Tide Pools

Intertidal Marine Invertebrates

Most of the animals in the tide pool are invertebrates - animals without backbones. Invertebrates are a diverse group of animals including many phyla and classes.

Phylum Porifera (Sponges)

Sponges are simple animals without internal organs, muscles, or nerves. Nearly all sponges are marine and sessile (attached to the bottom or a hard substrate.)

The more than 5,500 sponge species share many common characteristics including the following:

  • Water flows into the sponge through ostia (tiny pores in the outer wall), then through small canals into spongocoels (spacious chambers), and exits though larger holes called oscula (singular: osculum).
  • Chambers within a sponge's walls contain cells called choanocytes with flagella (long, slender projections) that beat to create a current pulling water through the sponge's tissue. Some sponges can pump as much as 20,000 times their volume through their tissues within 24 hours. Choanocytes filter out bacteria and other tiny nutrients from the water flowing through the sponge. Oxygen is also obtained from this current.
  • Sponges may be encrusting or found in a variety of upright forms and commonly colored red, orange, or yellow.
  • Sponge reproduction may be sexual or asexual. Many sponges are hermaphrodites and produce eggs and sperm at different times. The sperm is broadcast into the water through the sponges' osculum and taken in by other female-stage sponges of the same species to fertilize the eggs. Some sponges release larvae in the initial stages of development while others retain the larvae for a while.
  • Sponges have an asymmetrical body shape.
  • A sponge's structure is maintained by spicules (needle-shaped, skeletal fibers of calcium carbonate or silica) or a kind of collagen protein called spongin. Sponges are classified based on the presence and kind of spicules.


Class Hexactinellida (glass sponges) - Sponges of this class contain only six-rayed spicules made of silica.

Class Calcarea - Calcareous sponges have spicules composed of calcium carbonate. These sponges mostly inhabit shallow water areas in the tropics.

Class Demospongia - This class contains more than 90% of all living sponges. Nearly all of these sponges contain spongin and some also have siliceous spicules with one to four rays.

Some members of this class are found in fresh water, some in ocean depths, and others in intertidal areas.

  • crumb-of-bread sponge (Halichondria panacea) - This widely distributed sponge is found from the middle intertidal zone down to the subtidal zone in temperate waters along the eastern Pacific coast and along both coasts of the Atlantic. The crumb-of-bread sponge is green, yellow or orange with irregularly placed oscula. It often encrusts rocks in sheets up to several centimeters thick.


Phylum Ectoprocta

This phylum is composed of bryozoans—tiny, colonial animals that live in rectangular or cylindrical shaped boxes.

  • Each zooid (individual body) is about 1 mm (0.04 in.) and covered by a calcareous exoskeleton (outer covering).
  • Zooids often connect to form a colony with a basket-weave pattern. Encrusting bryzoans often colonize the underside of rock surfaces in the intertidal zone. Other zooids combine to form an upright, branching colony that resembles seaweed.
  • Zooids use circular lophophores (feeding tentacles) to filter tiny particles from the water.
  • Bryzoan colonies grow by asexual budding.
  • Many bryzoan species inhabit the intertidal, yet many species are often difficult to identify without a magnifying glass or microscope.


Phylum Cnidaria

The cnidarians are a diverse group; they include sea anemones, corals, jellyfishes, and their relatives. About 10,000 species are known worldwide - inhabiting oceans from the intertidal zone to the greatest depths.

Many cnidarians are large and brightly colored. Some common cnidarian characteristics include:

  • Radial symmetry: The body is symmetrical around a central axis. These animals lack a head and usually bear a crown of tentacles around the mouth.
  • Diploblastic: Cnidarians have two main cell layers — the ectoderm (outer layer) and the endoderm (inner layer). The layers are separated and supported by the mesoglea (a middle layer composed of a jellylike material.)
  • Nematocysts: These tiny stinging capsules are present in all cnidarians, and are especially abundant in the tentacles. A nematocyst consists of a bulbous double-walled structure containing a coiled-up, hollow thread with a minute barb at its tip. Projecting from the outside of the nematocyst is a tiny sensor. The entire structure is microscopic. When the sensor is stimulated physically or chemically, the cell explodes and the thread is ejected with considerable speed and force. The barb penetrates the victim's skin and injects a potent venom.
  • Gas exchange takes place through the body surfaces. Cnidarians do not have gills or gill-like structures.
  • Many cnidarians spend their lives in a polyp (stalked and sedentary state with a ring of tentacles around the mouth) or medusa (jellyfish-shaped) form.
  • After a cnidarian paralyzes its prey with the nematocysts, it grasps the prey with its tentacles and carries the prey to its mouth. Undigested food particles are expelled through the mouth and washed away with the tide

Two of the four cnidarian Classes are commonly encountered in tide pools.


Class Hydrozoa (hydroids)

This cnidarian class contains about 3,200 species.

The life cycle of most hydroids includes alternating polyp and medusa stages. A fertilized egg grows into a sedentary polyp. The polyp asexually buds off into one or more medusae. The medusae reproduce sexually and form polyps—continuing the cycle.

Most hydroids are colonial; many produce calcareous or chitinous exoskeletons.

Some hydroids form branched, sedentary colonies that resemble feathers or seaweeds.

Other hydroids, like the Portuguese man-o-war and the by-the-wind-sailor, form pelagic (open ocean), floating colonies.

Some hydroids form encrusting colonies. Members of the hydroid genus Hydractinia encrust shells occupied by hermit crabs and help defend the crabs from predators.

Many hydroid species (branching and encrusting) occupy the lower intertidal zones.


Class Anthozoa

This is the largest class of cnidarians and contains more than 6,000 species including sea anemones and corals.

Anthozoans are either solitary or colonial.

Anthozoans lack a medusa stage and remain in the polyp form throughout their entire life.

Much of a sea anemone's body is a saclike column. At the base of the column lies a flattened pedal disc that attaches the anemone to a substrate.

The top of the anemone's column is flattened into an oral disc, ringed by twelve or more tentacles surrounding a slitlike mouth at the center of the disc. Each tentacle contains numerous nematocysts and tentacle size and shape relates to how the anemone feeds.

Sea anemones feed on various invertebrates. Large species can capture fish. Other large intertidal species feed on crabs and bivalves driven down from higher intertidal levels by waves or predators.

