Zonation Patterns in the Rock Littoral Zone

Physical and biological factors create regular patterns in the ecology of
the rocky littoral zone.  These effects are displayed as wide bands, each
containing certain species. The upper limits of each band are maintained
by physical constraints, such as resistance (or lack thereof) to
desiccation and/or hypersaline conditions, while lower limits are set by
biological constraints, such as competition and predation/herbivorous
grazing.

	The first site examined was exposed to wave action, but had
several large tidal pools that would offer some shelter from strong
hydrodynamic forces. Within the supralittoral zone, or "splash zone",
lichen grow on the rocks, as well as any plants that are capable of
contending with occasional seas spray. Below this, in the supralittoral
fringe, blue-green algae grow in a slippery film, creating what some
biologists call the "black zone", due to the dark color of the
algae-covered rocks.  Near the lower border of this zone periwinkles begin
to appear, grazing on the algae and scraping it off the rocks with their
radulas. There were two species of periwinkle present at this site, the
common periwinkle, Littorina littorea, and the smooth
periwinkle, Littorina obtusata. They also feed on Ulva
lactuca, or sea lettuce, and Porphyra sp., or laver. These
algae can have extended ranges up the intertidal zone if there is water
runoff or small pools that will keep them moist. (Carefoot, 65)
Directly below this, the midlittoral zone begins, where approximately
equal time is spent submerged and exposed, and marked by the dominant
species Semibalanus balanoides, the Northern rock barnacle.
Barnacles are deeply affected by competition for space, and this is
evident in their morphology. Where there is more space, suggesting less
original recruitment, they take on a flat dome-like shape. When there is a
great deal of crowding, they take on a stalk-like columnar shape.
(Newell, 129-130) The barnacles are primarily fed upon by the dogwinkle,
Nucella lapilus. This predatory snail uses its radula to drill into
the hard shell of barnacles so it can reach the soft flesh inside.
Nucella also feeds upon blue mussels, Mytilus edulis, which
dominates the low midlittoral to infralittoral fringe area. In this
exposed area, most of the mussels found were small, and in dense beds as a
result of heavy recruitment. When not forming dense mats, they seem to
prefer living in the crevices between the rocks, held in place by byssal
threads.
This area was also home to the rockweeds, such as Fucus
vesiculosus, and the knotted wrack, Ascophyllum nodosum. Some
green filamentous algae, Chaetomorpha linum, was also found in
tangled bunches sporadically throughout the zone.  The Fucus here
had shorter fronds compared to the Fucus in the sheltered area, due
to wave action.  Fucus acts as a shelter to smaller organisms
because it grows in thick mats that retain moisture after the tide has
receded. Small green crabs, Cancer maenas, as well as Littorina
littorea were found amongst and under the fronds. Littorina
obtusata was found in a similar position in the Ascophyllum
nodosum, and is often associated with this species of algae.
(Bertness, 193) Also associated with Ascophyllum iis the red tubed
weed Polysiphonia lanosa, which lives on it as an epiphyte. This is
a detriment to Ascophyllum, since having the extra drag on the ends
of its fronds makes it more vulnerable to hydrodynamic forces, and thus,
more likely to break.
There were also tide pools in this region, forming slightly more protected
areas that were host to many species. Chondrus crispus, Irish moss,
grows in a fringe around the edge of most of the tide pools, noticeable
due to the bright blue iridescence it has when submerged. Tubular green
algae, Enteromorpha linum, also grow within the tide pool.
Encrusting coralline algae also grows on the surface of the rocks, both
Lithothamnium, which forms a pink crust, and Clathomorphum,
which is often found with tortoiseshell limpets, Acmaea
testitudinalis, feeding upon it. There were a few Forbes' sea stars,
Asterias forbesii, in the tide pools as well as a few Jonah crabs
(Cancer borealis). One of the tide pools had a maned nudibranch,
Aeolida papillosa. A few of the tide pools had incidental
Aurelia aurita, moon jellyfish, which are normally pelagic.
The next region is the infralittoral fringe. Here, Fucus grows
along with more Chondrus and scattered patches of dulse,
Rhodymenia palmata.  Ascophyllum and the species associated
with it grow mixed with the Fucus. Many of the species found in the
midlittoral zone are also present here, such as the periwinkles, the
tortoiseshell limpet, the Jonah crab, and the blue mussel.  Also present
