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 feeding strategies, click here.

The Cobble Beach at Snug Harbor is a north-west facing shore made up of various sized rocks, most of which can be turned over by hand. Water surge is variable depending on the weather, and some stones may be turned over by high wave action.

Upon the Rocky surface

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 on the rocky surfaces. They suspension feed by waving long, branched legs (called cirri) into the water column to capture food particles, especially small single-celled algae (phytoplankton). The cirri can be moved up-and-down, twisted from side to side, or individually wrapped around larger planktonic objects.
• 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 scutulata and 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 predators 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 through the hole to scrape out the soft tissue. The preferred barnacle is Balanus glandula which was already mentioned as being highly abundant in this habitat.
• Chitons are found both on top of and under the rocks. They are less abundant than the previous species mentioned. All of the chitons (Mopalia sp., Lepidochiton sp., Tonicella lineata) are grazers of algal material, detritus, and other organisms found growing low to the ground so they can be considered generalist grazers 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 underneath, just as long as you replace the rock gently.

• Crabs are abundant in this microhabitat. The most abundant are Hemigrapsus shore crabs (mostly H. nudus, some 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 digestive enzymes onto prey without swallowing the whole prey organism. This process 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 the 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.

Mid 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 below) 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 that live within the tissues of the anemone. These algae use sunlight to make sugars and other metabolic byproducts of photosynthesis, which leak into the surrounding tissues of the anemone for its own sustenance. This symbiotic relationship is particularly beneficial for anemones high in the intertidal that must protect themselves by closing up tightly at low tide (and consequently 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 suck in seawater and to capture small suspended food particles. Many species prey on mussels, including crabs, sea stars and some vertebrates, such as gulls.
• 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 concentrated in cracks and crevices.
• The goose-neck barnacle, Pollicipes polymerus, looks unusual but feeds like a typical acorn barnacle.
  

  
  Lower Intertidal (covered with water except at the lowest tides)
  

• The most notable lower intertidal organism is Balanus nubilus, which feeds in the same fashion as other Balanus.
• 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 predators.

Mud Flats of Garrison Bay are soft sediments within a protected bay with very calm waters that slowly creep in and out with 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 nuttallii, 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 selective deposit 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 used to collect detritus. Three families, Hesionidae, Glyceridae, and Phyllodocidae, are carnivorous predators on the surface or deep within the mud. Members of these families have jaws used for capture of prey (especially other worms). Species in the family Nereidae are scavenging omnivores that can be found either at the surface or burrowed underneath.

  Summary of Garrison Bay Mud Flats
  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 accumulate onto the mud’s surface, which may partly account for the rich nutritional content and high species abundance of the mud.
  

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