Anthozoans are mostly sedentary as adults. Although anemones usually remain attached to a substrate by a pedal disc, they can relocate by releasing the grasp of their pedal disc and then rolling or drifting with the current to a new location.

Stony corals possess similar body structure to anemones, but produce skeletons made of calcium carbonate. Most stony corals are colonial. In a colony, each polyp lies within a theca (skeletal cup), which it can contract into for protection. The polyps are interconnected by an outfolding of the body wall containing an extension of the stomach cavity. This sheet of tissue completely covers the colony's skeleton. Food obtained by one polyp can be passed to other polyps in the colony.

Anthozoans may be hermaphroditic (producing both male and female gametes—reproductive cells) or dioecious (individuals produce either male or female gametes). Hermaphrodites produce sperm first, then eggs.

Asexual reproduction is common in anthozoans—many reproduce by dividing into two equal parts. Another method is by pedal laceration, in which parts of the pedal disc are left behind as the animal moves. The pedal disc produces lobes that pinch off and then regenerate into small sea anemone clones.

When intertidal sea anemones are exposed to air at low tide, many species hold   rocks, shells and other debris on the outer surface of their closed in tentacles to maintain moisture.

The tissue of some shallow-water corals and anemones contain symbiotic zooxanthellae—a type of dinoflagellate (unicellular, golden-brown algae). These dinoflagellates provide their anthozoan host with some nutrients produced through photosynthesis while the host supplies the symbiont with carbon dioxide. Nitrogen and phosphorus are also transferred between the host and the zooxanthellae.

Some anthozoans commonly found in the intertidal zone include:

  • aggregating anemone (Anthopleura elegantissima)
    • This intertidal anemone ranges along the western coast of North America from Alaska to Baja California, Mexico. Individual aggregating anemones are approximately 2 to 5 cm (0.8–2 in.) in diameter. This anemone is usually light green with pink, purple, or blue tipped tentacles. The greenish color of this anemone is partly due to pigments and partially from zooxanthellae or zoochlorellae (unicellular symbiotic green algae) dwelling within the anemone's tissues.
    • Aggregating anemones occur in colonies of genetically identical clones. Each colony of clones maintains a territory and anemones on the borders actively defend the territory from other clones, creating anemone free zones between the colonies.
    • To prevent desiccation (drying up) when the tide is out, aggregating anemones pull their tentacles in and attach tiny rocks and shells to the outside of their bodies.
  • sunburst/starburst anemone (Anthopleura sola)
    • This anemone species shares the coloration of the aggregating anemone and was previously classified as the same species, only recently classified as a sister species. It ranges from central California to central Baja California, Mexico. Unlike the aggregating anemone, the starburst anemone is solitary and can grow larger—reaching up to 25 cm (10 in.) in diameter.
  • giant green anemone (Anthopleura xanthogrammica)
    • Giant green anemones inhabit the intertidal and subtidal zones along the Pacific coast from Alaska to Panama. They can reach a column length of 30 cm (11.8 in.) and the oral disc can be 25 cm (10 in.) in diameter.
    • Giant green anemones exposed to sunlight are often brilliantly green due to pigments and symbiotic zooxanthellae or zoochlorellae living in the anemone's tissues. Those that dwell in caves or heavily shaded areas tend to be pale in color.
    • Giant green anemones eat small fishes, mussels, crabs, and sea urchins. The intertidal hermit crab, Pagurus samuelis, often walks along the column and even over the anemone's tentacles and oral disc without being stung. Like anemonefish, an individual hermit crab might gradually become coated with mucus from the anemone and the anemone acclimates to the crab. The giant green anemone occasionally eats unfamiliar hermit crabs.
  • frilled/plumose anemone (Metridium senile)
    • This wide-ranging anemone is found along the Pacific coast from Alaska to southern California and along both coasts of the North Atlantic. Its column and oral disc can be uniform or varied in colors of white, orange, yellow, tan, or brown.  In the intertidal zone, its crown of tentacles is typically 2 to 5 cm (0.8–2.0 in.) in diameter. Yet subtidal anemones can have a crown diameter of 25 cm (9.8 in.) and reach 50 cm (19.7 in.) in height.
    • Smaller Metridium often produce dense colonies of clones. Individual clones on the borders of the colonies bear long, stretchy fighting tentacles filled with nematocysts that the anemone can use to attack nonclones.
  • orange cup coral (Balanophyllia elegans)
    • This bright orange, solitary coral is common in the lower intertidal and subtidal zones of the Oregon and California coasts. Individual polyps live in a cup-shaped, stony skeleton and grow to diameters of about 1 cm (0.39 in.).
    • Orange cup coral has sticky feeding tentacles to capture tiny prey.


Phylum Platyhelminthes

This phylum contains more than 20,000 species of flatworms, flukes, and tapeworms.

  • Members of this phylum have bilateral (two-part, mirror-image) symmetry, lack a body cavity, are triploblastic (have three main cell layers), unsegmented, and are dorsoventrally (top to bottom) flattened.
  • Aquatic flatworms (Class Turbellaria) are the largest of the free-living flatworms. One species even grows to 0.6 m (2 ft.) long.
  • Many aquatic flatworms have ocelli (eyespots) made up of clusters of light sensitive cells.
  • Flatworms are hermaphrodites and produce both male and female gametes.
  • Gas exchange takes place through the thin, body walls.
  • Many marine flatworms inhabit tide pools.
  • Notoplana spp.
    • Marine flatworms of this genus are common in intertidal habitats throughout the world. They are tan to dark brown with a pair of eyespots on the anterior end of the body. They often crawl beneath rocks and boulders or across algae searching for tiny bits of detritus or invertebrates to eat.


Phylum Mollusca

Mollusc is a Latin term meaning "soft-bodied." This group includes at least 93,195 described species of familiar animals like clams, oysters, mussels, limpets, snails, and squid. Molluscs are an important food source for many marine animals.

Animals in this phylum share many common characteristics including the following:

  • The mantle (underlying skin) secretes the shell.
  • A siphon (fleshy tube) draws in food and water.
  • Gills are organs used for respiration (to remove oxygen from water).
  • Most molluscs have a shell (hard, protective covering) or reduced shell.
  • Molluscs have bilateral symmetry


Class Polyplacophora

The Class Polyplacophora contains the chitons. There are about 1,000 polyplacophora species worldwide with about 120 living along North America's Pacific coast.

