 percent of the energy they take in for their own survival and
growth—leaving only about 10 percent for the consumer that eats it—there are fewer
organisms supported in successively higher trophic levels.
Scavengers and decomposers also play an important role in an ecosystem: They are its
primary recyclers. Scavengers are animals that feed on dead members of different
trophic levels. Decomposers, which include bacteria and fungi, break down organic
waste material and return essential elements, such as nitrogen and phosphorous, to an
ecosystem.
While a food chain shows just one possible pathway along which energy can move,
most consumers have more than one food source. To portray all feeding relationships in
an ecosystem, a more complex model, called a food web, is used. Each organism in a
food web plays an important role, so the loss of any one species due to environmental
threats such as disease and pollution can disrupt a strand and thus may potentially
threaten the health of an entire ecosystem.