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.
The moon is the primary factor controlling the temporal rhythm and height of tides. The moon produces two tidal bulges somewhere on the Earth through the effects of gravitational attraction. The height of these tidal bulges is controlled by the moon's gravitational force and the Earth's gravity pulling the water back toward the Earth.

At the location on the Earth closest to the moon, seawater is drawn toward the moon because of the greater strength of gravitational attraction.
On the opposite side of the Earth, another tidal bulge is produced away from the moon. However, this bulge is due to the fact that at this point on the Earth the force of the moon's gravity is at its weakest.

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.

The timing of tidal events is related to the Earth's rotation and the revolution of the moon around the Earth. If the moon was stationary in space, the tidal cycle would be 24 hours long.

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.
The second factor controlling tides on the Earth's surface is the sun's gravity. The height of the average solar tide is about 50 % the average lunar tide. At certain times during the moon's revolution around the Earth, the direction of its gravitational attraction is aligned with the sun's . During these times the two tide producing bodies act together to create the highest and lowest tides of the year. These spring tides occur every 14-15 days during full and new moons.
When the gravitational pull of the moon and sun are at right angles to each other, the daily tidal variations on the Earth are at their least . These events are called neap tides and they occur during the first and last quarter of the moon."

Local winds and climate as well as local geology also impact on the amplitude of the tides.
Strong offshore winds can batten down tides
Changes in barometric pressure can also impact height.
Shape of the coastline, as in Georgia which funnels in the water and also in the Bay of Fundy
Latitude has large impact as in Maine.

What does this mean to the organisms out there at the mercy of the tides?

1. On sandy shores, wave have great impact on the substrate itself, shifting. On the rocky shores that we are working with today, the impact is felt more as a physical force or pounding of life attached to the rocks.

Strong attachments, streamlined morphology, heavy walls and living in groups in which organisms can help buffer one another are adaptations to the tides and their waves.

Drag, is proportional the the area of the object and results from pressure differences on the sides of an object and acts to pull an object parallel to and in the same direction as a force. Thus organisms can not be too large or irregularly shaped.

Lift is also proportional to area but acts perpendicular to the force.Since lift is maximized by having a low profile, the organism must balance the two forces.

Acceleration is proportional to mass; as an object increaes in size accleration forces on them increase faster than their surface area or area of attachment. Thus if they grow lagrer without changing their body proportions they will be torn from their substrate.

2. One positive impact is that incoming water bring in food and nutrients to the organisms in the community.Filter feeders such as barnacles and mussels have food delivered, while alage gain necessary gas exchange as well as mineral nutrients.

3. Another factor is exposure when the tide is out. Exposure to intense, reflected light and changes in temperature as great as 20-30C daily can easily stress most life. This leads to zonation of organism with differing capacities to tolerate the more extreme exposures.
Tight communities help a lot, and those that are mobile escape under rock surfaces when feasible.
The rocks themselves may moderate who lives upon them. Darker rocks tend to heat up faster and restrict the organsims that can survive there.

Dessication as water move out have resulted in incredible adaptations; certain barnacles and mussels can live without water 20-40 days.They can close their shells tight to restrict water loss and hold in supplies. However this creates breathing problems. Some have small slits to let air in, others air gap, while still others can respire anaerobically for a while.

Algae may lose up to 75-90% of their water.Within an hour of resubmergence, they rehydrate and start up photosynthesis.

4. In more northern latitudes, winter winds may cause temperatures to plummet. The community may be encased in ice for weeks. In New England as much as 50-70% of the internal water content of the invertebrates and algae may freeze. To deal with this organisms hold increased salt loads ( like us salting the roads) or use glycerol to prevent cells from bursting with crystal formation. The ice sheeting may make the algae or organism so heavy that they can be torn from their rocks.

5. Algae are ususally larger than the animals. To combat the drag, they tend to be very flexible, even when very large. They are covered with mulcilagenous layers to prevent dessciation ( see alage sections). Their flexbility results in a whipping action, which creates whiplash halos, clearing out invertebrates close by ( and their competition for space)


Feeding the shores:

There are two sources of primary production which support the rocky intertidal food webs:

  • benthic algae and
  • the microscopic free-moving planktonic diatoms.

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.
In addition to serving as a food source, the larger algae serve to buffer other biota against desiccation and the pounding forces of waves as well as form a physical platform for growth of other organisms.

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.

Functional Roles:
It is thought that the different algal groups play roles consistent with their structures.