Chitons are oval-shaped with a convex dorsal surface. Their shells consist of eight overlapping, calcareous plates. The plates are anchored in a thick, muscular girdle. A chiton's entire flattened, ventral surface is a single, large muscular foot. They are generally found clinging to a rocky substrate. When disturbed or pried loose a chiton curls up into a ball, like a pill bug.

Most chitons use their radulas to scrape algae off rocks. The diets of individuals within a species vary with different habitats. Some chitons' diets consist of animal matter, and one species, the veiled chiton, is a carnivore that captures tiny, active animals like crustaceans 2 mm (0.08 in.) or less in size.

The chiton's powerful, muscular foot is used for clinging and creeping.

Chitons typically have external fertilization with gametes released into the water.

Some common intertidal chitons include the following species:

  • gumboot chiton (Cryptochiton stelleri)
    • This is the world's largest chiton—reaching a length of 33 cm (13 in.). It inhabits intertidal and subtidal zones along the coasts of northern Japan to the Aleutian Islands and to the Channel Islands off southern California.
    • Its eight plates are completely covered by a thick, leathery, brick-red mantle.
    • The reddish brown color of the gumboot chiton may be enhanced by the many species of red algae that grow on its mantle and by its diet, which consists mainly of red algae.
    • When exposed to air in the intertidal zone, this chiton can absorb atmospheric oxygen although at a much slower rate than it respires when submerged.
  • mossy chiton (Mopalia spp.)
    • The mossy chiton is common in the middle to low intertidal zones along the Pacific coast from British Columbia to central Baja California, Mexico. It is one of the few chitons also found in estuaries. Its girdle is covered in stiff brownish hairs and its grayish plates are often coated with algae and other encrusting organisms.
    • During daylight or when exposed at a low tide, these chitons remain firmly attached to a rock and nestled in an indentation with algae or a reservoir of water to prevent desiccation. When submerged and at night, they move within their 50 cm (20 in.) home range to graze on algae.
  • Nuttall's chiton (Nuttallina californica)
    • Nuttall's chiton commonly clings to rocks in the mid to high intertidal zones and ranges from Puget Sound, Washington to San Diego, California. Its girdle can be solid brown or have whitish stripes or blotches and has short bristles. The girdle and valves are often encrusted with algae.
    • Nuttallina prefer to graze on erect forms of coralline algae, but will also eat other kinds of red and green algae. This is one chiton species that carves out oval-shaped depressions in soft rocks to burrow into to help prevent desiccation.


Class Gastropoda

The gastropods comprise more than 70,000 species of living molluscs that include the limpets, snails, abalone, and sea slugs (nudibranchs and sea hares). Many features common in this class are not found in other classes.

The following characteristics are widespread among gastropods:

  • Most species have a mouth and a radula.
  • Eyes are on tentacles on each side of the head.
  • Many gastropods have a shell or a reduced shell. The shell opening can be tightly closed with an operculum; a "trap-door," that seals the opening of the shell when the animal pulls in its foot.
  • The radula is most diverse in the gastropods. Members of this class use the radula for scraping the substrate, grasping and biting, tearing flesh, rasping, boring through shells, and even harpooning prey. A gastropod uses its radula similar to the way you use your tongue to lick an ice cream cone. When a gastropod feeds, it places its radula against the substrate and the radula carries the scraped off particles into its mouth.
  • A gastropod crawls by gliding along its foot. The foot is a broad, flat muscle on the gastropod's ventral surface. The foot also adheres to rocks and other surfaces. As a gastropod moves, waves of fine muscular contractions sweep along the foot. The contractions lift the animal's foot, then returns it to the surface a little farther ahead, pulling the animal forward.
  • Gastropods produce eggs and sperm. The fertilization method and larval development varies for each group of gastropods.


Some common gastropods seen in the intertidal zone include:

  • rough limpet (Collisella scabra)
    • Unlike most other marine snails, limpets have flattened, caplike shells. The rough limpet's shell is waved with ribs extending from the off-centered apex to the edge. This species is common to the high intertidal and splash zones of the Pacific coast from Oregon to southern Baja California, Mexico. At high tide, the rough limpet scrapes diatoms (microscopic plants) and algae off rocks. At mid to low tides, the rough limpet returns to a specific home site that it has carved out to exactly fits into the edge of its shell providing a tight seal to prevent desiccation.
  • giant keyhole limpet (Megathura crenulata)
    • The giant keyhole limpet is the largest of the many keyhole limpet species—each characterized by the hole at the highest point of their cap-shaped shell. The giant keyhole limpet's pink, whitish, or black shell reaches lengths of 13 cm (5 in.), while its yellowish to black body extends beneath the shell to 25 cm (9.8 in.) long. Giant keyhole limpets range in the subtidal and intertidal areas along the Pacific coast from Monterey, California to Baja California, Mexico.
  • owl limpet (Lottia gigantea)
    • This large limpet reaches lengths of 9 cm (3.5 in.). Owl limpets cling tightly to cliff faces and rocks in the high to low tide zones of the Pacific coast from Washington to Baja California, Mexico. An owl limpet's shell is brown and white, although it is often eroded due to frequent exposure to crashing waves. The inside of the shell contains an owl-shaped marking.
    • Owl limpets exhibit strong territorial behavior—occupying a home range of about 1,000 cm2 (155 in2), which they keep clear of smaller limpets, mussels, and anemones by scraping them off or even pushing them away. By this behavior, an individual owl limpet is able to maintain a 1 mm (0.04 in.) thick algae film within its territory to graze upon.
  • abalone (Haliotis spp.)
    • The abalone has a caplike, oval-shaped shell characterized by a row of three to ten holes near the anterior margin that water exits from after passing over the mantle cavity and gills. The outer layer of the shell is often muted in color and often mottled with tiny encrusting marine invertebrates. The shell's nacreous (inner) layer is iridescent in a rainbow of colors. An abalone roams over rocks to graze on marine algae and can cling tightly to rocks with its muscular foot if exposed to crashing waves or a predator.
    • Abalones range along all continental coastlines except for the eastern coast of North, Central, and South America. The red, pink, black, and green abalones are species common to the temperate Pacific coast of North America. Abalone meat is a prized delicacy in many regions and the shell's iridescent inner layer is valued for use in intricate inlay for jewelry, instruments, and a wide variety of other objects.
    • Due to declines in abundance, several abalone species are protected. The northern or pinto abalone (H. kamtschatkana) is listed as "endangered" by the IUCN/World Conservation Union and the white abalone (H. sorenseni) is listed as "endangered" under the United States Endangered Species Act. The black abalone (H. cracherodii) population has plummeted since the mid 1980s because of a withering syndrome in which the abalone's foot shrinks and can no longer cling to rocks. Because of this decline, the IUCN lists the black abalone as "critically endangered.
    • To prevent unsustainable declines many countries and states impose regulations on commercial and recreational abalone collection for food. In California, abalone sport fishing is only allowed north of San Francisco and is strongly regulated and enforced by the California Department of Fish and Game. The Australian government regulates the Tasmanian abalone fishery, which makes up about 25% of the world's abalone harvest. In some regions of the United States and other countries, abalone is farmed, which helps meet the commercial demand for marine delicacy.
  • periwinkle snail (Littorina spp.)
    • These tiny snails tightly cluster and cling to rock crevices in the splash zone. The eroded periwinkle, L. planaxis, ranges from Oregon to Baja California, Mexico and inhabits a higher vertical position on the shore than any other California mollusc. It mostly lives out of the water and secretes mucus which hardens and cements it to a rock to prevent desiccation when water is not present. Periwinkle snails eat by scraping diatoms and microscopic algae off rocks.
  • black turban snail (Tegula funebralis)
    • This black-shelled snail commonly inhabits tide pools and rocks in the middle intertidal zones from Vancouver Island, British Columbia to central Baja California, Mexico. Several other Tegula species inhabit the intertidal zones within the same range, and other Tegula species inhabit other coastal regions.
    • When the tide is out, black turban snails often cluster in groups of hundreds of snails in rocky crevices or shaded areas. At high tide, the snails graze on a variety of algae species. Black turban snails are preyed upon by a number of animals including sea otters, rock crabs, and ochre stars. Occasionally, humans collect black turban snails for food. Hermit crabs often move into any empty turban snail shells.
  • wavy turban snail/ wavy top snail (Megastraea undosa)
    • The wavy top snail has a large, heavy shell with undulating ridges spiraling along the outside of its reddish shell. The wavy top's shell can grow to 11 cm (4.3 in.) in diameter making this one of the larger snails to inhabit the lower intertidal and subtidal zones.  Wavy top snails range from Point Conception, California to Baja California, Mexico. A similar species, the red turban snail, is smaller and ranges along the Pacific coast from British Columbia to Baja California, Mexico.
  • tube snail (Serpulorbis spp.)
    • This unusual snail lives in a calcareous tube and often intertwines with other tube snails to form a mazelike colony encrusting the sides of rocks. The scaled worm snail, S. squamigerus, ranges from Monterey to Baja California, Mexico. However, other Serpulorbis species inhabit other coastal regions. Tube snails are sessile and individuals filter feed by extending a mucus net from their feet to catch plankton and other drifting particles.
  • sea hare (Aplysia spp.)
    • Sea hares are large sea slugs with two anterior projections and two upward tentacles that resemble rabbit ears. Aplysia species are found in low intertidal and subtidal waters of most temperate and tropical coasts. The California black sea hare (A. vaccaria) is probably the world's largest gastropod reaching lengths of 76 cm (30 in.).  Sea hares eat algae and are often tan, brown, black, or red in color—most are well camouflaged in their environment. When threatened by a predator, a sea hare can release a dark purple ink cloud that acts as a smoke screen to allow the sea hare to escape.

Class Bivalvia

A smaller and less diverse group than the gastropods, the class Bivalvia includes about 20,000 species including mussels, clams, oysters, and scallops.

In bivalves, the mantle fold encloses the visceral mass, head, and foot.

The shell consists of left and right calcified valves, an uncalcified hinge, and elastic ligaments joining the two valves dorsally.

Bivalves are filter feeders that siphon and filter water.  Bivalves feed by passing water through a pair of gills. Gills not only extract oxygen from the water but also filter out tiny food particles. Bivalves lack a radula and most other structures of the molluscan head.

Most bivalves are relatively sedentary and remain attached to wood pilings or rocks. Some bivalves can use their muscular foot to burrow into mud or sand, others can glide along the substrate in much the same manner as snails. Some bivalves, such as scallops, can swim by repeatedly and rapidly clapping their valves.

Most bivalves have separate sexes, but in many oysters and scallops, individuals can produce both eggs and sperm. Fertilization is external; eggs and sperm are released into the water.

Bivalves are used as food or bait by commercial and sport fishermen, and millions of pounds are harvested yearly.

During the warmer months, some filter-feeding animals incidentally filter in biotoxins produced by certain kinds of phytoplankton, making them poisonous to humans and to other marine animals. In regions where this occurs, seasonal quarantines prevent humans from consuming mussels and other filter-feeding shellfish that may contain these toxins.

Mussels are one type of bivalve that commonly occupy mid to low intertidal zones.

  • California mussel (Mytilus californianus)
    • This mussel commonly attaches in clumps to pier pilings and forms large mussel beds on rocks exposed to heavy surf in the middle to low intertidal zones. It is also found offshore to 24 m (79 ft.) depths. The California mussel's range extends from the Aleutian Islands, Alaska to southern Baja California, Mexico.
  • blue mussel (Mytilus edulis)
    • This widespread mussel is found in clumps and beds of the low intertidal on rocks and even mudflats of sheltered bays. It ranges along North America's Pacific coast, the North Atlantic coasts, and also the shorelines of Japan, Australia, and western South America. Blue mussels are harvested and cultivated for food in parts of Europe, Australia, Canada, and New England.


Class Cephalopoda

This group of molluscs includes more than 1,000 species of octopuses, nautilus, cuttlefish, and squids.

In cephalopods, two large eyes and other sense organs are located on the head. Portions of the foot are modified to form eight or more sucker-bearing tentacles that radiate from the head.

Camouflage is well-developed, and many cephalopods blend in by instantly changing color to the hue, intensity, and even texture of the background. They use color patterns that contrast with the background for social signaling and defense.

When alarmed, an octopus may flatten its body and present an elaborate defensive color display, including color changes sweeping over its body, and large dark spots around its eyes. A large ink sac releases ink to confuse predators.

Most cephalopods are active predators, well equipped for sighting prey and catching it with their tentacles. Salivary glands in their beaklike jaws inject toxin to subdue prey. They can then use their jaws to tear apart their catch. Cephalopods also have a radula, which most use for delicate cleaning. The octopus' radula can even drill into mollusc shells.

