Rocky Intertidal Species of Fort Ross

The Rocky Intertidal Zone on the California Coast is one of the most magical, diverse habitats for intertidal marine life in the world. From the border of Oregon all the way down through Baja you can find hundreds of species and subspecies in the Rocky and Sandy Intertidal zones. There are hundreds, maybe even thousands of locations along the coastline of California to go tide pooling, many with a common species theme, some with flora and fauna endemic to that location. Whether you are a kid going out on a class field trip or just a kid at heart, still thrilled by discovering something new, tide pooling can be for everyone.The beautiful thing about tide pooling is it requires very little. Other than knowing when to go – this makes a big difference, carry a tide book in your car – all you need now is a place to go.

Fort Ross State Historic Park is known for its cultural history, but it also has a lush and diverse natural history. The park contains about 3,400 acres of wildlands from coastal hills covered in Redwood and California Oak to grass lands, all the way down to the marine terrace. On any given low tide adventure, here are some of the jewels you can find:

Anemones, Mussels, Gooseneck Barnacles, Sea Urchins, Sea Stars, Hermit Crabs, Shore Crabs, Chitons, Bat Stars, Limpets, Black Turbin Snails, Top Snails, Seaweeds, and hundreds more!

Sea Anemones

Sea Anemones are round, solitary polyps with a tube shaped body that lack a skeleton (invertebrate). All species are predators with a mouth surrounded by feeding tentacles that have stinging cells capable of capturing small crustaceans, fish, plankton and algae, other marine fauna, such snails, crabs, and anything that happens to touch them that cannot get away.

Each tentacle is armed with minute, stinging structures called nematocysts. When you touch the tentacles of an anemone, they feel sticky to humans, because our skin is too tough for the nematocyst threads to penetrate. For the small critters that are the anemone’s prey, the sting is paralyzing or fatal.

Sticky bumps on the outsides of their bodies collect sand and bits of shells, which provide camouflage, reflect light to keep the anemone cooler, reduce water loss at low tide, and prevent them from drying out (desiccation).

While we think of anemones as being sessile (does not move during it’s life stages), they can in fact creep slowly on their pedal disk to a nearby spot or detach completely, float away to reattach at a new site. Sea anemones typically host unicellular photosynthetic, plant like symbionts (an organism that lives in close association with another) zooxanthellae and/or unicellular algae zoochlorellae. The symbiotic algal type present within a given anemone depends on a number of factors, including light condition, tidal height, and temperature.

The algae live within the cells of the anemone host, in the tentacles, oral disk and column. The relationship is not crucial to the survival of the anemone. Zooxanthellae give the anemone a brown color. The bright green seen in many anemones could be from the photosynthetic, chlorophyll containing zoochlorellae. Anemones can exist as male, female or they can be hermaphroditic.

When an anemone in an intertidal zone is exposed, it contracts, shortening the sensitive tentacles, and pulls the distal margin of the column over them. They also hold a large reservoir of seawater inside their body to keep from quickly drying out. They choose their location carefully; you can often find them beneath an overhanging rock, somewhere they are semi protected from the crashing waves, desiccation in the sun, and predators.

The sea star Dermasterias imbricata is the main predator of Anthopleura, others include the nudibranch Aeolidia papillosa, and the wentletrap snail Epitonium tinctum.

In captivity, they have lived over 80 years old, in the wild, there is evidence to suggest, Anthopleura could live over a 150 years! With over 1000 species described from nearly pole-to-pole, here are the two most commonly seen at Fort Ross.

Aggregating Anemone

Anthopleura elegantissima

The name says it all: Elegantissima means most elegant.

Beds of these hardy animals, definitely the most abundant anemone on the coast, make the intertidal rocks so squishy underfoot, the pressure of a step causing jets of water to squirt from their mouths and from pores in their body walls.

In a contracted state elegantissima is rarely more than 4 cm in diameter; expanded, the disk may be more than twice that size. The tentacles are often banded or tipped with pink or lavender.

Two types of microscopic algae live in the anemones’ tissues and give them their green color—anemones without algae are white. The algae supply food to the anemones, and the anemones bend toward or away from the light to provide the algae with the proper amount of light needed for photosynthesis.

Anthopleura elegantissima is able to persist practically indefinitely and in great abundance under normal conditions because genetically identical individuals are periodically produced by longitudinal fission, a type of asexual reproduction.

At high tide, this species can split down the middle, pull apart and reform, resulting in two identical (but smaller) individuals. Each of these divides repeatedly until there are hundreds of aggregating anemones, all crowded together (thus the name aggregating). Each animal in the group is genetically identical, a clone. These anemones are separate-sexed so each clone is either all male or all female.

