Geoscience Reference
In-Depth Information
These organisms cover the upper intertidal rocks in slippery mats, which are grazed on by the rough peri-
winkle.
Periwinkles are molluscs, a phylum that has two distinguishing anatomical structures—the mantle and the
radula—found nowhere else in the animal kingdom. The mantle is a fold in the body wall whose function is to
secrete the calcareous shell typical of molluscs. It also serves to house the gills and protect the larvae in their
planktonic stage. The radula is a ribbonlike tongue, wound like a clock spring on the floor of the pharynx. It is
made of chitin (the same substance lobster shells are made of) and is armed with sharp teeth arranged in sever-
al hundred rows. The radula acts as a conveyor belt for scraping algae from the rocks.
The highly vascularized lining of mantle cavity, which makes it akin to a lung, is the vital adaptation for liv-
ing at the top of the intertidal zone, allowing the periwinkle to survive the period when it is not covered by the
tide, which may be as long as a month. It can also breathe anaerobically—that is, without contact with the at-
mosphere—for up to a week when it seals its shell against the rock, a behavior sometimes necessary to prevent
it from drying out. Unlike other periwinkles, it also produces live young. Eggs encased in a cocoon develop in-
side the mother and emerge as fully formed shelled creatures the size of coffee grounds.
In sharp contrast to the black zone, the barnacle zone is glaringly white because of the limestone shells of
the barnacles. Rock or acorn barnacles are stationary and therefore must be exposed to the tide every day to
feed. The barnacle is a crustacean; it glues itself to the rocks and waits for the tide to deliver its meal of micro-
scopic life. It lives head-down in a volcano-shaped shell consisting of six calcareous plates. The top is fitted
with a pair of trapdoors that shut at low tide to prevent the barnacle from drying out, but open as soon as water
covers the shell. Then six pairs of feathery appendages called cirri appear and begin to sweep the water, creat-
ing a vortex that draws food particles into the crater where the little animal passes its sheltered life. The great
19th-century naturalist Louis Agassiz described the creature and its lifestyle well: “Nothing more than a little
shrimplike animal, standing on its head in a limestone house and kicking food into its mouth.”
Barnacles thrive in the upper intertidal zone and often monopolize it because of their many adaptive mech-
anisms for coping with its extreme environmental conditions. They can survive perhaps the greatest temperat-
ure range of any animal, from 44°C (111°F) to -15°C (5°F). At extremely low temperatures, barnacles and oth-
er invertebrates protect themselves against the tissue-damaging effects of ice crystals by secreting antifreeze
compounds, such as glycerol and trehalose, into their body fluids. In times of extreme heat, they cool them-
selves by means of transpiration, or evaporation of bodily water. When not covered with seawater, the barnacle
can breathe humid air through a small passage, the micropyle, in its shell. And when the air is too dry, it can
close its shell completely and survive on small amounts of stored oxygen.