  • Green algae are the ephemerals, with light, fast growing tissues. These rapidly colonizing plants are found in in recently disturbed, early successional habitats.

  • The other groups invest more in tougher tissues, which persist over time: they resist long-term grazing, the pounding of storm waves, and competition from other species. The tough leathery brown are great examples of resistant structures, while the corralline reds have great structural defenses.

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.

Seaweed Defenses:

Like land plants, seaweeds have both biochemical and structural defenses.
The green as mentioned earlier invest little in these defenses and pay the price, being the preferred food of most intertidal herbivores.
The tough cuticles of most browns and some reds ( chew some), form a great barrier to grazing by small crustacean grazers.
The least edible are the those who form a crusty layer on the rocks. Snails forced to feed on them actually suffer tooth damage. Still more resistant are the coralline crusts which only limpets, chitons and urchins will attempt to eat.

Chemical defenses include:
Sulfuric acid ( ie. Desmarestia) that dissolves the calcium carbonate teeth of those fool hardy enough to try. The cells of Desmarestia contain sulfate ions. When these cells are disrupted, these ions interact with seawater to produce sulfuric acid.

Secondary metabolites such as phenolics and halogenated compounds. Secondary compounds are like wise used by most land plants for defense ( i.e. familiar examples of such chemical include the alkaloids ( morphine and other poisons)).It is thought that these defenses are aimed primarily at fish rather than the smaller herbivores, and algae with heavy biochemical defenses are used by smaller critters most likely to escape their own demise by larger visiting organisms as well as to obtain these same biochemicals to be used in their own bodies for personal defense.



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.



Brown Alga

Brown algae 1500 species 250 genera
Where are they found?
Marine, preferably cold; agitated and aerated found on rocky coasts in the littoral zone between the region of high and low tides (intertidal zone) and are periodically exposed to air and completely exposed to sunlight at low tide.; others submersed all the time from surface to depths of 220m

What do they look like? the most complex algae anatomically and morphologically, some more complex than the mosses/liverworts. All forms are multicellular-
Cell walls: cellulose, and alginic acid not found in other algae. This acid is slimy or gummy and causes the filaments of cells to adhere into a compact body. May have other function as some species are as much as 24% by weight of algininc acid.
Specialized tissues: some spp. have an epidermis like outer covering, a parenchyma-like cortex area and a cylinder of trumpet cells that resemble phloem cells so much that some people call them seive tube members. Carry long distance transport of carbohydrates; elongate with large seive like arrangement in end walls; holes lined with callose. Flow rates of 65-78 cm/hr
Colors:Differ from higher plants in that they contain chlorophyll a & c and contain a number of xanthophyll pigments ie. fucoxanthin Also contain carotenes. This complex of pigments allows them to photosynthesize at numerous levels in the sea; similar to dinoflagellates and golden-browns.
Storage product: laminarian (polymer of glucose) up to 34% of body weight, mannitols or fats but not starch. Generally though not a lot stored since environment is stable all year round so photosynthesize and grow continuously.


1. Kelp eaten as vegetable in East; provide salts, vitamins, trace elements, Also harvested for ash (Na,K) for industry, iodine. and used as fertlizer.
2. Alginates from walls used as thickening agents and colloid stabilizers in the food, textile, cosmetic. pharmaceutical, paper & welding industries.
Algin is now used in a wide range of foods including desserts, gels, milkshake mixes, dairy products and canned foods. It is used in bakery products from cake mixes to meringues to improve texture and retain moisture. Algin is used in frozen foods for its stabilizing properties to assure smooth texture and uniform thawing. It is also used to stabilize beer foam. The primary industrial applications for algin are in paper coating and sizing, textile printing and welding-rod coatings. Pharmaceutical and cosmetic applications for algin include its use in products including tableting, dental impression compounds and anti-acid formulations.
3. The entire marine sport-fishing industry depends on it to provide shelter and habitat for many game fish as well as the numerous smaller prey-items on which the game fish survive on.

Fucus distichus
This seaweed is common and often occupies all of the primary space of middle, upper and even lower rocky shores of the west coast of North America. Wave action often causes the thalli to lash about, which inhibits successful recruitment of other seaweeds and invertebrates.

This brown seaweed is common on the outer coast of the Pacific northwest of the United States and dominates the lower intertidal and very shallow subtidal. Its stipe and flat blades allow it to flex and bend to minimize drag on the entire plant when waves hit the shore.

