Protection & Defense on the FHL Floating Docks

Within the floating dock habitat, several methods are used by different organisms for protection from predators and physical disturbance.  The float substrate we examined was composed of rubber tires; therefore, lifestyles of the float community are adapted for living on a relatively hard surface.  For example, organisms on floats cannot escape aerial exposure or predators by burrowing into their substrate (for more information on living in soft-sediment substrates, see False Bay). Many of the organisms living on the floating tires are sessile (see locomotion) and therefore cannot move away to protect themselves from danger.
Some sessile organisms construct hard exoskeletons which they use for protection. Polychaete worms, such as members of the serpulid, spirorbid and sabellid families, secrete tubes into which they withdraw when threatened.  Barnacles also secrete a hard exoskeleton, including several plates at the top opening that can be closed tightly. 

Numerous other organisms within the float environment use exoskeletons as a means of protection or defense. Many molluscs (such as mussels, limpets and chitons) have calcareous shells that protect them from predators. Crabs also have hard exoskeletons and are equipped with pinching claws to defend themselves.

Decorator crabs are commonly found on the floats. They accumulate other organisms (such as algae, bryozoans and other sessile animals) on their carapaces for camouflage. Here is a decorator crab well-camouflaged in a covering of  the "potato chip bryozoan", Dendrobeania lichenoides.  For more information on decorator crabs, click here. Also present on the floating docks, the small cancer crab Cancer oregonensis tends to rest in small holes and can often be found hiding in empty Balanus nubulis barnacle shells on the float tires. 


Although the bryozoan Bugula pacifica may resemble a branching plant, it is actually a colony of many tiny animal units (zooids) that build and live in the branching structure shown in the picture to the left. Many bryozoans have different types of zooids that perform special functions--such as feeding, protection and reproduction--for the colony.  Bugula pacifica has specialized zooids, called avicularia, that help to defend the colony from unwelcome settlers. Resembling bird beaks on long stalks, avicularia keep the colony surface clean by pinching visitors who settle on it. 

Click here to see a (fuzzy) movie of the highly magnified avicularia and feeding polyps of a Bugula pacifica colony.  In the movie, you will see the beak-shaped avicularia swinging and snapping in the water.  Look at the bottom right corner of the screen to watch a feeding zooid (a polypide with delicate tentacles) retract into its chamber. 



Other organisms inhabiting the floats defend themselves by chemical means. Sea anemones contain nematocysts, or stinging cells, which are used for capturing prey but also for self-protection (see feeding for more information about nematocysts). Commonly found on the FHL floats, Metridium senile (the white sea anemones shown above) often exudes white threads called acontia (containing nematocysts) when it is disturbed.  Nematocysts are fired from the acontia to sting all that come in contact with them.  To see a movie of firing nematocysts, click here and watch for the slender filaments as they corkscrew away from the edge of the tentacle.


The nudibranch Phidiana crassicornis has an interesting method of defense. This gastropod is able to ingest the stinging capsules of the hydroids it eats and store the unexploded capsules in the tips of its cerata (the fleshy projections along its body) for its own defense.
Another organism which has a very unique means of defense is the sea cucumber Eupentacta quinquesemita. This small cream-colored echinoderm (up to 10 cm long) can be found hiding in crevices on the float tires. In times of high stress, the animal can purge, or eviscerate, some of its internal organs out the mouth.  It leaves the ejected material behind as a distraction to the predator, and the organs later regenerate within the animal.

For more information on how different marine invertebrates protect themselves in their natural habitats around San Juan Island, visit the following web pages:

Argyle Creek                Cattle Point                False Bay                Plankton