Giant Green Anemone

Quick interesting facts:

• Solar-powered green: it hosts photosynthetic symbionts (zoochlorellae and zooxanthellae) that contribute to its color and nutrition.

• Shade makes it fade: individuals living in caves/shady zones often have fewer symbionts and look much less green.

• Two different “moods” depending on tide: submerged, it can look bright green; out of water it often closes up and looks darker/olive-brown.

• Dinner-plate sized: the tentacle crown can reach about 25 cm across; the column can reach ~30 cm tall.

• Mussel-bed opportunist: detached mussels are often described as a main food source.

• Rare “what did I just read” moment: there are rare reports of it consuming seabirds; it’s unclear whether the birds were alive or dead when engulfed.

Anthopleura xanthogrammica is a species of intertidal sea anemone of the family Actiniidae.

Description

A large intertidal sea anemone with a broad, flat oral disk and a dense crown of short, conical tentacles. It can appear vivid green when healthy and well-lit, but may also look brownish/olive (especially when closed during low tide).

Size (reported maxima): column up to ~17.5 cm wide and ~30 cm tall; tentacle crown up to ~25 cm across.

Distribution

Northeastern Pacific: commonly reported from Alaska to southern California, with some references noting occurrences farther south where cold-water conditions occur.

Habitat

Mostly low–mid intertidal on rocky or sandy shores, especially in tide pools and surge-exposed areas. It can also occur on concrete pilings in bays/harbors. Reports mention tide pools down to about ~15 m in some areas.

Diet

Opportunistic predator/scavenger. Uses stinging cells (nematocysts) to capture prey. Commonly listed prey includes detached mussels (often cited as a main food), plus sea urchins, small fish and crabs.

Predators

Often listed predators include:

• Leather seastar (Dermasterias imbricata)
• Nudibranch (Aeolidia papillosa)
• Wentletrap snail (Epitonium tinctum) and other snails (e.g., Opalia spp.), plus a sea spider (Pycnogonum stearnsi)

Biology

• Symbiosis: hosts photosynthetic algae (zoochlorellae) and dinoflagellates (zooxanthellae) within tissues; this supports nutrition and strongly influences coloration.
• Movement: can slowly creep using its basal disk, though it typically stays put once settled.
• Reproduction: described as sexual reproduction via external fertilization; sources note pelagic larvae that later settle (mussel beds are mentioned as a settlement habitat).
• Feeding → waste: after digestion, waste is expelled back through the mouth opening.