Feeding at Eagle Cove

Eagle Cove tidal zonation and tide pool

Animal form provides a number of clues about the types of foods that an organism can profitably capture, ingest, and digest.  The great diversity of body forms among marine invertebrates at Eagle Cove correlates with a diverse array of feeding strategies.
As heterotrophs, animals depend on primary production by photosynthetic organisms, or secondary production by other animals, for their nutritional needs.  Examples of heterotrophy include carnivory, herbivory, and omnivory. These groupings can be further subdivided functionally according to how and where an organism procures food: by scavenging, parasitising, grazing, suspension feeding, deposit feeding, or predation.  Each of these feeding modes in turn is associated with differences in body design and microhabitat use.
Several environmental gradients at Eagle Cove influence the distribution of organisms and feeding modes.  Steep faces on the seaward side of the rocky promontory parallel steep gradients in the length of tidal exposure.  Small pools that remain filled at high tide lie adjacent to stretches of well-drained, bare rock.  Smaller boulders that jut out of the sand have both seeward and landward sides that experience different exposures to sun and waves.   Rock walls toward the landward side of the intertidal have a patchwork of overhangs beside more exposed surfaces.  Each of these gradients can influence the distribution of feeding modes over different spatial scales.

Feeding modes for common taxa at Eagle Cove

Here we describe basic feeding modes for higher-level taxonomic groups of organisms that are abundant at Eagle Cove. Our survey focused most extensively on organisms in tide pools, on rock faces, and under cobble east of the sandy beach.

Porifera (sponges)
Cnidaria (sea anemones)
Nemertea (ribbon worms)
Annelida (polychaetes: marine segmented worms)
Mollusca (bivalves, snails, sea slugs)
Arthropoda (crustaceans: decapods, isopods)

Porifera (sponges): sessile heterotrophic suspension feeders

Demosponge Haliclona sp.
Lacking a protective covering, sponges are excluded from areas that become dry at low tide, but they are common in protected areas of the low intertidal.   Because sponges are sessile suspension feeders, they must either constantly create their own water currents to deliver suspended food particles or take advantage of water currents available in the environment. Sponge bodies contain a series of channels with flagellated cells that are used both for creating currents and concentrating food from suspension.

At Eagle Cove, look for:  Haliclona sp. (purple), Halichondria sp. (green/yellow), Ophlitaspongia sp. (orange-red)

Phylum list

Cnidaria (sea anemones): sessile carnivores with supplementary autotrophy

Anthopleura elegantissima

The phylum Cnidaria includes both medusoid (jellyfish) body forms and polypoid body forms.  Sea anemones are part of a class, the Anthozoa, that include only the polyp form, which is sedentary and feeds using a ring of tentacles that encircle the mouth.  The tentacles bear specialized stinging cells (cnidocytes) that contain capsules (cnidae, or nematocysts) that forcefully eject a long thread used to stick to or subdue prey.  The animal then passes the prey item to the mouth or folds all of the tentacles inward to deposit prey into the gastrovascular cavity.  As in most corals, some anemones also harbor photosynthetic Zooxanthellae, which live symbiotically inside the cells of the anemone's tentacles.

At Eagle Cove, look for: Anthopleura elegantissima (common green anemone), Anthopleura xanthagrammica  (larger green anemone), Urticina crassicornis (red-green striped anemone)

Phylum list

Nemertea (ribbon worms): active predators, sometimes scavenging

Ribbon worms are generally hunters that follow a chemical trail left by their prey. They have an extensible proboscis, which rests inside of an internal fluid-filled body cavity and is shot out above the mouth region by the muscular contraction of the cavity.  At the tip of their proboscis, some nemerteans have a poison-filled stylet that can be used to subdue prey. The proboscis can be alarmingly long, and the sting at the end sufficient to capture animals many times the size of the worm.  

At Eagle Cove, look for: Paranemertes sp. (dark brown/purple), Amphiporous sp. (white)

Phylum list

Annelida (marine segmented worms): diverse feeding strategies

Segmented worms, from the annelid class Polychaeta, include some of the most diverse feeding modes found within one taxon.  The worm shaped body has been adapted for different types of food capture, particularly by modifying characteristics of the head region and of parapodial appendages found on each segment. The following families can be found at Eagle Cove:

Phylum list

Mollusca: filter-feeders and grazers

The mollusc community at Eagle Cove reflects several basic feeding strategies that take advantage of different aspects of the mollusc body design.  Chitons (Class Polyplacophora) use the hard teeth of the long tongue-shaped radula to rasp at algae encrusted surfaces.  Snails and nudibranchs (Class Gastropoda) can similarly use the radula for grazing, but radular teeth can also be modified for a predatory lifestyle for chewing through soft tissue or rasping through calcified shells.   On the other hand, bivalves (Class Bivalvia) have lost the radula, and many have gills (ctenidia) that are instead modified for straining food particles from suspension.  For a listing of species to look for at Eagle Cove, see the master species list.

chiton  Katharina tunicata


Prosobranch snail  Calliostoma sp.

nudibranch eating the
sea anemone Anthopleura

Phylum list

Bryozoa (Ectoprocta): filter feeders

membranipora Membranipora membranacea
movie of polyps retracting/extending

Bryozoans appear as scaly honeycomb patches on algae or rocks or as epibionts on the surfaces of other animals.  Nearly all of the individual colony members (zooids) have a ring of tentacles called a lophophore, although some zooids may be specialized for cleaning, defense, or reproduction.  The colony members, which are genetically identical and physiologically integrated, use cilia on the lophophore tentacles to capture suspended food particles and to transfer them to the mouth.

At Eagle Cove, look for: Membranipora membranacea , Dendrobeania sp. (potato-chip shaped), Heteropora magna (yellow, stony, tubular).

Phylum list

Arthropoda (Crustacea): diversity in feeding strategies associated with diversity in appendages

The crustacean arthropods provide one of the most extensive examples of how variation in form is associated with variation in function.  A hallmark of the phylum is a set of serially-repeated jointed appendages, one pair per body segment.  These appendages have been modified evolutionarily along the length of the body, and across different taxa, for different functions in locomotion and food handling.   The eucarid "decapods" (shrimp, hermit crabs, and true crabs)--so named because the first three pairs of thorax appendages are modified into mouthparts, leaving five pairs of legs (= 10) on the thorax--are commonly encountered at Eagle Cove, but the more cryptic peracarids (amphipods, isopods) and the ubiquitous cirrepedes (barnacles) also provide useful examples of variation in feeding strategies. 

red hermit crab Elassochirus gilli
Anomuran crabs (hermit crabs, porcelain crabs, galatheid crabs) have a soft, flexible or twisted abdomen, and the last pair of appendages on the thorax is highly reduced. These animals generate a feeding current using special appendages near the head region which they use to filter particles from suspension.  Others typically use the chelate appendages to scavenge drift material that collects around cobble.

At Eagle Cove, look for: Pagurus caurinus (hermit crab) and Petrolisthes cinctipes (porcelain crab) .

red rock crab Cancer productus

decorator crab Pugettia gracilis

Brachyuran ("true") crabs have a symmetrical abdomen, with all 10 thorax appendages visible.  As in anomuran crabs, the abdomen is highly reduced, with appendages modified to hold eggs (in females).  Many are active hunters that can use the powerful chelate appendages to open bivalves, snails and barnacles as well as to catch and manipulate smaller crustaceans.  Most will also scavenger live or dead animal or plant material.

At Eagle Cove, look for:
Hemigrapsus oregonensis & Hemigrapsus nudus (shore crabs), Pugettia gracilis & Pugettia producta (kelp crabs), and Cancer productus.

green isopod Idotea wosnesenskii
Isopods and amphipods are often herbivores on live algal material or diatoms or opportunistic scavengers on decaying animal or plant material.  

At Eagle Cove, look for these isopod species:
Gnorimosphaeroma oregonense (grey, rolls up like pillbug), Idotea wosnesenskii , and Idotea resecata (translucent green).  

acorn barnacles Semibalanus cariosus
The body plan of cirrepedes (acorn and stalked barnacles) is highly modified for suspension feeding.  These unusual crustaceans spend their lives attached at their anterior end to a hard substrate, kicking their jointed appendages--extended by long, articulated cirri--up into the water to strain out food particles.  Because all arthropods lack cilia, feeding is done entirely but muscular activity, distinguishing these suspension feeders from those that use ciliated tentacles (e.g., bryozoans, molluscs, annelids, sponges--see above).

At Eagle Cove, four acorn barnacle species are common: Chthamalus dalli (small size, brown and flattened, high subtidal), Balanus glandula (medium size, dome-shaped, mid-intertidal), Semibalanus cariosus (larger size, lined plates, lower intertidal), and Balanus nubilis (large size, low intertidal to subtidal).  In addition, look for the gooseneck barnacle ( Policipes polymerus) in dense colonies within crevices high in the intertidal.

Phylum list

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