Giant Green Anemone (Anthopleura xanthogrammica): Species Profile

At low tide on the Pacific coast, the most confident animal in the tidepool is a green flower that eats crabs. The giant green anemone, Anthopleura xanthogrammica, spreads a tentacle crown the size of two open hands across pools from Alaska to Baja California, green as aquarium neon, patient as geology. It is one of the largest anemones in the region, a genuine predator of the intertidal and shallow kelp-forest edge, and — the thing first-time visitors never expect — partly a plant. Its colour and much of its energy come from symbiotic algae living inside its tissues: an animal that photosynthesises. That combination of predator and solar cell makes it one of the most instructive invertebrates on the coast.

Fast facts

Scientific name	Anthopleura xanthogrammica
Group	Sea anemone, class Anthozoa, phylum Cnidaria
Size	Column to about 17 cm wide; tentacle crown spreading to 25 cm in large, submerged animals
Lifespan	Long — decades; among the longest-lived intertidal invertebrates where undisturbed
Depth range	Intertidal pools and surge channels to shallow subtidal, roughly 15 m
Range	Eastern North Pacific: Alaska to Baja California
Status	Common; not commercially fished, but affected by collection pressure and warming pools

The green partnership

The colour advertises the strategy. The anemone's tissues host two populations of photosynthetic symbionts: zooxanthellae (dinoflagellate algae of the same broad family that powers corals) and zoochlorellae (green algae). Their proportions vary with the light regime of the pool the anemone happens to live in — deeper or shaded animals lean on zooxanthellae, brightly sunlit ones can run emerald with zoochlorellae. The animal benefits from the photosynthetic products the algae leak into its tissues; the algae get nitrogen, CO₂ and a predator's immune protection at the address of a permanent sun-facing pool.

This is the same general contract as reef corals, concluded independently in a temperate intertidal animal — and it comes with a coral-like vulnerability. A. xanthogrammica expelled from light, or stressed by heat, bleaches: the tissues transluce white as the symbionts leave, and survivorship drops. Unlike corals, it bleaches quietly and out of public notice, because its pools are not bathed in satellite attention. Warmer, longer marine heatwave summers — the pattern our explainer on marine heatwaves tracks — land on intertidal symbiotic animals with particular force at midday low tides, when a pool stops being the ocean and becomes a pan of sun-warmed water.

Anatomy: the flower with a file inside

The visible animal is a muscular column attached to rock by a broad pedal disc, topped by several concentric rings of short, banded tentacles around a central mouth. Three anatomical details repay attention:

1. The tentacles are armed battery by battery. Each tentacle is studded with nematocysts — stinging capsules that fire on contact and harpoon or entangle prey. To a crab, small fish or detached mussel that stumbles into the crown, the flower closes like a fist, and the pull is toward the mouth. To a human fingertip, the same tentacles register only as a faint velcro click; our skin is too thick and the nematocysts too small to deliver a meaningful sting.
2. The column wears rubble on purpose. In the wild the upper column is studded with bits of shell and gravel, attached deliberately by the animal using adhesive verrucae — camouflage and, more importantly, a sunshade that protects the column from desiccation when the animal contracts at low tide, pulling its oral disc inward and shrinking into a greenish, pebbled dome.
3. There is a pharynx that works like a file. Prey is worked into the mouth and acted on by a ribbed pharynx; the indigestible contents — crab shell, mussel chips, urchin spines — are compacted into a pellet and ejected from the same opening, hours later. An anemone is a one-opening animal; the same mouth eats and excretes. Tidepool regulars learn to tell the closed, digesting animal from the open, hunting one at a glance.

What it eats, and who eats it

For a sessile animal, the menu is surprisingly carnivorous: crabs, small fish, detached mussels, chitons, sea urchin fragments — whatever flows or stumbles in. In surge channels beside kelp forest, the anemones effectively toll a perpetual current tax on intertidal animals passing between the forest and the surf zone; the crown of a large anemone can handle prey the size of a human thumb without hurry.

Predators, in turn, are few and specific. The leather star (Dermasterias imbricata) makes a speciality of anemones, and the aeolid nudibranch Aeolidia papillosa grazes the tentacles and hijacks the nematocysts it ingests into its own defensive batteries — one of the more elegant thefts in Pacific natural history, played out at animal speeds measured in days. A few spider crabs and sculpins will take tissue from damaged animals. Otherwise, a large anemone is close to untouchable in its pool, which is the ecological explanation for how old they get: animal lifespans of several decades are routine estimates, and the biggest, oldest specimens in sheltered pools are probably older than most of the divers who look at them. The ochre sea star's keystone experiments — told in our ochre sea star profile — run in the same pools, a few metres away, and the tidepool's lesson overall is that every species in it is old, slow, specialised — and closely watched by everything else.

