Page 144 - (DK) Ocean - The Definitive Visual Guide

142 SHALLOW SEAS

Rocky Seabeds ROCKY SLOPE
These underwater rocks in British
Columbia, Canada, are covered with
marine life. A sunstar and leather star
FROM THE WARM TROPICS TO COLD POLAR SEAS, many distinctive search for prey among pink soft corals and
sponges, while urchins graze below.
communities of marine life develop on the rocky floors of shallow seas.
Underwater rocks provide points of attachment for both seaweeds and
marine animals and are often covered with life. Seaweeds thrive in the sunlit
shallows and provide a sheltered environment for animal communities.
Firmly attached animals extend arms and tentacles to catch planktonic food
from water currents, or pump water through their bodies to filter out
nutrients. Mobile animals graze seaweeds or prey on fixed animals or each
other. The life on a rocky reef depends on many environmental factors.

The Seaweed Zone
Seaweeds rely on sunlight for growth, and thrive only on the shallowest rocks.
The depth in which they can grow depends on water clarity, from a few
yards in turbid seas, to more than 330 ft (100 m) in the clearest waters.
In colder waters, huge forests of kelp and other large brown seaweeds
dominate the shallows, with smaller seaweeds in deeper water. Large
seaweeds are often scarce on rocks in the tropics—instead, the Sun’s energy is
harnessed by tiny unicellular
algae inside coral tissues.
Seaweeds harbor a plethora of
associated animals. Some live
permanently in the seaweed zone,
while others use it as a breeding
ground or nursery before moving
into deeper water.
BALLAN WRASSE
FOOD SOURCE In summer, adult ballan
Energy from sunlight captured by wrasses lay eggs in nests built of
seaweeds is used by grazing seaweed, secured in rock crevices.
animals. Here, green seaweeds Young wrasses are often patterned,
cover rocks in Orkney, Scotland. providing camouflage.

ROCK GRAZERS Animal-dominated Deeps
In deeper water, light levels are too low for most seaweeds,
Sea urchins are highly successful although encrusting red seaweeds need little light and grow
marine invertebrates, well farther down. Much of the plantlike growth in deeper water
defended by sharp spines. They
graze the seabed, eating virtually actually consists of fixed animals, which are most abundant in
everything except hard-shelled places with strong tidal currents. For mobile animals living here,
animals and coralline seaweed the seabed is a minefield of toxic substances, released by fixed
crusts. They have a profound animals to deter predators. Below 160 ft (50 m), water
effect on seabed communities. movement from waves is much less, and fragile animals such as
If urchins are abundant, they can sponges and sea fans can grow to a large size. Here, and in places
seriously reduce the diversity of
life on the seabed, leaving urchin more sheltered from water movement, a smothering layer of fine
“barrens.” Conversely, where silt continually settles on the rock surfaces, restricting the animal
urchins are sparse, they can life to forms that can hold themselves above the rock or can
increase diversity, by clearing remove the silt. On the most heavily silted rocks, animals may
spaces for new life to settle. grow only on vertical or overhanging surfaces. dorsal spines
OCEAN ENVIRONMENTS Stonefish have a textured in sediment camouflage

with poison
protruding
ROCKY-BOTTOM
glands
eye used
PREDATOR
when hiding
skin color
skin and irregular shape,
and texture
making them difficult
to spot. A huge mouth
engulfs prey, while the
dorsal spines
contain venom
that can be fatal.
large
mouth
tail fin