All cephalopods swim by jet propulsion. They force water from the mantle cavity out through the siphon. The siphon is flexible, and a cephalopod can propel itself in any direction through the water.

Octopuses swim by squirting water jets, but more frequently use their tentacles to crawl around the rocks and ocean bottom.

Octopuses have internal fertilization. Males have a modified arm with a spade-shaped tip. The male transports sperm to the female oviduct with this arm tip. After mating, female octopuses deposit their eggs in clusters that resemble a bunch of grapes. They remain to care for the eggs, continuously removing debris and sediment from the eggs by washing water over them.

While most cephalopods are subtidal, many smaller octopuses inhabit tide pools.

  • two-spotted octopus (Octopus bimaculoides)
    • The two-spotted octopus is named for the bright blue ring encircling a dark spot located beneath each eye. Young two-spotted octopuses live under rocks, crevices and holes of the middle to low intertidal zones on rocky shores or mudflats. Mature individuals live in shallow subtidal waters to 20 m (66 ft.) depths. This species ranges from central California to Baja California, Mexico.
    • Two-spotted octopuses eat a variety of crustaceans, molluscs, and fishes.
    • Like other octopuses, the two-spotted octopus can instantly change the color, pattern, and texture of its skin so that it is well camouflaged with its environment to prevent detection from predators such as scorpionfish and moray eels.
  • blue-ringed octopus (Hapalochlaena spp.)
    • While not found in North American coastal waters, the ten species of blue-ringed octopus inhabit tide pools and shallow reefs of Australia and Asia. When alarmed, these small octopuses display numerous brilliant blue rings or spots all over their bodies. This display advertises octopus's deadly venom which contains tetrodotoxin—a powerful neurotoxin.  Blue-ringed octopuses are usually shy, but if one is encountered, you must take care not to disturb it since a single bite can cause paralysis or even death to humans.


Phylum Echinodermata

Echinoderm is a Greek word meaning "spiny-skinned." This phylum contains many different classes and about 7,000 species. Animals in this phylum all share a few common characteristics:

  • Endoskeleton (internal skeleton)—The echinoderm endoskeleton is made up of ossicles, a type of calcareous structure. The compactness of the ossicles determines the degree of flexibility of the animal. In sea cucumbers the ossicles are not very compact, making the cucumber very flexible. The ossicles of the sea urchin are much more compact and make up the hard endoskeleton or test. The flexibility of the sea star is intermediate to the sea cucumber and the sea urchin.
  • Tube feet—An echinoderm's tube feet are used for locomotion and respiration. The tube feet operate by using a hydraulic, or water-vascular, system.
  • Radial symmetry(adults)—The adult body symmetry radiates around a central axis. The pattern of symmetry is pentamerous—it occurs in fives or multiples of five. In their larval stages, echinoderms have bilateral symmetry.

Class Ophiuroidea

The approximately 2,000 species of brittle stars and basket stars make up this group of echinoderms.

Like asteroid echinoderms, ophiuroids, typically have five arms; although brittle have longer, more flexible arms and move by wiggling their arms.

Most brittle stars scavenge for food.

The delicate arms of brittle stars are easily broken, but also quickly regenerate.

Many species of brittle stars can be found under rocks of the middle to low tide zones.

  • spiny brittle star (Ophiothrix spiculata)
    • The spiny brittle star shelters in crevices or under rocks in the intertidal zone. Its range extends along the Pacific coast from central California to Peru and offshore to the Galápagos Islands.
    • The spiny brittle star is named for the distinct thornlike spines on its disc and arms. To feed, the brittle star uses the spines of at least one arm to anchor into a crevice and then extends its other arms into the water to entrap food particles on a sticky substance emitted by the spines and tube feet. The tube feet along each arm then coordinate to transfer food particles to the brittle star's mouth.

Class Asteroidea

Scientists have identified at least 1,500 species of sea stars in the class Asteroidea. Most sea stars have five rays ranging in length from 10 to 25 cm (4–10 in.). Some species may be much larger and have more than five rays.

Sea stars regenerate lost rays. Regeneration is typically slow and may take as long as one year. Some stars may have six or seven rays because two rays regenerate instead of one.

Some sea stars, such as the giant-spined sea star, have pedicellariae—tiny pincherlike structures covering their aboral (top) surfaces. Pedicellariae help grind algae and other tiny pieces of debris that collect along the skin of the sea star.

Oxygen exchange takes place at the tube feet and at papulae, tiny bumps scattered over the aboral surface. Papulae give the animal a fuzzy appearance. Small hairlike structures create a current of water over the papulae so respiration can occur.

At the tip of each arm is one tube foot that cannot be retracted. This is a tactile organ. Just above this tactile organ is a small white eyespot that detects changes in light intensity. In searching for food the sea star relies on chemoreception, a combined sense of taste and smell, more than on touch or sight. If meat juices are poured into a tank containing sea stars, the sea stars become active and behave as they would in the presence of food.

The mouth is located on the oral surface, or underside. A sea star feeds by a process called evagination: it pushes its stomach through its mouth and outside the body wall. Stomach enzymes then digest the food externally. When digestion is complete, the stomach muscles contract, retracting the stomach back into the body cavity.

All sea stars use their tube feet for locomotion. The tube feet are in grooves on the undersides of each arm. The tube feet operate on a water vascular system. The tube foot is a hollow muscular structure with a balloon-like reservoir called an ampulla on top of the tube. The ampulla is covered with a meshwork of muscle fibers. When these muscle fibers contract, the ampulla is deflated and fluid is forced into the tube foot. This stretches the tube foot outward beyond the groove. Contraction of other muscles creates suction and forces the fluid back into the ampulla, allowing the tube foot to cling to a solid substrate.

With a few exceptions, most sea stars are either male or female. They release eggs and sperm into the water where fertilization takes place. The presence of eggs or sperm in the water stimulates sea stars of the opposite sex to release their sex cells. Sea stars usually spawn once per year. Sea stars that live in temperate waters spawn in the spring. During spawning a female may release up to 2.5 million eggs.

Sea stars commonly occupy the middle to low intertidal zones.