Kelps and other large brown algae:
  • Their large bodies are made up of 3 parts:
    *Holdfast (in dark, poorly aerated water) a rootlike mass of tissue that functions, as the name suggests to anchor the stipe in some suitable substrate.
    *Stipe-or stem may contain an enlarged air bladders called pneumatocysts to form flotation blades. These may number up to 100
    Other of the stipe cells become reproductive and produce sporangia which give rise to haploid spores. Successful spores form a tiny gametophyte (hetermorphic generations) which is perennial and may last for 17+ years,
    *Blades or the leaf-like extensions are deciduous (annual), During the time period when the blades have dropped, the algae, must survive on stored food till new blades form in spring. Some species produce blades that measure up to 45 m (=130 ft; but are not bulky)
    All blades have an:
    outer meristoderm that is meristematic(adds new cell layers every year) and photosynthetic
  • a cortex of parenchym
  • central zone of trumpet cells and elongate cellstrue
  • apical meristem which produces true parenchyma tissue
  • intercalary meristem which produces blades By intercalary we mean that there exists a region not located at the tip ( which could be eaten or bashed) which has the ability to produce new cells as needed.( This same design is found in grasses, so when you mow the tips, the intercalary meristem below can keep on generating new tissue for you to mow the following week)




Division Rhodophyta or the red algae: 3900 spp and 400 genera

Really different from other algae and higher plants
*Colors: like cyanobacteria, they have phycobilins- red color due to phycoerythrin but they can be black, purple or brown due to phycocyanins; this in addition to chl a and cartenoids;
Recently an encrusting coralline algae was found at a depth of 268m a greater depth than that recorded for any photosynthetic organism; this more than 100m lower than the depth to which light can penetrate water
* Storage product is floridean starch (branched polymer of glucose) but this is never stored in chloroplast ( in cytoplasm) plus other weird sugars only found in the reds;[carrageenan below]
*Cell wall of cellulose overlayed by thick layer of slimy mucilage (sulfated galactans) agar and carrageenan; the complete wall is quite thick, often as thick as protoplast is wide; since all but a few are multicellular, even internal cells have direct contact with the water
1. These are important commercially as thickening agents, suspending or stabilizing agents.
Agar is used to make capsules that contain vitamins, drugs;dental impression material; base for cosmetics; culture medium (agar plates). Also anti-drying agent in bakery goods, preparation of fast setting jellies and desserts, temporary preservative for meat and fish in tropics.
Carrageenan is used for to stabilize paints, cosmetics and dairy products ( in puddings, ice creams, cheeses, salad dressings).
2. Whole plant is used as food -Nori- employs 30,000 people in Japan alone value $20M; sushi, soups etc.

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.
* Pit connections; a ring with a lens shaped protein plug held in the walls by equatorial grooves; don't know whether intercellular communication or transport? if 2 separate filaments meet they form this pit.
* Numerous red algae are parasitic on other red algae (40 genera); basal cell penetrates into host forming secondary pit connections with host cells.
* Type of cell division is weird so majority don't form parenchyma tissue; only bodies of filaments held together by their mucilaginous walls. Tissue thus is pseudoparenchymatous. True parenchyma only found in few species Porphyra.
* No true cell specialization; inner cells smaller, outer larger. Grow rhizoids to rocks etc.




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
This is a flowering plant, found in moderately exposed rocky shores, usually on flat and protected small areas on which sand accumulates


Rocky intertidal Herbivores:

As mentioned previously, there are two types of herbivores:

  • Slow moving resident grazers which feed on benthic diatoms and seaweeds examples which include: periwinkles, amphipods, isopods, sea urchins, and limpets.
  • Sessile filter feeders which capture from currents phytoplankton, cyanobacteria, zooplankton and which include: mussels and barnacles

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.

Higher on the shore, species of the periwinkle, Littorina are the dominant grazers. They feed with a tongue like radula, raking or scraping off diatoms, small algae, and soft tissue algae into their mouths. They don't attempt to the heartier well defended species.

Littorina littorea or the common European periwinkle, is a relatively new introduction, introduced about two centureis ago. This inch long alien has become the most abundant intertidal herbivore throughout its range on the east coast and has changes the habit in its wake. It has replaced native species of snails, cut down on populations of edible green algae, and in areas overrun the shores with densities of 1000 per square meter. The two endemic species avoid competition by moving at high intertidal heights where the European species can not survive.



These rocks are covered completely by a population of barnacles. Adults are hermaphroditic but planktotrophic larvae disperse in the plankton for several weeks. As a consequence, it is a necessary condition that the dense populations on these rocks are derived from a dense larval settlement. In fact, newly settled S. balanoides arrive in densities far greater than shown here. As they grow, barnacles undercut and smother each other, resulting in high mortality.

