Eating and being
eaten is not the only way living things interact on our shores. Many
live in close association with one another to increase the chances
of survival of one or both of them. This includes animals which are
quite different and even plants living inside animals. Such a relationship
is called symbiosis.
In mutualism, both partners benefit.
Reef-building hard corals
harbour in their tentacles, microscopic single-celled algae (called
algae undergo photosynthesis to produce food from sunlight. The food
produced is shared with the coral polyps, which in return provides
the algae with shelter and minerals. This arrangement allows
hard corals to build reefs that in turn form the basis of shelter
and food for the vast variety of reef life.
Many other animals have a similar arrangement with zooxanthallae,
clams, carpet anemones
and some sponges.
transparent shrimps (about 1cm) are sometimes seen among the Carpet
anemone's tentacles. It is not certain how they avoid being stung
by the tentacles. The shrimps find refuge and in turn, they may help
keep the Carpet anemone clean. A similar arrangement is found between
and sea anemones.
In commensalism, one living thing benefits at no expense
or gain to the other.
brittle stars may find shelter inside a sponge. While the brittle
stars enjoy a continuous flow of food and oxygen, it probably makes
no difference to the sponge.
snail (Crepidula sp.) is white and looks like a clam. It
often settles on the inside of a shell occupied by a hermit crab.
The snail enjoys the food and oxygen brought in with the flow of water
generated by the hermit crab. The snail's presence, however, probably
makes no difference to the hermit crab. Small
sea anemones may also settle on a shell occupied by a hermit crab.
In parasitism, one living thing gains considerably at
the expense of the other.
For example, the parasitic
barnacle Thompsonia littoralis grows through the host crab
like a root system, eventually breaking through the joints to produce
egg sacs as shown in the photo at right. The parasite does not kill
the crab but affects its reproductive system so it becomes infertile.
Tan, Leo W. H. & Ng, Peter K. L., 1988. A Guide to Seashore
Life. The Singapore Science Centre, Singapore. 160 pp.