The most exciting thing happens when two clones meet. Members of the same clone extend their tentacles at high tide and do not mind touching members of their own clone … but, should a member of another clone be touched, they fight until one moves or dies.

During sexual reproduction, once a year they release eggs or sperm into the water as a broadcast spawner. Its planktonic larva is the source of the original anemone on the rocky shore that forms each clone.

Elegantissima contains a compound (anthopleurine) in their columnar tissues that when under attack, is released as an alarm pheromone. This chemical causes other anemones to contract defensively. Clonemates of elegantissima are the first beneficiaries, but nearby anemones like A. Xanthogrammica and A. Sola also respond to anthopleurine.

Giant Green Anemone

Anthopleura Xanthogrammica

Beautiful solitary, bright emerald green to turquoise, these sea flowers are incredible animals to find when tidepooling. They are usually found in a depth range of 0-15m, in the intertidal/subtidal zones along rocky shores.

Although they are solitary, they are known to cluster in groups of up to 14 individuals in one square meter. Individuals living in the same area often maintain physical contact through their tentacle tips.

Xanthogrammica, unlike elegantissima typically are not aggressive towards each other.


They can reach up to 12” (30 cm) in diameter and the tentacle crown has at least 6 rings of tentacles. Disk and tentacles are green or blue to white depending on how much sunlight the anemone receives. When sunlight is abundant, the algae grows, producing the bright green color.

Xanthogrammica classically live in surge channels below mussel beds (their primary diet) and depend on wave action to dislodge mussels and deliver them to the anemone’s waiting mouths. They also enjoy sea urchins and shore crabs.

First observed in Alaska in 1835, they have an enormous range from Sitka to Panama, though they are uncommon from southern California to Panama. They have also been found on the eastern coast of Russia.


California Mussels

Mytilus californianus

The mussel is one of the nearly 10,000 species in the bivalves (a large group of animals each of which is covered by a pair of shells – valves) worldwide, including oysters, clams, scallops, and shipworms.

When tide pooling on the Sonoma coast you will find beds of mussels covering the surf-pounded rocks, in other areas of California, on pilings and deep seamounts over 300’ deep.

The California mussel’s shell is blue-black with radiating ridges of brown fanning across the shell. They can reach up to 10” long!

The soft tissue inside the shells is called the mantle, a hinge joins the two shells, sometimes with two interlocking teeth, with two gills on the side that filter feed.

They feed by collecting suspended particles from seawater, taking advantage of the rich waters found off our coasts, especially in the spring when upwelling occurs, full of bacteria and phytoplankton.

Interestingly, bivalves have modified their gills from being strictly a respiratory structure into a bifunctional breathing/filter feeding apparatus.

If you find a dislodged mussel in the rocks below it’s companions, you’ll notice a cluster of tan ‘threads’ at the joint or base. These are called Byssal threads, they are proteinaceous strands that they use to securely attach themselves to rocks.

The foot on a mussel is specialized, no longer having a burrowing function like other bivalves.

This species has a very high reproductive capability: a single female can produce 100,000 eggs annually. The California mussel can grow 9 cm (3.5”) in one year’s time, so it’s no wonder we have vast beds of them on the coast.

They do have many predators, including sea stars, snails, crabs, gulls, sea otters and humans.

The harvesting season for mussels is November through April.

The meat of mussels is pumpkin orange. Native people used these extensively as a food source and the shells as tools.

Gooseneck Barnacles

Pollicipes polymerus

Also called goose barnacle, or leaf barnacle, these live rather high in the intertidal zone, alongside mussels, for 20 years of more!

Barnacles are crustacean arthropods, which means they are distantly related to crabs, lobsters, and shrimp. They are sessile animals, meaning once they settle in a location after their juvenile, free-swimming larval stage, they remain in that location for the rest of their lives. Like mussels, they live in the middle to high Intertidal zone, usually on the rocks, but have been reported on the skin of Humpback whales.

A barnacle’s body or capitulum is encased in semi-triangular, creamy white/gray plates of a hard exoskeleton of chitin. They secrete new plates when happy and healthy, so look for that in groups of them. The number of plates also grows as the animal ages, and can reach over 100 mini plates.

Their bodies are generally dark brown, but sometimes appear deep to bright red, due to the haemoglobin in the animals blood. Their body sits on top of a permanently attached stalk, covered by a thick skin called a peduncle, that can grow to 6”.

