Page 219 - (DK) Ocean - The Definitive Visual Guide

BOTTOM-LIVING 217

Burrowing and Boring

Much of the seafloor is covered in soft sediments, such as sand and mud. Living on the
surface of the sediment is both difficult and dangerous, and most animals burrow below
or build tubes in which to live and hide. Bivalve molluscs and segmented worms cope
especially well in this habitat, and many different species can be found in sediments all
over the world. Safe under the sediment surface, a bivalve draws in oxygen-rich water
and plankton through one of its two long siphons, expelling waste through the
other. It never has to come out to feed or breathe. Piddocks and shipworms bore
into rocks and wood, then use their siphons in
a similar way. Sediment is not a completely safe
home—predatory moon snails dig through sand
and bore into bivalve shells, eating the contents.
Ragworms are also active predators, hunting through
the sediment for other worms and crustaceans. Some
worms build flexible tubes from sand grains, their
own secretions, or both. The tubes stick out of the
sand, and they feed by extending feathery or sticky REPLACING SIPHONS BORING INTO ROCK
The siphon tops of buried
The boring sponge uses chemicals
tentacles from the tube to catch plankton. If danger bivalve mollusks are to dissolve tunnels in calcareous
threatens, they can withdraw rapidly. A similar sometimes nipped off by shells and rocks, creating a living
strategy is adopted by tube anemones and sea pens. flatfish but can regrow. space for itself.
FIXED TO THE BOTTOM
Christmas-tree worms live attached to
the bottom in hard tubes that they cement Symbiosis
into coral reefs. They feed by filtering
plankton from the water, using their Bottom-living is a challenge for marine organisms.
beautiful double spiral of tentacles. A safe crevice on a coral reef, for instance, is valuable,
but fiercely fought over. The solution to finding a
home is often to enter an intimate relationship with
a different organism—a situation called symbiosis.
When only one partner benefits, the relationship
is called commensal, and often involves one
animal providing a home for the other.
MUTUAL RELATIONSHIP
Small pea crabs live inside mussels,
The Banded Coral Shrimp earns its place
gaining shelter and food, while the in the moray eel’s well-defended crevice
mussel merely tolerates their by cleaning the teeth of its host.
presence. Symbiosis in which
both partners benefit is called mutualism. Many tropical gobies
live in such relationships with blind or nearly-blind shrimp. The
shrimp digs and maintains a sandy burrow that accommodates
both, while its sharp-eyed partner goby acts as a lookout. Some
anemones adhere to the shells of hermit crabs, gaining from
the crab’s mobility and access to its food scraps. The crab is
protected, in return, by the anemone’s stinging tentacles.
The third type of symbiosis is parasitism, in which one
partner, the host, is harmed. The crustacean Sacculina
spreads funguslike strands through its host crab’s body
to extract nutrients, weakening or killing the crab.

A HOME IN EXCHANGE FOR CLEANING OCEAN LIFE
Large reef anemones often provide a
haven for clownfish and tiny cleaner
shrimp. The anemone benefits from the
housekeeping activities of its guests.