Tour of Rocky Shoreline Habitats


Nearshore sea life, Costa Rica:

Above left - a school of yellow-tailed snapper and other fish pass close to the rocks at the base of Eladio's Cliff.  Upper right - a spotted eagle ray (Aetobatius narinari) in the same location.  Right:  Egg case of a skate (below).  The egg cases are sometimes referred to as "mermaid's purses"; the hooks are supposed to entangle them on the bottom of the ocean and precent them from being cast onto the shore as this one was.

Rocky Shores:

In contrast to the shifting ephemeral nature of a sandy shore, rocky shores give an aura of permanence with their prominences boldly standing up to the crashing of waves.  While this permanence is illusory in the long run, over the short lifespan of many plants and animals it is real enough and competition for space becomes critical.  Here too we see plants make a reappearance as the plants can anchor themselves to the rocks.  Often, the rocky shores are interspersed with stretches of sandy beach, as the photo of Playa Nancite, lower right, illustrates.


The rocky coast of New England comes to mind when one thinks of rocky shores, and a visit there, such as out expedition in 1998, does not disappoint.  From the top of Mt. Cadillac in Acadia National Park, Maine (photo at right), one can look out over Bar Harbor at the rocky Maine coast (below left). A closer look at the rocks in the intertidal zone (the area between low and high tides, lower right) gives a glimpse into the intense competition for space that occurs on any hard substrate.  The bulk of the rock surface is coated by a layer of grayish barnacles (Balanus sp.), which, for whatever reason, are able to compete more effectively in that area.  Perhaps they can hold onto the exposed rock face, perhaps they are more resistant to desiccation, whatever the reason the open rock face is covered with barnacles.  A close examination of the crevices in the rocks, however, reveals a different story.  Here the black mussels, attached to the rock with byssal threads, are able to outcompete the barnacles.

Let's take a closer look at barnacles (left).  The large barnacle in the center is surrounded by a number of smaller barnacles, some of which are growing on the large barnacle's shell.  Think of a barnacle as a shrimp which glues its back to the rock and then secretes a shell of moveable plates around itself.  It can close the plates up to protect the barnacle at low tide, or open them up, etend the legs, and wave them through the water where they filter out microscopic plants and animals (plankton) that form the barnacle's diet (the large barnacle to the left is just unfolding its legs).

Barnacles start out as small microscopic planktonic animals themselves.  They settle onto a (hopefully bare) substrate, glue themselves down, and set up housekeeping.  Obviously, if they can't find an open place they are doomed, as are these smaller barnacles in the photo at the left.  They have as much a chance of getting food as Stephen Hawking has of getting rebounds away from Shaquille O'Neil.

Where do the bare spots come from?  Any number of causes.  A piece of rock may break off, taking the established barnacles away and exposing a new, clean substrate.  Maybe a wave will toss a rock or log against the cliffside where the existing barnacles will get scraped off.  Whatever happens, a new surface will be colonized rapidly.  The photo at right shows a few older barnacles and a host of new ones, all about the same age, occupying formerly bare space.  The better competitors among these will grow to cover the substrate and squeeze out their less competitive peers.  Barnacles are not limited to Maine, of course - the lower pictures show some taller acorn barnacles (and a few snails) from the Washington Coast.  Likewise, the competition between barnacles and mussels isn't limited to the east coast of North America; the picture below right is from the Olympic Peninsula of Washington.

To the left you can see more western barnacles with their legs extended.  Barnacles in the intertidal zone may only be exposed for an hour or so a day and must do all their feeding in that short time.

Goose barnacles (left, below) have a slightly different strategy.  They attach themselves to the substrate via a flexible structure that gives under the impact of strong waves.  These individuals were found on the Olympic Peninsula.


Seaweeds are multicellular algae that often grow attached to rocks along a coastline.  They are from several different divisions of the algae.  The rockweed (above) is from the brown algae (Phaeophyta), the red algae (Rhodophyta) pictured above right is Chondrus crispus (Irish Moss) and the Codium sp. (Green Fleece Algae) is a type of Chlorophyta (Green Algae).  Among the differences between these groups are the types of chlorophyll and accessory pigments used.  

On our trip to New England in 1998 we came across many types  of rockweed (see pictures below).  This brown algae covered the rocks in some places in great profusion; it was quite a challenge to walk across it as it was very slippery.  If you look at the photo above you will notice mussels under the rockweed; the wet rockweed helps organisms under it to survive the periods of low tide.

Left: Laminaria digitata is a type of kelp found on the eastern coast of the United States.  Western species of kelp are up to 40m (120 feet!) long.

More rockweed (above) and Ulva sp., commonly called Sea Lettuce, a green algae (right)

Tides are a particularly conspicuous part of life in New England, home to some of the greatest tidal differences in the world (the largest tides are in the Bay of Fundy just to the north of Maine; unfortunately, we were there at high tide).  The pictures to the left and below are from Wolf Neck Woods State Park in Maine on our 1998 expedition.

In the lower photos you can see the large expanses of rockweed exposed at low tide.

Let's go out to Machias Seal Island (left) to see some of the seabird colonies.  In 1998, we rode out to Machias Seal Island on the Chief, operated by the (late) Barna Norton and his son John.

Barna and John Norton

Barna and John Norton



Machias Seal Island is administered by the Canadian goverment, which strictly regulates visitors to the island so as not to disturb nesting seabirds.  These birds used to nest up and down the east coast, but human development has restricted them to a few islands today.  That's an Artic Tern (Sterna paradisaea) to the right, its nest marked by a flag lest anyone step on it.  Below are Common Terns (Sterna hirundo); the pair on the right is courting, with the male offering the female a fish he has caught.

