Rocky Intertidal Shores
The rocky intertidal shores of both the east and west coasts are filled with unique communities, complex in their relationships and incredibly well adapted to the rigors of the pounding oceans. This introduction should give you an appreciation of the life which has successfully survived the interface of land and ocean.
The abiotic environment and its impact on life.
"An ocean tide refers to the cyclic rise and fall of seawater. Tides
are caused by slight variations in gravitational attraction between the
Earth and the moon and the sun in geometric relationship with locations
on the Earth's surface. Tides are periodic primarily because of the cyclical
influence of the Earth's rotation.
At the location on the Earth closest to the moon, seawater is drawn toward
the moon because of the greater strength of gravitational attraction.
Considering this information, any given point on the Earth's surface should experience two tidal crests and two tidal troughs during each tidal period. The moon's gravitational pull is the primary force responsible for the tides on the Earth.
However, the moon is in motion revolving around the Earth. One revolution
takes about 27 days and adds about 50 minutes to the tidal cycle. As a
result, the tidal period is 24 hours and 50 minutes in length.
Local winds and climate as well as local geology also impact on the amplitude
of the tides.
Feeding the shores:
There are two sources of primary production which support the rocky intertidal food webs:
The latter although small, are critical to the life of of the sessile filter-feeding organisms, including the barnacles and mussels found throughout this ecosystem.
The benthic algae attached to the rocks and the aqueous bottom range
from one-celled algae to the mid and quite large seaweeds and kelps. Forming
in some cases, underwater forests, they feed an equally diverse set of
grazers which include snails, urchins and incredible variety of fish.
The world of the seaweeds
Three major taxa dominate the rocky shores: the simpler greens, along with the more complex reds and brown algae.
Green Algae or chlorophytes pigment-wise most resemble the land plants. Commmon intertidal greens include sea lettuce ( Ulva), filamentous Entermorpha, dead man's finger ( Codium).
The brown algae or phaeophyta use xanthophyll pigments in addition to the chlorophylls used by all other plants. Common brown algae include the rockweeds ( Fucus), knotted wrack ( Ascophyllum) and the magnificent kelps ( Laminaria). Some of the latter can reach lengths of hundreds of feet in the cold West coast waters.
The red algae or rhodophytes use phycobilin and carotenoids combinations along with the requisite chlorophylls, to form colorful surfaces of red to black. Common reds include the gelatinous Irish moss ( Chondrus) and calcium skeletal corralines.
At one time, it was though that the seaweeds distributed themselves according to their ability to use different wavelengths of light. Longer red and IR are abosrbed near the surface of water. Only shorter green and blue penetrate deeper water. ( why water is blue). As green algae pigments absorb both in the red & blue region it was thought that is why they were restricted to the higher sections of the water column.Reds with their phycobilins that absorb blue and green wavelenghts were thought to compete best in the deepest waters, relegating the brown to be intermediate with their xanthophylls which absorb yellows and greens. Logic lost this argument. It is now thought to be a factor of quantity of pigments rather than quality.
Green Algae: The most fragile of the algae, they endure major grazing and shredding by the waves. They are considered the opportunists of the rocks; they move in after disturbances have cleared the rocks and survive long enough to reproduce.
Division Rhodophyta or the red algae: 3900 spp and 400 genera
Calcareous algae are algae that deposit calcium carbonate
(limestone) in their tissue. When the algae die, they leave a fossil "skeleton"
behind. These plants do not have real skeletons, but the limestone deposits
make it appear so. These skeletons build up sediment in tropical lagoons
and reefs as well as on our shores.
Diatoms live in "glass houses." They have beautiful, ornamented, silicified cell walls. This is quite evident if the protoplast is removed by acid or heat
In most diatoms, the cell wall, called a frustule, is made up of pectin impregnated with hydrated silica. Electron microscopy has shown that the fine markings of the diatom walls are actually pores which give the protoplasm access to the external environment.
The walls consist of two overlapping portions (valves) which fit together like a petri dish or a gelatin capsule. The outer valve, the epitheca, fits over the inner valve, the hypotheca. The two valves may be attached to a girdle band instead of overlapping.
The valves are either pennate or centric. Pennate diatoms ( fresh water generally) have bilateral symmetry in valve view. The general outline may be boatshaped or rod-shaped. In the center or the valve of most pennate diatoms is either an unsilicified groove, the rapine, or a hyaline median line, the pseudoraphe. The rapine seems to be associated with the movement of many pennate diatoms.
Surf Grass, Phyllospadix
Rocky intertidal Herbivores:
As mentioned previously, there are two types of herbivores:
Fish actually do not play an important role as herbivores in this system, unlike the residents of coral reefs.
Sea urchins feed with a movable jaw of 5 continuously growing calcium carbonate teeth. The can bite or scrape most algae except the corallines crusts. Sometimes forming fronts of hundreds of thousands of urchins, they consume near everything in sight, leaving behind a barren rocky outcrop.
They are ggressive grazers that eats more than 20% of the ecosystem's kelp plants, making space for new plants and animals.
They however have been consumed in large quantities, especially in the last 2 decades, by humans. Harvested for their eggs ( considered a delicacy by Japanese), their populations have been hit hard with overharvesting and by diseases. In the N. Atlantic, outbreaks of a water borne amboeiod pathogen associated with warming waters have been responsible for shifting subtidal urchin barrens dominated by crusts into kelp beds.
Predators: Intertidal predators range from snails and crabs to birds and fish