Their stalks are incredibly strong, and withstand the force of breaking waves without a problem; in fact their presence indicates you are in an area prone to harsh ocean waves.The stalks and bodies are edible, and have been exported from North America to Europe as a delicacy.

Predators of P. polymerus include ochre sea star and sea gulls. Their tight clustering groups make them more resistant to predation. They filter-feed on amphipods and other creatures up to the size of a housefly. You may notice that groups of barnacles tend to line up with the direction of the waves or current movement, making it easier to capture food with their cupped feeding appendages.

Adult gooseneck barnacles are generally hermaphroditic (meaning they have both male and female reproductive organs). A mature barnacle produces 3 to 7 broods per year, with about 100,000 to 240,000 larvae from each brood. They reach sexual maturity within one year.

Their range is from Sitka Alaska to Baja Mexico.

Sea Urchin

Sea urchins are considered a keystone species, meaning ‘in ecology, a species that has a disproportionately large effect on the communities in which it occurs. Such species help to maintain local biodiversity within a community either by controlling populations of other species that would otherwise dominate the community or by providing critical resources for a wide range of species.’

– Encyclopedia Britannica

There are more than 800 species of sea urchins and sand dollars worldwide, all echinoderms – meaning spiny-skinned animals. They do not have a brain or a heart.

They are primarily subtidal, but some live up to the lowest intertidal levels.

The round inner shell that the spines come out of is called the ‘test’. It is covered with pincers, tube feet and spines that move on ball and joint sockets. The spines both protect the animal and spear food.

Though we think of the test as being the exoskeleton, urchins do in fact have an internal skeleton composed of calcite and magnesium. The amount of each is determined by the temperature and location in the body. Each plate or spine is a single calcite crystal.

The mouth, called Aristotle’s Lantern named after the Greek philosopher who first described it, has five teeth that can be seen on the bottom surface.

‘Sea urchins use a water vascular system to manipulate their tube feet by controlling water movement to and from the feet through muscular tubes. As the tube feet press against an object, removal of water from the tube creates a vacuum. When the tube is refilled with water, the vacuum is broken and the grasp of the foot is released.’ – Aquarium of the Pacific

They are very light sensitive; this is why we tend to see them in holes and under rocks.

Purple Sea Urchin

Strongylocentrotus purpuratus

Strongylocentrotus actually means ‘ball of spines’. True to their name, adult purple sea urchins are purple in color but juveniles have green spines, or spines transitioning from green to purple.

Purple sea urchins are very well studied due to their voracious appetites, specifically on kelp beds. They can have a great impact on habitat structure. When kept in a delicate balance, they help maintain a healthy habitat, but can quickly run amuck when there are too few predators. They are also used as an indicator species to monitor pollution levels as they are very sensitive to changes in water quality.

Their primary food source is seaweed, but enjoy any fragments of animal or other plant material they collect with their tube feet and spines. Captured food is transferred to the mouth by a coordinated movement of the tube feet and spines.

These animals have both male and female sexes. The gonads are five-lobed and you’ve probably heard of them; Uni. Uni is a very popular delicacy served in many sushi bars, said to be highly nutritious and considered to be an aphrodisiac. It ranges from rich gold to pale yellow with a creamy consistency. It’s available seasonably, from late fall through winter.

It is estimated that one female can produce up to 20 million eggs in one year!

Certain sea urchin species including the Strongylocentrotus spp. excavate holes in the rocks where they live. The current theory is that they use their feeding apparatus and spines to bore the cavities out of the rock. Research has shown that even quartz sand grains and glass can be cracked by their teeth. They tend to make a hole the perfect size from themselves. The hole provides protection from the pounding waves and it is thought a certain amount of grazing material to eat.



Mark W. Denny and Steven D. Gaines, Encyclopedia of Tidepools & Rocky Shores , UC Press, 2007.

Edward F. Ricketts, Jack Calvin, and Joel W. Hedgpeth Revised by David W. Phillips, Between Pacific Tides, Stanford University Press, 1985.

J. Duane Sept, The Beachcomber’s Guide to Seashore Life of California, Harbour Publishing, 2002.

Norman A. Meinkoth, National Audubon Society: Field Guide to Seashore Creatures, North America, Knopf

Seaworld Parks & Entertainment

UCSC Ecology & Evolutionary Biology – Pacific Rocky Intertidal Monitoring: Trends & Synthesis

Monterey Bay Aquarium

Central Coast Biodiversity

California Tidepools

Joint Nature Conservancy Committee

Aquarium of the Pacific