Page 446 - Environment: The Science Behind the Stories

Intertidal zones undergo constant change

Where the ocean meets the land, intertidal, or littoral, eco-
systems (Figure 16.10) spread between the uppermost reach
of the high tide and the lowest limit of the low tide. tides are
the periodic rising and falling of the ocean’s height at a given
location, caused by the gravitational pull of the moon and sun
(see Figure 21.21, p. 619). High and low tides occur roughly
6 hours apart, so intertidal organisms spend part of each day
submerged in water, part of the day exposed to air and sun,
and part of the day being lashed by waves. These creatures
must also protect themselves from marine predators at high
tide and terrestrial predators at low tide.
The intertidal environment is a tough place to make a liv-
ing, but it is home to a remarkable diversity of organisms. Life
abounds in the crevices of rocky shorelines, which provide
shelter and pools of water (tide pools) during low tides. Sessile
animals such as anemones, mussels, and barnacles live attached
to rocks, filter-feeding on plankton in the water that washes
over them. Urchins, sea slugs, chitons, and limpets eat intertidal
algae or scrape food from the rocks. Sea stars (starfish) creep
slowly along, preying on the filter-feeders and herbivores.
Crabs clamber around the rocks, scavenging detritus.
The rocky intertidal zone is so diverse because environ-
mental conditions such as temperature, salinity, and moisture
(a) Tide pools at low tide
change dramatically from the high to the low reaches. This
environmental variation gives rise to horizontal bands domi-
nated by different sets of organisms arrayed according to their
habitat needs, competitive abilities, and adaptation to exposure.
Sandy intertidal areas, such as those of Cape Cod, host less bio-
diversity, yet plenty of organisms burrow into the sand at low
tide to await the return of high tide, when they emerge to feed.

Supratidal zone
(splash zone)

Level of high tide

Intertidal zone CHAPTER 16 • M AR in E A nd Co A s TA l s ys TEM s A nd R E sou R CE s

Level of low tide

Subtidal zone

(b) Tidal zones
Figure 16.10 The rocky intertidal zone stretches along rocky shorelines between the lowest and
highest reaches of the tides. Fish and invertebrates inhabit tidal pools (a), and many types of algae cover
the rocks. The intertidal zone (b) provides niches for a diversity of organisms, including sea stars (starfish),
barnacles, crabs, sea anemones, corals, bryozoans, snails, limpets, chitons, mussels, nudibranchs (sea slugs),
and sea urchins. Areas higher on the shoreline are exposed to the air more frequently and for longer periods,
so organisms that tolerate exposure best specialize in the upper intertidal zone. The lower intertidal zone is
exposed less frequently and for shorter periods, so organisms less tolerant of exposure thrive in this zone. 445

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