  • ochre star (Pisaster ochraceus)
    • Ochre stars cling to middle intertidal and subtidal rocks in coasts exposed to heavy surf from Alaska to northern Baja California, Mexico. Ochre stars are yellow, orange, tan, brown or purple with an aboral surface patterned with numerous small, white spines.
    • Ochre stars commonly hunt for mussels but will eat barnacles and gastropod molluscs if mussels are absent. Occasionally, sea otters or gulls prey on ochre stars.
  • northern sea star/sea star (Asterias vulgaris)
    • The northern sea star dwells in the temperate waters of the northeastern Atlantic occupying rocky regions of the intertidal and offshore to depths of 600 m (2,000 ft.).  A similar species A. rubens is found along the Atlantic coast of northern Europe.
    • The northern sea star is usually orange, tan, pink, red, or purple with white spines abundantly scattered all over its aboral surface. It preys mainly on bivalves, especially mussels and oysters, and also on other molluscs and barnacles.
  • bat star (Asterina miniata)
    • Bat stars inhabit the low intertidal and subtidal zones to depths of 290 m (951 ft.) along the Pacific coast of North America from Alaska to Baja California, Mexico. The bat star's aboral surface is brightly colored in a single color or mottled in shades of cream, red, orange, brown, or purple. Instead of having long arms like most sea stars, bat stars have webbing between their short, triangular arms. They scavenge on a variety of marine plants and animals—especially surfgrass, algae, and colonial tunicates.
  • sunflower star (Pycnopodia helianthoides)
    • This unusual sea star, native to the Pacific coast of the United States, has up to 24 rays. Its aboral surface is typically pink, purple, brown, orange, or yellow. The sunflower star is the largest Pacific coast sea star—reaching more than one meter (3.3 ft.) in diameter.
    • The sunflower star uses its more than 15,000 tube feet to quickly pursue its preferred sea urchin or bivalve prey or to capture other marine invertebrates such as polychaete worms, snails, limpets, crabs, sea cucumbers, dead or dying market squid (Loligo opalescens), and even smaller sea stars.

Class Echinoidea

The echinoids include about 950 species of sea urchins and sand dollars.

Sea urchins are rounded animals with pointed spines covering their entire body except the oral surface.

The skeleton of the sea urchin is composed of closely joined calcareous plates forming a rigid case around the vital organs. People often refer to this as a "shell," but the structure is more accurately called a test. Tube feet emerge from pores in the test.

Sea urchins use their spines for protection, defense, movement, and burrowing. The spines rest on ball and socket joints along the test. Sea urchins can rotate their spines and burrow into rocks or other hard substrates. They wear the surface down with their spines and pull away the bits of rock with their tube feet. Since the spines grow longer with age, an urchin can become imprisoned if it grows too large for the hole it burrows.

There are several highly developed types of pedicellariae among the spines and around the mouth. The pedicellariae keep algae and debris from collecting on the urchin.

Most sea urchins have external gills on the oral surface. They also respire through their tube feet.

Most sea urchins have a remarkable system of hard jaws and teeth for grinding food. The Greek philosopher and naturalist, Aristotle, described the structure now known as Aristotle's lantern. Aristotle's lantern is made of five long, chisel-like teeth that meet at the mouth opening. Sea urchins feed mainly on kelp but also eat other types of algae. When eating kelp, the urchin usually eats the stipe (stem). Once the sea urchin has eaten through the stipe, the rest of the plant floats away.

Sea urchins move in much the same way as sea stars, using a water vascular/tube foot system.

Sea urchins are either male or female. An urchin releases large quantities of eggs or sperm into the water for fertilization.

In many parts of the world the gonads are considered a delicacy. Male gonads are whitish and the female gonads are a deep yellow or orange.

Many kinds of sea urchins inhabit the intertidal zone including the following species:

  • purple sea urchin (Strongylocentrotus purpuratus)
    • These sea urchins commonly inhabit rocks in the low intertidal and subtidal zones on the eastern Pacific coast from Vancouver, British Columbia to Baja California, Mexico. Adults are vivid purple while juveniles are often light green.
    • Purple sea urchins often use their spines and teeth to carve out rounded depressions in rocks of the low tide zone. They can even excavate rusty patches to burrow into steel pier pilings. Broken spines regenerate, while teeth are replaced in about 75 days. The urchins cling to the rocky substrate with their tube feet—only their spines project out of the burrows providing a defensive barrier against predators.
    • Purple urchins graze on marine algae and detritus. If excessively abundant, these urchins can overgraze and affect the growth of giant kelp.
  • green sea urchin (Strongylocentrotus droebachiensis)
    • This mostly light green sea urchin is found in the low tide and subtidal zones in temperate and arctic waters on both sides of the Atlantic coast and from Alaska to Puget Sound on the Pacific coast. This species is the most abundant species on the New England coast.


Class Holothuroidea

The approximately 1,150 sea cucumber species don't seem to conform to the generalized concept of echinoderms. They have cylindrical radial symmetry. (Imagine an elongated sea urchin without spines, with a softer body, and turned over on its side.) From their external appearance sea cucumbers seem to have no trace of a skeleton, but they do have tiny, nonarticulated, calcareous ossicles embedded in their skin. 

Sea cucumbers have various reactions to predators. Most sea cucumbers contract their body wall, pulling the ossicles closer together. This makes the sea cucumber smaller and more rigid, and perhaps less appetizing or harder for a predator to swallow. Many sea cucumbers secrete toxic substances that can be harmful to animals that ingest it.

Some species defend themselves by eviscerating (expelling their entrails), leaving the entrails to the predator while the sea cucumber slips away. The entrails are regenerated in a few days. Some species expel special Cuvierian organs that form masses of sticky tubules, which entangle predators.

Inside the sea cucumber are large respiratory trees where oxygen exchange occurs. Water is forced into the respiratory trees through the cloaca (a common opening of the digestive, excretory, and reproductive systems). This process may take six to ten minutes. All the water is expelled in one contraction.

The mouth of the sea cucumber is anterior and not directed downward like other echinoderms. A ring of mucus-covered tentacles circles the mouth. Food particles are trapped in the mucus and are then wiped off as the sea cucumber pulls its tentacles out of its mouth.

Sea cucumbers move using tube feet. The tube feet of some sea cucumbers are distributed over the entire body, some are concentrated on a "ventral" surface, and some are restricted to rows along each radius. As in sea stars, the tube feet operate using a water-vascular system. The body cavity is filled with fluid. The sea cucumber uses its tube feet for moving along the rocky substrate and for burrowing into sand and mud.

Sea cucumbers are either male or female, and both eggs and sperm are released into the water.

In some parts of the world, sea cucumbers are harvested by humans for food and traditional Chinese medicine.

