San Juan Island includes a wide variety of habitats that have some
animal species in common and some that are found only in specific
habitats. This page summarizes feeding strategies by the most
prominent fauna at three field sites surveyed during the MIZ 2004
marine habitats project. We visited one field site on each of
three low tides to work with the habitat team, in order to gain a
better understanding of differences among
these three intertidal habitats. We used these visits to gain an
understanding of physical differences among sites as well as the most
abundant or prominent animals encountered using typical survey methods
for each site. For a reference to common terms used in comparing
The Cobble Beach at Snug Harbor is a north-west
facing shore made up of various sized rocks, most of which can be
over by hand. Water surge is variable depending on the weather, and
stones may be turned over by high wave action.
Upon the Rocky
When looking closely at the surface of the beach,
almost every rock is littered with a combination of limpets, snails,
barnacles, and chitons. With the exception of a few snail species that
prey on shelled organisms, these animals from the upper rocky surface
feed on small particles that are suspended in the water column, or
organisms and material that have settled onto the rocks (such as algae,
or the very young stages of other
animals) and can be grazed off.
- Barnacles (Balanus glandula, Chthamalus
dalli, Semibalanus cariosus) are by far the most abundant organism
rocky surfaces. They suspension feed by waving long, branched legs
cirri) into the water column to capture food particles, especially
single-celled algae (phytoplankton). The cirri can be moved
twisted from side to side, or individually wrapped around larger
- Limpets (Lottia scutum, Lottia pelta, Lottia
digitalis ) are also very abundant on the surface of rocks. The
three species of limpets each have a modified, hard tongue (radula)
that contains multiple rows of chitinous teeth used for scraping algal
material off the rocks so it can be ingested.
- Snails abundant on the rocks employ multiple
feeding strategies. Species of the family Littorinidae (Littorina
L. sitkana) are herbivorous grazers that also employ a radula to
scrape algae off the
rocks. All of the species of Nucella
present (N. canaliculata, N. lamellosa, N. ostrina) are
of barnacles. The snail uses the radula to drill a hole in the barnacle
shell at the seam between two plates, and then extends its proboscis
the hole to scrape out the soft tissue. The preferred barnacle is Balanus
glandula which was already mentioned as being highly abundant in
- Chitons are found both on top of and under the
rocks. They are less abundant than the previous species mentioned. All
the chitons (Mopalia sp., Lepidochiton sp., Tonicella
lineata) are grazers of algal material, detritus, and other
found growing low to the ground so they can be considered generalist
of very small matter.
Hidden Just Beneath the Rocky Surface
Whenever you are on a cobble beach, never
resist the urge to turn over a rock or two and peek at what is
as long as you replace the rock gently.
- Crabs are abundant in this microhabitat. The most abundant are Hemigrapsus
shore crabs (mostly H.
H. oregonensis), which graze the green thin alga (Ulva)
present in high abundance in the lower intertidal zone. Other highly
abundant crab species are Petrolisthes eriomerus (which is a
suspension feeder) and Lophopanopeus bellus (an omnivorous
predator/scavenger). Other crab species (Cancer productus, Cancer
gracilis, Cancer oregonensis) are carnivorous predators
on bivalves, limpets, snails, and other crabs.
- Sea stars are also abundant under and between
rocks. Both Pisaster ochraceus
and Evasterias troschelii
are carnivorous predators on barnacles, bivalves, mussels and even
other sea stars. Sea stars capture mostly sedentary or
slow moving prey. Some are able to evert the stomach and secreted
enzymes onto prey without swallowing the whole prey organism. This
can take many hours to complete, after which the sea star will spit out
any hard and indigestible parts, such as shells.
Summary of Cobble Beach.
Most of the prominent or abundant animals at this site are predatory
carnivores, herbivorous grazers, or suspension feeders, with a few
omnivorous scavengers. Deposit feeders (which ingest sediment) are
rare, possibly because most
of the surface is loose rock (more deposit feeders might be found if
top rock layers were excavated to reveal the sand and mud beneath).
Rocky Promontory of Eagle Cove is a south-facing
cove exposed to the rough waters of the Strait of Juan de Fuca.