On outer coast rocky shores of western North America, Mytilus californianus forms extensive mussel beds, providing that the sea star Pisaster ochraceus is not too abundant and wave shock is not strong.

Scores and even hundreds of invertebrate species live on and among the mussels, which function analogously to trees in a rain forest. Mussels provide a substratum upon which to live, but they also provide a wet microclimate and even food for species that live in the interstices of the mussel bed.

Note the seaweed. As it turns out, Mytilus larvae often settle preferentially upon filamentous algae - so much so that mariculturists use ropes to encourage settlement, owing to their similar mechanical surface properties to the seaweeds.

Limpets graze heavily on Endocladia, which would reduce the settlement of mussels. The sea star Pisaster ochraceus, however, preys on limpets, which indirectly enhances Endocladia growth and the settlement of mussels, which are the main prey of Pisaster.

Limpets such as the eastern Pacific mid-intertidal Notoacmea scutum are important grazers on rocky shores and strongly modulate the balance of occurrence of rocky shore species. By their grazing activities they may bulldoze aside and cause early mortality of newly settled barnacles. That may be the explanation for the near absence of barnacles in this picture. If limpets are removed, thick diatom mats develop within days.

Predators: Intertidal predators range from snails and crabs to birds and fish

Snails such as dog whelks( nucella) on rocky shores and Moon snails ( Polinices) on sandy shores drill into shelled prey with a radula. Others smother their prey with the foot and use the radula to rasp the flesh out of their victims.

The dog whelk is a drilling gastropod that feeds mainly on barnacles on northeastern American rocky shores. Like many other mollusks, populations are polymorphic for color and banding pattern. Above you can see orange, black and white color morphs, as well as completely colored and banded individuals.

Studies on other gastropods demonstrate that these color patterns are strongly heritable and that the colors are an adaptation partially to conceal the snails from predators by matching them with the color of the background rocks (these were gathered together for this photograph).

Nucella lapillus needs to be moist when the foot is outside the shell and it is restricted to the lower part of a rocky shore, where the snails can be submerged and wet for a large part of the day. They often retreat to the lower intertidal or to wet cracks in the rocks at the time of low tide, and move upwards to find prey when the tide rises. Owing to this restriction, predation by dog whelks is concentrated on the lower part of the shore.

The dog whelk feeds on the mussel Mytilus edulis on rocky shores of New England. When it mounts a mussel it commences to drill a hole in the shell. Mussels however can fight back and respond by attaching byssal threads to the snail and ensnare it, which traps them and exposes them to crab predation.

The shape of the snails and other gastropods is not left to chance. Studies of crushing by their predators reveals:

Crab, most likely a Dungeness ( Cancer magister)+
Ochre star ( Pisaster ochraceous) or Asterias

Crabs including the introduced European green crab and the rock crabs on the east coast and Dungeness and others on the west coast..They can locate buried prey by chemoreception and physical contact, dig them from the substrate and use their strong claws to break open shells.
On the rocky shores of the N. Atlantic, green crabs have omnivorous diets but are major predators of blue mussels. They often live associated with the mussels, and have well developed shell opening behaviors for preying . With their unequal claws the larger master arm with its enlarged abductor muscle generates great force, and a heavy dentition helps to crush shelled prey. The smaller claw is much thinner and is used to pick the flesh They prefer the medium sized mussels, as the larger require more effort to pick, and the smaller don' yield as much meat for the effort.

The Dungeness Crab or Cancer magister gets its common name from the town of Dungeness, Washington, where the first commercial harvesting of the crab was done. They are cannabilistic ( as other crab species are suspected to be) when need be, but other wise Dungeness crab feed primarily on fish, shrimp, molluscs, and crustaceans. . They themselves are eaten by humans, harbor seals, and sea lions.

On rocky shores, gulls, crows and oystercatchers either use their beaks to hammer through prey or drop hard shelled prey onto rocks to break them open. On sandy shores shorebirds use their long beaks to forage for mostly soft bodied prey.

In salt marshes and soft substrate habitats, killifish and flounder predate on virtually all smaller animals restricting many of their prey to high intertidal refuges.


This asteroid starfish occurs on rocky shores from Baja California to Alaska and was used by Robert Paine as the prime example of a keystone species. It consumes a wide variety of prey and its consumption of mussels often allows the coexistence of competing sessile rocky shore species