Some sea cucumbers inhabit the rocky intertidal although many other species inhabit soft-bottomed habitats.

  • warty sea cucumber (Parastichopus parvimensis)
    • The warty sea cucumber is found in the intertidal and to depths of 27 m ( 89 ft.) on the Pacific coast of North America from Monterey, California to Baja California, Mexico. This species is brown or orange with numerous, conical black-tipped projections all over its body. The warty sea cucumber's tube feet lie on its ventral side. Tentacles around the sea cucumber's mouth produce sticky mucus and are used to dab up detritus and other particles from the sea bottom.


Phylum Arthropoda

Arthropod is a Greek word meaning "jointed appendage." With more than a million described species, this is the largest and most diverse invertebrate phylum. This large group includes crustaceans, insects, and spiders. Arthropods make up more than 85% of all animal species.

Animals in this phylum share these characteristics:

  • Segmented bodies
  • Jointed appendages - A rigid exoskeleton (outer covering) made ofchitin—As it grows, an arthropod must molt—shedding its old exoskeleton and producing a larger, new exoskeleton.
  • Bilateral symmetry
  • Triploblastic (contain three main cell layers)

Subphylum Crustacea

Crustaceans are the overwhelmingly dominant arthropods in the sea, with about 67,829 described species including amphipods, isopods, lobsters, crabs, shrimps, and barnacles. They are enormously important to the ocean's ecology. The most abundant crustaceans in the sea are zooplankton (animal plankton)—the small microscopic herbivores which graze on phytoplankton or other zooplankton. These small species provide the food base for a host of small fishes and invertebrate carnivores. Larger crustaceans are essential to the diets of many larger fishes and mammals, and they also provide food for humans. 

The exoskeleton of crustaceans contains calcium carbonate in addition to the chitin, giving these animals their crusty texture.

All crustaceans breathe with gills, but there is great diversity in the shape and location of the gills among the different orders, families, and species.

Most crustaceans are marine. Some species inhabit freshwater or are terrestrial.

Most crustaceans have three main body parts; the head, thorax, and abdomen. On many crustaceans, the head and thorax are often fused into a cephalothorax.

A crustacean's head bears a pair of compound eyes, two pairs of antennae, one pair of mandibles for biting, grinding, or compressing food; and two pairs of maxillae, variously modified for collecting and ingesting food.

The thorax and abdomen have a number of lateral appendages. Many crustaceans, such as lobsters and shrimp, have a tail-like section called the telson at the end of the abdomen.

Crustaceans swim with oarlike or propellerlike beating of swimmerets (swimming appendages on the abdomen). The swimmerets usually have fringed, hairlike structures that increase the water-resisting surface. Although most crustaceans can swim, on many species certain appendages are heavier and adapted for crawling and burrowing.

The crustacean's rigid exoskeleton poses a problem for growing animals. The solution to this problem is to periodically shed the skeleton, a process called molting. A crustacean's exoskeleton cracks, and the animal wriggles out. Its new exoskeleton is soft, and for a short time the animal is vulnerable. The new exoskeleton soon hardens. The faster a crustacean grows, the more often it molts.

Female crabs can only be fertilized when they are newly molted. The male usually molts before the mating season, and must locate a female prior to her molting so that he can be present to fertilize her eggs before her new exoskeleton hardens. The female probably releases a chemical attractant, known as a pheromone, at the appropriate time. Once together, the male grasps the female and holds her until she begins to molt. He releases her as she emerges from her old exoskeleton. After her new exoskeleton becomes partially firm, she lowers her abdomen and allows the male to fertilize her using his modified appendages. The male continues holding the female until her exoskeleton hardens. Females usually brood eggs, which may be attached to appendages, contained within a brood chamber, or retained within a sac formed when the eggs are expelled.

Most crustaceans are either male or female, but barnacles and some others are hermaphrodites. Internal fertilization is the general rule, and males usually have certain appendages modified for grasping the female.

The Subphylum Crustacea contains animals from several different classes and orders that occupy the intertidal zones.

Class Maxillopoda, Infraclass Cirripedia

This crustacean infraclass consists of the barnacles. Most barnacles are free-living although some species are parasitic and live on certain species of whales and other animals. A barnacle is a sessile, shrimplike animal that, following its larval stages; settles in a head-down position, permanently attaches to a rock or other hard substrate, and then grows a hard shell around its body for protection. Barnacles extend their featherlike feet into the water to catch food particles.

  • acorn barnacle (Balanus spp.)
    • Acorn barnacles are common and widespread residents of coastal and inshore temperate and tropical waters. The numerous acorn barnacle species are roughly symmetrical and shaped like miniature volcanoes. Many have ridges on the outer surface of the shell.
  • goose barnacle/leaf barnacle (Pollicipes spp.)
    • The leaf barnacle is a type of stalked barnacle in which the body is divided into two sections. The capitulum or top section contains the barnacles feeding appendages and most of its organs.The peduncle (stalk) attaches the barnacle to a substrate.  The capitulum and occasionally the peduncle are covered in calcareous plates.
    • Pollicipes polymerus inhabits rocky shores along the Pacific coast from British Columbia to Baja California, Mexico. Clumps of leaf barnacles often inhabit wave swept rocks in the middle intertidal zone and often cluster with California mussels. Another Pollicipes species is found along the west coast of Mexico to South America and yet another occupies the eastern Atlantic coastline south from France.

Class Malacostraca, Order Decapoda

This crustacean order contains the crabs, prawns, crawfish, shrimps, and lobsters. The decapod thorax region has three pairs of maxillipeds (feeding appendages) and ten leglike appendages. Often one pair of the leglike appendages, called chelipeds, has large chelas (claws)

