Protection at Eagle Cove

Within Eagle Cove there exist several zones containing different assemblages of animals.  Each zone provides special challenges related to protection and defense. The major threats are posed by predators, wave action, heat, and desiccation.  Here we focus on a few examples from each of the three habitats, paying attention to differences in environmental conditions, availability of resources, and predatory threats.

Rocky Promontory

The rocky intertidal area of Eagle Cove undergoes the most extreme temperature changes of the zones that we surveyed. During high tide the temperature remains stable, but at low tide the temperature can rise dramatically in a shallow pool. Most animals found in the intertidal zone are ectotherms that cannot regulate their internal body temperatures. Because many physiological processes rely on stable body temperatures, the activity of many organisms is regulated by daily changes in conditions, and some are known to produce heat shock proteins (HSPs) as a way to limit thermal damage.

Aeolidia hunting Anthopleura

Many intertidal organisms are confined to areas that remain relatively moist and cool at low tide.  The sea anemone Anthopleura elegantissima, for example, is commonly found in shallow pools or crevices.  At low tide they retract their tentacles and compress their soft bodies to conserve water. When submerged, anemones are subject to predation by the beautiful sea slug Aeolidia papillosa , which stores the stinging cells of the anemone (nematocysts) for use in the slugs' own self defense.  It is not known how the slug is able to consume nematocysts without being stung, or how it manages the transfer of stinging cells to its own tissue.

Soft bodied polychaete worms are also especially vulnerable to desiccation.  The soft parchment-like tubes secreted by sabellid worms are useful for protection from predators and desiccation. Sabellids anchor their tubes in rock crevices where they are protected from the rough surf. When exposed at low tide they retract their feeding structures into their tubes. Many other polychaetes take shelter in crevices or under the moist algae to prevent dehydration.

Sabellid worms retracted into their tubes at low tide

Katharina blends in well with its environment

Molluscs like snails and limpets use shells for protection from predators and environmental fluctuations. At low tide these animals can pull their shells down against the rocky surface. If pried loose by a hungry predator, snails will retreat completely into their shells and use the operculum to close the aperture and shut off access to the soft tissue.

Chitons can be difficult to spot in tidepools because they often blend in well with the rocks and algae. They are protected by calcareous plates that run down their backs and can also resist predation by clamping the mantle area around the shell down against the rocks. This makes it nearly impossible to pull the animal off of its substrate and has the added benefit of conserving water.

Hermit crabs (genus Pagarus) use empty snail shells for protection. This saves them the trouble and expense of secreting a tough shell of their own, but it also means that as the crab grows it must continuously locate new shells.  We noticed that many of the hermit crabs found at Eagle Cove have ill-fitting or damaged shells, suggesting that optimal shells are in short supply. Hermit crabs will typically retreat into their shells when handled, but crabs at Eagle Cove often drop out of their shells and scurry away because they are too large to retract completely inside.  Perhaps the empty snail shells are broken up when they are tumbled on the rocks by heavy wave action.

Bryozoan polyps and the rectangular skeletons they retreat into

Bryozoans are soft-bodied sessile animals that can often be found encrusting on rocks or algae. To the naked eye, the colony looks like rough scales. Under the microscope one sees a crown of tentacles associated with each zooid, in addition to a calcareous outer skeleton.  Bryozoans can retract their polyps into their skeletons when they aren't feeding to protect the fragile polyps from desiccation and attack.

Echinoderms like starfish can protect their soft tube feet from hungry crabs by pressing their bodies firmly against the rocky substrate when at rest.  They are able to press their stiff bodies to irregular surfaces or wedge them into crevices by shaping the body using mutable connective tissue.  Echinoderms have a water vascular system that they use in locomotion and this system loses water as they move so conservation is important during low tides. During low tide they will retreat into crevices and halt their foraging activities.

Sea cucumbers have a rather unique form of defense. If harassed by a predator the cucumber can release sticky strands called Cuverian tubules from its anus. The strands detach from the cucumber and entangle the predator, allowing the sea cucumber an opportunity to escape.

Pisaster starfish resting at low tide

Cobble Area

Anomuran crab Petrolisthes

The cobble area is subject to rough wave action and cobble tumbling, and consequently many of the animals found there have some type of tough exterior.  Animals in the cobble area, which is high in the intertidal zone, are also susceptible to desiccation.  There are some small pools under the rocks where most creatures are found, but the low diversity of algae indicates that the area is dry compared to the rocky promontory. Crustaceans and limpets are well represented in this area.

Porcelain crabs (genus Petrolisthes) are abundant under the cobbles. They cling tightly to the rocks and if harassed they are likely to autotomize (selectively release) a limb.  This remarkable ability helps the crab escape predation by freeing it from the predators grasp.  The limb will eventually regenerate during a subsequent molt.

The isopod Gnorimosphaeroma oregonense is also found in high numbers here. These crustaceans resemble the mottled terrestrial roly-polys to whom they are closely related. This isopod is found in aggregations under the cobbles and will roll up into a ball to protect the vulnerable underside when disturbed.  Idotea wosnesenskii is another isopod found under cobbles.  It's powerful jointed appendages allow it to grip tightly to the rock surface.

Stuck to the underside of many cobbles is a clump of wet sand that is, in fact, a tube containing a terebellid worm. Terebellids construct their tubes out of mucus and sand. The tube helps keep the worm moist, and keeps the worm securely underneath the rock.

idotea isopod Idotea wosnesenskii

Sandy Beach

opposum shrimp, Order Mysidacea
Sweeping a net through the sandy shoreline will reveal a surprising number of small crustaceans, including mysids ("opossum shrimp") and cumaceans, which are closely related to isopods. These animals are protected by their sandy coloration and ability to burrow into the sand. There are also larger shrimp here of the genus Crangon, about the length of a little finger.  They are extremely well camouflaged against the sand, and are best seen after being caught in a dip net.

Between the sand grains is a community, the meiofauna , that exists and must gain protection at an entirely different scale. These interstitial animals include several of the phyla with larger representatives (like polychaetes, cnidarians, flatworms and crustaceans) as well as phyla that are mostly small bodied (like nematodes and tardigrades).  They avoid abrasion from sand grains and wave action by adhering to sand particles or by developing a tough exoskeleton.

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