Intertidal With Tide Pools
Both Eagle Cove and Cobble Beach are
predominantly rocky substrates and therefore share many of the same
species of limpets,
snails, barnacles, chitons, and sea stars. Some others (described
were prominent at Eagle Cove but not at Cobble Beach.
- The sea anemone Anthopleura elegantissima
is an abundant carnivorous suspension feeder. The anemone will wait for
food particles in the water column to touch its sticky tentacles (which
adhere to objects using both mucus and small stinging structures called
nematocysts), and then use the tentacles to move food into its mouth.
Both small planktonic organisms and larger prey (such as mussels that
fall from the rocks above) are eaten. Any indigestible material (such
as shells) will then be spit
out. These anemones also have symbiotic single-celled algae
live within the tissues of the anemone. These algae use sunlight to
sugars and other metabolic byproducts of photosynthesis, which leak
the surrounding tissues of the anemone for its own sustenance. This
relationship is particularly beneficial for anemones high in the
that must protect themselves by closing up tightly at low tide (and
cannot feed until the high tide returns).
- The mussels Mytilus californianus and
M. trossulus are suspension feeders that (like most other
bivalves) use the ciliated gill (in molluscs called a ctenidium) to
seawater and to capture small suspended food particles. Many species
prey on mussels, including crabs, sea stars and some vertebrates, such
- While climbing over the rocks, one will notice
many tubes approximately 1 cm in diameter and 7-10 cm long. These are
suspension-feeding sabellid polychaetes, Eudistylia sp. They
feed using mucus-covered feather-like tentacles that are projected from
the top of the tube during high tides and retracted into the tubes
during low tides (this can also
be done quickly when stimulated with touch). These worms are
in cracks and crevices.
- The goose-neck barnacle, Pollicipes polymerus,
looks unusual but feeds like a typical acorn barnacle.
(covered with water except at the lowest tides)
- The most notable lower intertidal organism is
Balanus nubilus, which feeds in the same fashion as
- The abundance of barnacles, mussels, and other
invertebrate life encrusting the rocks supports several species of
carnivorous sea stars (Leptasterias
hexactis, Pisaster ochraceus, Henricia leviuscula).
- Several species of bryozoans can also be found,
but the most abundant is Membranipora
membranacea which grows on algae and is a suspension feeder.
This species is also found
growing on algae at Cobble Beach.
Summary of Rocky Promontory
of Eagle Cove
Eagle Cove is the home to some of the same species as Cobble Beach,
which are all dominant species on any rocky beach in Washington. The
most prominent species in this habitat are suspension feeders or
Mud Flats of Garrison Bay are soft sediments
within a protected bay with very calm waters that slowly creep in and
the tidal oscillations.
- Clams are evident at the surface of the mud in
several ways: as shells, as holes, or as the exposed tips of the
siphons (the intake and outflow for seawater). Species include: Clinocardium
Macoma nasuta, Tresus capax, Protothaca stamanea, Transennella tantilla
, and Gemma gemma. All are suspension feeders.
- Polychaete worms are plentiful at the surface and
within the mud. Abundant subsurface direct deposit feeders include the
families Arenicolidae, Capitellidae and Opheliidae. Members of these
families have mouths built for sucking in mud. One very interesting
feeder found initially at the surface but actually buried in the mud is
in the family Terebellidae. Worms in this family are easily identified
by the long,
hair-like tentacles that are extended over the surface of the mud and
to collect detritus. Three families, Hesionidae, Glyceridae, and
are carnivorous predators on the surface or deep within the mud.
of these families have jaws used for capture of prey (especially other
Species in the family Nereidae are scavenging omnivores that can be
found either at the surface or burrowed
Summary of Garrison Bay Mud
The two most prominent animal groups are bivalves and worms. The
seawater of this bay is rich in suspended material (for bivalves), and
much of this nutrient ends up in the mud (for worms). The gentle
movement of water in
and out of the bay allows sediments and nutrients to settle and
onto the mud’s surface, which may partly account for the rich
content and high species abundance of the mud.