Page 172 - (DK) Ocean - The Definitive Visual Guide

170 THE OPEN OCEAN AND OCEAN FLOOR

Plankton and Nekton The Twilight Zone

In spring, as phytoplankton blooms begin to develop, zooplankton start In the twilight zone, there is just enough
multiplying. They follow the phytoplankton into the sunlit zone to feed. light for animals to see—and be seen. As
Most are herbivores that feed on phytoplankton; some are carnivores a result, predators and prey are in constant
that hunt other zooplankton. Many are classed as meroplankton—the battle. Many species are almost totally
young of animals like crabs, lobsters, barnacles, and some fish—which translucent, to avoid casting even a faint
have a planktonic larval stage and use the currents to spread. By taking shadow. Others are reflective, to disguise
advantage of the summer phytoplankton feast, they avoid competing themselves against the light from above,
for food with adults of their own kind. While plankton drift with the or have wafer-thin bodies that reduce their
currents, many free-swimming animals (collectively called nekton) silhouette. To cope with dim light, many
gather to feed on them: fish, animals in this zone have large eyes.
squid, marine mammals, COPEPOD The main source of food here is detritus.
Copepods are herbivores. They
and turtles. These, in turn, Many animals therefore migrate upward into
make up 70 percent of the total
are food for predatory fish zooplankton population, with the sunlit zone, where food is plentiful, at
and seabirds. Some larger thousands in a cubic yard. night, returning to the twilight zone as the
animals, such as basking Sun rises. Millions of tonnes of animals,
sharks, also feed on equivalent to around 30 percent of the total marine biomass, make this
zooplankton and nekton. daily trek—by far the largest migration of life on Earth. The length of
the journey is a matter of scale. Small planktonic animals measuring less
1
than 1mm ( /25 in) in length may only migrate through 20 m (70 ft),
SARGASSUMFISH
Here, two Sargassumfish are hiding but some larger shrimp travel 600 m (2,000 ft) each way,
in Sargussum seaweed, floating on every day.
the surface of the Sargasso Sea.

GIANT FILTER-FEEDER
More than 36 ft (11 m) long, basking sharks
like this one scoop up shoals of plankton,
then filter them from the water with the
white gill rakers inside their jaws.

OCEAN ENVIRONMENTS