  • cancer crabs/rock crabs (Cancer spp.)
    • About 23 species of this genus occupy temperate coasts throughout the world—with nine species inhabiting North America's western coast. The carapace (outer covering of the cephalothorax) of a Cancer crab is broad and oval in shape and bears numerous small, sawlike teeth along its front margin.
    • Although many Cancer crabs are recreationally and commercially caught by humans, the most commercially important species is the Dungeness crab, C. magister, which supports a large fishery extending from British Columbia to California.
  • striped shore crab/lined shore crab (Pachygrapsus crassipes)
    • These abundant crabs dominate the high intertidal zone of rocky coastlines. Their range extends along the eastern Pacific coast from Oregon to Baja California, Mexico and into the Gulf of California. The species also occupies the coasts of Japan and Korea where it was probably introduced in the late 1800s. The lined shore crab's carapace is dark purple to reddish and marked with whitish or green lateral stripes.
    • These crabs are well adapted to the high intertidal and can remain out of the water for up to 70 hours in shaded areas such as rocky crevices or overhangs. Lined shore crabs do require access to water to moisten their gills and feed. They mainly use their claws to scrape algae off the rocks, although they also opportunistically feed on dead animal matter and even hunt live snails, limpets, hermit crabs, isopods, and flies. Lined shore crabs will actively defend a food item from other shore crabs.
  • hermit crab (Pagurus spp.)
    • This widespread hermit crab genus is a common occupant of intertidal zones throughout the world. Hermit crabs are named for their unusual behavior of inhabiting empty gastropod shells to protect their soft abdomens. They have a soft anterior part of the carapace and an abdomen that is adapted and curved to fit within a shell. The shell is held on through contraction of the abdominal muscles and gripped with modified uropods (the sixth pair of abdominal appendages) and the last two pairs of thoracic legs. The chelipeds block the shell opening when the hermit crab withdraws into the shell for extra protection or can be used to pick up food particles. As the hermit crab grows, it must find a larger shell to move into. Although, hermit crabs often steal shells from each other, they do not take shells occupied by live snails.
  • snapping shrimp/pistol shrimp (Alpheus spp.)
    • Snapping shrimp are found in coastal waters worldwide. A. clamator often inhabits holes and crevices of tide pools in the low intertidal zone on rocky shores from San Francisco to Baja California, Mexico. A snapping shrimp possesses an asymmetrical pair of front claws; one of which is greatly enlarged and specialized to produce loud popping sounds that are often audible underwater or at low tide. This snapping sound makes these one of the noisiest animals in the ocean. When the shrimp's claw snaps closed, this produces a pistol-like, superheated, bubble blast that can stun prey, such as small fishes and shrimps, or deter predators.
  • spiny lobster/rock lobster (Palinurus spp.)
    • The California spiny lobster (P. interruptus) ranges from central California to Baja California, Mexico and smaller individuals can occasionally be found at night in surfgrass beds or in rocky crevices of the low intertidal zone. Many other Panulirus species inhabit tropical and subtropical waters and, like the California spiny lobster, are commercially fished. The California Department of Fish and Game sets the season, minimum size, fishing methods, and daily bag and possession limits for the California spiny lobster recreational fishery. Unlike true lobsters, spiny lobsters lack enlarged front claws. Instead, spiny lobsters have numerous sharp spines on their tail, carapace, and antennae that can cause painful cuts and scrapes if handled by humans or other potential predators.

Class Malacostraca, Order Isopoda

Pill bugs, sow bugs, rock lice, and other freshwater and marine counterparts make up this crustacean order. The body of an isopod is elongated and dorsoventrally (top to bottom) flattened. The head is fused with the first thoracic segment and contains a pair of compound eyes, two pairs of antennae (one pair is larger) and four pairs of mouthparts. The thorax is then divided into seven equal segments each with a pair of unbranched walking legs. This is followed by six abdominal segments with the last fused to a terminal plate (telson). In most isopods, a pair of branched legs used for respiration and, for some species, swimming, extends from each of the first five abdominal segments.

  • rock louse (Ligia spp.)
    • This isopod crustacean is often spotted quickly scurrying over and beneath the rocks, pier pilings, and algae of the high intertidal and splash zones.  Ligia occidentalis inhabits the Pacific Coast from Sonoma County, California to Central America. Other Ligia species occupy intertidal areas of the North Pacific coast and other regions including Hawaii. The rock louse scavenges for dead plant and animal matter and also scrapes algal film off intertidal rocks. Although essentially terrestrial—the rock louse cannot survive if continuously submerged—it relies upon the intertidal zones for access to moisture. The rock louse dips its uropods and telson into tide pools to keep its gills moist and replenish body water. The water is drawn up into the gills and body through capillary action.


Phylum Annelida

This phylum contains about 16,500 species of segmented worms including the familiar earthworm found in many gardens.

  • Annelids are bilaterally symmetrical.
  • All annelids have a coelom (a fluid-filled body cavity) in which the organs are suspended. The body cavity is divided into separate compartments called septa that give the worms their segmented appearance.
  • Many annelids are hermaphrodites, although polychaetes are usually separate sexes. Some annelids reproduce via asexual budding.

Class Polychaeta

The name polychaete means many bristles and refers to the extensions on the parapodia (paddlelike appendages) that the 8,000 species of this class possess. The bristles function in swimming, walking, or digging.

Some polychaetes are sedentary and secrete a mucus tube to protect their bodies. They extend featherlike or long, sticky tentacles to collect food.

  • sandcastle worm (Phragmatopoma californica)
    • Sand castle worms construct their colonies of honeycombed, sand-encrusted tubes at the base of large rocks in the middle and low intertidal zones along the Pacific coast from central California to northern Baja California, Mexico. When submerged, these tube worms extend their lavender-colored tentacles into the water to catch plankton and detritus for food and collect sand for tube construction.
  • Ophiodromus pugettensis
    • If you turn over a bat star you may see this commensal (one species benefits and the other is unaffected) polychaete, which typically lives in the tube feet grooves of sea stars, especially bat stars. It is also free-living on pier pilings and muddy bottoms and inhabits the west coast North America from British Columbia to the Gulf of California, and the coasts of Peru and Japan.


Phylum Chordata, Subphylum Urochordata

The approximately 3,000 species of tunicates or sea squirts form this subphylum of animals. The urochordates are intermediate between the invertebrates and vertebrates.

  • Many species have a free-swimming tadpole-like larval stage in which the larvae possess the chordate characteristics of a dorsal nerve cord, gill slits, and a post-anal tail.
  • As most urochordate larvae mature, they attach to a solid substrate and their tails and nervous systems degenerate.
  • Adult tunicates are simple animals possessing doubled saclike bodies with an incurrent and an excurrent siphon through which water flows. Adults develop a tunic or thick outer covering made of cellulose.
  • Adults filter feed by drawing water in through the incurrent siphon and then strain particles through a gill basket. Water then flows out the excurrent siphon.
  • Some tunicate species are solitary although many form colonies composed of genetically identical individuals that share a common tunic and often encrust rocks in the low intertidal zone.