Page 146 - (DK) Ocean - The Definitive Visual Guide

144 SHALLOW SEAS

Sandy Sea Beds

MOST OF THE CONTINENTAL SHELF is covered with thick
sediments, accumulated from millennia of land and coast
erosion. The calcareous remains of marine life are continually
added to the mix. Unlike deep-sea sediments (see pp.180-81),
shelf sediments are stirred up by waves during storms, re-
suspending nutrients and profoundly affecting marine life
and productivity. Sediments are largely the domain of animals,
as seagrasses and seaweeds grow only in limited, shallow areas.
Buried beneath the surface of a sandy sea bed, there may be
vast numbers of animals hiding from, or waiting for, prey.

Gravel and Sand coarse bristles
(chaetae) on sides
The coarsest sediments from coastal and land erosion are usually
deposited inshore by rivers and glaciers as they enter the sea.
Frequently shifted by waves and tides, clean, coarse sand and
gravel make a difficult habitat; typical inhabitants include
tough-shelled molluscs, sea cucumbers, burrowing
urchins, and crabs. A wider range of organisms live in
the more stable sand and gravel, where purple-pink beds
of maerl can be found. This unattached, calcareous seaweed
is made up of coral-like nodules. The open structure of live SANDY HABITAT felt-like dorsal
maerl twiglets is ideal for sheltering tiny animals, newly settled A marine segmented chaetae
worm, the Sea Mouse
from the plankton, while the dead maerl gravel underneath supports
lives in muddy sand.
burrowing animals. Beds of seagrass and green seaweeds thrive in
shallow sand, harbouring a wide range of life. Embedded shells and stones provide
anchors for various seaweed species. Many fish have adapted to life on sandy sea beds,
the most familiar being flatfish. EXPLOITING SANDY BEDS
Stingrays are among the many animals
Shallow-water anglerfish wave their
that hide in the sand of the sea bed;
fishing lures to tempt prey within this Southern Stingray does so both to
striking distance of their huge mouths, escape predators and to ambush prey.
while garden eels live permanently in
sand burrows, partly emerging to eat
plankton. Sand eels and cleaver wrasse Mixed Sediments
dive into the sand to avoid predators.
Most sediments on the continental shelf are a mix of
GRAVEL DWELLER
This Flame Shell lives in a nest of gravel, pebbles, coarse and fine materials. An important part of these are
and shells. It pumps seawater through the nest, calcareous fragments, derived from hard-shelled animals.
extracting food with its sticky, acidic tentacles. Mixed sediments offer a wider range of building materials
for tubes and burrows than sand or mud and are easier
Soft Mud to traverse, so a far greater variety of animals live here.
Seaweeds and hydroids cover the bed, attached to shells
In sheltered waters in enclosed bays, estuaries, and pebbles. Visible life includes tube worms, brittlestars,
and fiords, and in the deeper parts of the and burrowing anemones; most of these withdraw into
continental shelf, the finest particles of the sediment if threatened. Below the surface, hidden
sediment settle as soft mud. Easily stirred animals, including bivalves
up, the fine particles smother newly settled LIFE ON THE SEDIMENT and crustaceans, provide a rich
larvae and clog gills. There is little oxygen Its mouth fringed by tentacles, source of food for animals that
this half-buried sea cucumber
OCEAN ENVIRONMENTS are covered with small in the mud, raising sticky
just below the mud surface, so buried
can find and excavate it, such
(left) and a hermit crab inhabit
animals must find ways to obtain oxygen
as starfish, crabs, and rays.
these mixed sediments.
from seawater. Despite these challenges,
mud can be very productive. Bacteria and
diatoms are often abundant on the mud
surface, providing food for hoovering
animals such as echiuran worms. Stable
burrows are more easily built in mud than
in sand or gravel. Animals such as sea pens
and burrowing anemones anchor themselves
ANCHORED IN MUD
polyps and tentacles
This sea pen’s branches
to catch the raining
plankton or to ensnare a
polyps that feed on
passing fish or crustacean.
the plankton.