Aggregating kin, still green

Visitors regularly confuse the giant green anemone with its small, carpet-forming relative, the aggregating anemone (Anthopleura elegantissima), which clones itself by splitting and covers rock in dense patches of individuals a few centimetres across. The rule of thumb: xanthogrammica is the large, solitary, broad-crowned animal of the deeper pools and surge channels; elegantissima is the small, clonal, war-waging carpet of the higher, often more exposed rock. The aggregating species fights its clonal neighbours across anemone no-man's-lands in a slow-motion intertidal trench warfare that rewards a patient hour of watching. Both share the green symbiosis; only the giant is the kelp-forest-edge giant.

Why this matters to kelp

The giant green anemone lives at the seam between the rocky intertidal and the shallow kelp forest — the zone where the forest's grazers, scavengers and refugees all traffic. It is not a kelp-forest engineer like an urchin or a lobster, but it belongs to the tightly knit cast that makes the edge of a kelp forest rich, and it shares its sunlit, symbiotic signature: like the kelp itself, it is an organism whose visible colour is the signature of a partnership. In the way our kelp forest food web guide frames it, the tidepool anemone is a mid-web node: feeding on what the forest's edge supplies, feeding in turn on nothing much at all, outlasting everything in its pool, and quietly converting sunlight into the slow biomass of decades.

Tidewright's own long-term monitoring program surveys intertidal indicator species alongside canopy cover, and green anemone bleaching has become one of the early-warning signals the diving crews learn to photograph before the canopy itself declines: the animal is sensitive, conspicuous, and easy to census. In coastal citizen-science programs — the kind our beach-walk citizen science guide introduces — a green anemone count is among the most robust data a careful walker can contribute.

Key takeaways

• The giant green anemone (Anthopleura xanthogrammica) is a large, solitary, photosymbiotic anemone of Pacific tidepools, Alaska to Baja.
• Its green colour is two living partnerships — zooxanthellae and zoochlorellae — and heat stress can bleach it coral-fashion.
• It is a real predator — crabs, mussels, small fish — but its sting cannot harm a human fingertip.
• Column rubble, a rubbished camouflage, and decades-long lifespans make it one of the most observable, most durable intertidal animals.
• Its aggregating, clonal cousin is the small carpet anemone; the giant is solitary and kelp-forest-adjacent.
• As a symbiotic, heat-sensitive intertidal indicator, it is one of the first animals in a pool to show warming stress.

FAQ

Can a giant green anemone sting you?

Not meaningfully. The nematocysts that paralyse crabs and small fish cannot penetrate ordinary human skin; touch a tentacle and you feel a mild adhesive tug, like touching the loop side of velcro. People with very thin or broken skin might feel a tingle, and the standard handling rule applies to all wild animals — look, don't squeeze — but this is among the gentlest large invertebrates in any tidepool.

How do they get so green — is it paint?

It's symbiosis, not pigment. The animal hosts photosynthetic algae inside its cells — dinoflagellate zooxanthellae and, in sunlit individuals, green zoochlorellae — and the visible colour is the algae showing through the animal's translucent tissues. It is the same broad arrangement as corals, arrived at independently, and it is why the same species can look olive in dim pools and vivid emerald in sunny ones.

How long do giant green anemones live?

Decades, where undisturbed — they grow slowly, have almost no predators, and live in one spot. Exact ages are hard to measure, but large specimens in undisturbed pools are reasonably estimated in their multiple decades, making them among the oldest residents of any intertidal community.

Why is the anemone I found closed, and covered in pebbles?

It is low tide. When a pool drains, the anemone contracts: tentacles retract, the oral disc folds in, and the column closes into a pebbled dome. The shell and gravel stuck to its upper column are deliberate — a self-applied sunshade against desiccation and UV. Wait for the rising tide or come back submerged, and the same animal will open into the full green crown.

Do giant green anemones clone themselves?

No — that is their close relative, the aggregating anemone (Anthopleura elegantissima), which splits by binary fission into carpets of clones. The giant green anemone reproduces sexually, releasing eggs and sperm into the water, and remains a solitary individual for its whole life. If the rock is carpeted in small green anemones pressed edge to edge, you are looking at a clone war of the aggregating species, not a population of small giants.