Herring gulls (Larus argentatus), left, are common up and down the coast.  The real attraction at Machias Seal Island were the Murres (below, left) and, of course, the Puffins (below, right and far below, center)


Left: A mixed flock of Razorbills as well as thin-billed (Uria aalge) and thick-billed (Uria lomvia) murres.

A pair of Razorbills (Alca torda).

Above and Left - Puffin Fratercula arctica.

To the right, Candace Tuxhorn (MC 1998) stands on rocks exposed at low tide on Great Wass Island, Maine.  We spent a lot of time in 1998 exploring the New England tidepools.

Below:  Green Crab (Carcinus maenas) on the rocks at Bar Harbor, Maine.  This is a female, carrying her eggs under the carapace.  In this way, she is able to protect the eggs and make sure they remain aerated.

Left: Sandworm (Nereis sp.).  We saw several of these in Maine; the one pictured here is a preserved specimen posed on a piece of coral in the lab.  The specimen at lower left was a living individual photographed on the beach at Great Wass Island.

A limpet.  A relative of the snail, the limpet glides over rocks on its muscular foot, scraping algae off as it goes.  

Below:  A nudibranch, another type of gastropod, with eggs.  Below left:  an Orange-footed Sea Cucumber (Cucumaria frondosa) Digital images by Tanya Jarrell.

Candace Tuxhorn (MC 1998) and Dr. Almuth Tschunko at Bar Harbor.

Let's go out west!  The pictures above and below are from a beach on the Olympic Peninsula in Washington State,  We'll focus on the rocky areas.

Anenomes can be found in the deeper waters, of course (the individuals to the left are obviously a tropical species) but some anenomes are found on rocks exposed at low tides.  An example of this is the Giant Green Anemone (Anthopleura xanthogrammica), below right.

Giant Green Anemone (Anthopleura xanthogrammica), above left; Aggregating Anemone (Anthopleura elegantissima).  The anemones pictured below are found in deeper waters.

The Frosted Nudibranch (Dirona albolineata) is a gastropod which lives on the northwestern coast of the US.  It feeds on a variety of invertebrates including snails and anemones.  It can be found on the rocks below the low tide zone.


Grainyhand Hermit Crabs (left) live completely underwater, unlike the hermit crabs mentioned earlier living on the beach in Costa Rica.


Below: a tiny Yellow Shore Crab (Hemigrapsus oregonensis) defends itself in Vancouver, British Columbia.

The Purple Sea Star (Pisaster ochraeus) ranges in color from purple (above) to orange (right).  An important predator, it is usually found on the rocks where it feeds on a variety of invertebrates, particularly mussels.  Robert Paine, in landmark studies, determined that the Purple Sea Star is a keystone predator.  He found that the sea stars tend to focus their predatory attentions on the most dominant competitors and thus opening up the ecosystem to other competitors which would otherwise be crowded out.  In other words, presence of the sea star actually increases diversity at the next lower trophic level.  Below is a close-up of the skin of this echinoderm, and below right is the underside.  On the ventral surface (bottom)  you can see the tube feet which the sea star uses to move and to grip its prey.  Each tube foot has a tiny suction cup at its tip.

Another Echinoderm from the Pacific Northwest is the Purple Sea Urchin, which feeds on algae much the way its relatives in the Caribbean do (Left).

Below: Octopii are important predators among the rocky inlets of the Pacific northwest.  This specimen was at the Cleveland zoo and I'm not sure if it comes from the northwest or not.  The most highly developed of the molluscs (arguably of all the invertebrates) cephalopods like octopuses and squid are highly intelligent and alone among the invertebrates have eyes similar to those of the vertebrates (the eyes having originated separately; there is do direct evolutionary relationship).  Octopii are highly intelligent and quite a handful in captivity.


This limpet is from the west coast and gives you a better idea of what the animal under the shell looks like.  The apex of the shell is open; water flows over the gills and exits through that hole.

The shrimp, below right, is another west coast inhabitant of the rocky shore.

A long lost relative?  The tunicate, poking out from under the rock below left, is also a Chordate just like fish, frogs, snakes, humans and birds to name a few.  All share a post anal tail (you lost yours before birth, I hope), pharyngeal gill pouches, a notochord and a dorsal hollow nerve cord.  The free-swimming larval tunicate looks more like a tadpole and is more recognizable as a chordate than its highly modified, sessile, filter-feeding adult form, shown here.

A few oddities to finish our tour.  The Harbor Seals (above) were spotted on a rocky outcrop in Maine.  Mammals, like the west coast Sea Lion (above right), they feed on fish and invertebrates.  Finally, the algae to the right are from the west coast - sea hair and sea lettuce.


Environmental Considerations of Sandy and Rocky Shores:

Our rocky and sandy coasts are in peril because we love them so much.  They are a very attractive place to live and work, as this pictures of the waterfront at Castine, Maine attest.  A large and increasing percentage of the human population lives near the coast, and this puts a lot of pressure on those ecosystems.  Mangrove swamps were destroyed in Louisiana (for oil and gas exploration) and in Indonesia (for shrimp[ farms and other aquaculture) and the human shoreline inhabitants thus bore the brunt of Hurricane Katrina and the Indonesian tsunami.  Boats leak things (remember the Exxon Valdez?).  Fishing gear gets lost (below left).  And construction, agriculture and forest clearing - even miles inland - bring pollution and sediment to the coasts through our rivers, as shown below right where the Fraser Rivers empties into the Pacific near the Vancouver BC airport.


Proceed to Coral Reefs Proceed to Mangrove Swamps
Proceed to Coral Reef Fish Proceed to Sandy Shores
Proceed to Coral Reef Invertebrates Proceed to Rocky Shores