Geoscience Reference
In-Depth Information
luxuriant heads of hair. Both plants have air-filled bladders along their fronds, which serve as flotation devices
as the tide rises over their rocky homes. Bladder wrack is more dominant in energetic environments, exposed
to wind and waves, and knotted wrack is better adapted to quieter, more sheltered places. These plants—the
fucoids—have also evolved strategies to cope with the stresses that come with a territory where temperatures
fluctuate and desiccation is a threat. Chemicals in their cell walls, called alginates, counter these stresses but
also lend flexibility and strength to the plants as a buffer against the waves' often violent force.
Barnacles can live at all levels of the intertidal zone, but in the brown algal zone, new organisms outcom-
pete the barnacles, which are so dominant in their own domain. The algae overgrow the barnacles and impede
their ability to feed, thus reducing their numbers. The rock weeds also shelter the primary predator of
barnacles, the dog whelk. And the blue mussel outcompetes the barnacle, largely replacing it in the brown al-
gae zone.
Blue mussels colonize an area faster than any other organism, in part because of their great fecundity. A fe-
male produces up to 12 million eggs, which are fertilized when they are broadcast widely into the water
column. The larval mussel settles to the bottom, where it metamorphoses into a miniature mussel and moves
up the shore looking for a place to attach itself. It does so by means of byssal threads produced from a protein
substance in its foot. These act as cables or guy wires, anchoring it to the rocks. Like most bivalves, mussels
are filter feeders, drawing in water through one siphon and passing it over mucus-coated gills that trap the food
particles as well as extract oxygen, and then expelling the cleaned water through a second siphon.
Populations of the fecund blue mussels and barnacles are held in check by the predatory dog whelk, whose
beautiful spiral shells are a collector's delight but a mussel's or barnacle's nightmare. The dog whelk pries
open the top trapdoors of the barnacle, then inserts its radula between them. It does not accomplish this feat by
sheer physical force, however. Covering the barnacle with its foot, it first secretes a highly poisonous purple
dye, purpurin, once prized by native North Americans, to kill the barnacle. It adopts a different strategy with
the much larger and stronger mussel. It drills a small hole in the shell, then inserts its proboscis, which houses
its mouth. Again, however, it gains entry by first secreting a substance that softens the shell, then uses its many
toothed radula to drill a larger hole.
These dramatic predatory acts take place under cover of the fucoids. The luxuriant growth of these seaweeds
has evoked the terrestrial metaphors of a “forest” or even a “jungle.” The comparison is also apt because of the
diversity of animals that live there.
These plant communities perform a number of vital ecological functions. At high tide, they act as canopies
that shade and shelter some twenty-two juvenile fish species, including tomcod, pollack, sculpin, cod, alewife,
white hake, and flounder. At low tide, the canopies become wet mats that protect many species of tiny inver-
tebrates, which, in turn, attract some fifteen species of seabirds and shorebirds in search of food. The most
common creature sheltering under the fucoid forest is the amphipod Gammarus oceanicus, or as it is more
commonly known, “sideswimmer” or “scud.” Pull apart any clump of rockweed, and hundreds of these crusta-
ceans scatter for cover. These rockweeds also contribute to offshore productivity when parts of the plants break
off and drift seaward. There, they can form extensive mats that shelter zooplankton, which are eaten by larval
lobsters and juvenile fishes such as lumpfish, sticklebacks, rockling, and hake, as well as by seagoing birds
like phalaropes and terns.
Below the brown algae, in the low intertidal zone, we enter the red algae or Irish moss zone. The namesake
plant is Irish moss, a ruddy, many-fingered plant that forms dense mats, especially on flat rocks, where its ten-
acious holdfasts allow it to flourish in this turbulent zone. (Processed Irish moss is a common ingredient in ice
cream and other foodstuffs. Another edible red algae is dulse, which is sun-dried and eaten as a snack or condi-
ment.) Blue mussels may occupy areas where the Irish moss fails to take hold. The common periwinkle is a
frequent grazer in the red algae zone but concentrates on ephemeral species, to the benefit of the Irish moss.