were scattered pieces of Alaria esculenta, edible kelp that grows
in the infralittoral zone, which is almost always exposed except for at
the lowest of spring tides. A lacy encrusting bryozoan, Membranipora
prolifera, lives on the fronds of Alaria, making it more
brittle and likely to break under high hydrodynamic forces. The green sea
urchin lives in this region as well, where it is less susceptible to
physical stresses and bird predation. (Bertness, 190)
The other site explored was sheltered from direct wave action. The
expected trends of larger size and more delicate species were observed.
The supralittoral zone and the supralittoral fringe were very similar to
the corresponding regions in the wave-exposed area. However, an additional
species of periwinkle was present, Littorina saxatilis. The
noticeable differences occurred in the midlittoral zone.  Mytilus
had a greater body size in the sheltered area, and was not restricted to
protective crevices to prevent detachment. A different species of sea
star, the Boreal sea star, Asterias vulgaris, was present in many
of the tide pools. There were many more sea stars than at the exposed
site, and they were much larger. At the exposed site the largest sea star
was about 5 inches in diameter, while the largest sea star at the
protected site was almost a foot in diameter. The crabs here were also
larger, both Cancer maenas and C. borealis. The pink
encrusting algae Lithothamnium found in the first site was also
present in the second site, but covered more area. Another encrusting
algae was present as well, Hildenbrania rubra, which was bright
reddish-orange with occasional black spots. Four new types of organism
occurred within this area that did not have correlating organisms in the
exposed area. The first was Metridium senile, the frilled anemone.
This was living on the underside of an overhanging rock face, and was
unrecognizable at first glance, since the tentacles were inverted and it
was somewhat shrunken into itself to conserve water. The second was the
red beard sponge, Microciona prolifera. This bright orange sponge
was living patchily on rock faces within the tidal pools. ThThe third
species was Bougainvillea carolinensis, a hydroid that lived in the
shallow pools of water on the rocks, in dense colonies.  Finally, the
fan-shaped coralline alga Corallina officinalis was present in some
of the tide pools, growing on the surface of the rock in a similar manner
to the other three sessile and somewhat delicate organisms. All of these
species would have been ripped off the rocks had they been living in the
wave-exposed site.
In general, the "big picture" of the intertidal zonation patterns could be
discerned by looking across the water at the opposite rock face. From that
distance, it appears as if there is a wide band of white, then a band of
varigated greens and browns, then a darker brown band. There are patterns
both on a close-up level, within zones, as well as the larger intertidal
zonation observed. In both sites, predation and competition played
important roles in maintaining the distinct zonation patterns. For
example, the predation of Strongylocentratus on various algae and
of Nucella on Balanus defined the lower barriers of the
lower and upper midlittoral zones, respectively. The upper limits of each
zone were defined by competition. Organisms that are more tolerant of
harsher conditions such as desiccation and heat are able to live in a
niche higher in the littoral zone, and thus outcompete their less able
neighbors.  Certain elements of the intertidal zone, such as the
protection afforded by tide pools and thick beds of Fucus, enable
organisms to extend what their range would otherwise be. The general
pattern of algal growth, with green algae higher, giving way to red and
brown is due to the fact that while green algae only have chlorophyll,
brown and red algae have pigments which allow them to live in deeper water
where red wavelength light cannot penetrate. (Carefoot, 73) 
Overall, the patterns of zonation in the rocky littoral zone come from a
complex interplay of physical and biological factors, which allow each
organism to create a specific niche. This way, no singular organism
dominates the entire area between low tide and high tide, and species
diversity is encouraged.

Works Cited
Bertness, M. D. 1998. The Ecology of Atlantic Shorelines. Sinauer
Press.
Carefoot, Thomas. Pacific Seashores: A Guide to Intertidal Ecology.
University of Washington Press.
Gosner, Kenneth L. 1978. Atlantic Seashore. Houghton Mifflin Co.
New York,New York.
Newell, R.C. 1979. The Biology of Intertidal Animals. Marine
Ecological Surveys. Faversham, England.