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Invertebrate of the Week #6 – Giant Green Sea Anemone (Anthopleura xanthogrammica)

If you’re a young seabird, this week’s Invertebrate of the Week is right out of your nightmares.

A recent article in Marine Ornithology demonstrated that these flashy tide-pool denizens are capable of consuming young avian prey that happens to get too close.  On at least two occasions, researchers discovered the bodies of seabird chicks in the grip of these large anemones.

In perhaps the most dramatic instance, a very young cormorant chick had apparently fallen from its nest and become trapped head-first in the clutches of a nearby A. xanthogrammic that was residing in a tide pool.  A similar observation was made several years earlier in which a nestling gull hybrid was completely engulfed by a giant anemone, leaving only the bird’s feet exposed.

Giant Green Anemone consuming seabird
Dorsal (A) and ventral (B) views of a Giant Green Anemone consuming a cormorant chick in a tide pool in Cannon Beach, Oregon, on 24 July 2013. Underwater photographs by Lisa Bullis Habecker. Source: Marine Ornithology 42: 1–2 (2014)

So what else is there to know about this giant anemone aside from the fact that it is able to make the occasional snack out of a wayward tertiary consumer?  A lot!

Blood Star and Giant Green Anemone, Olympic National Park
Blood Star and Giant Green Anemone, Olympic National Park. Source:

At a maximum column size of 30 cm high x 17 cm wide with a crown reaching a diameter between 25cm and 30 cm,  A. xanthogrammic is certainly a mammoth when compared to most other temperate anemones.  They typically live a solitary life in intertidal and subtidal zones (though they are also found in clusters) and are able to survive at depths as great a 15m as long as they remain within their estimated optimum temperature range of 15-22.2° C.

Anthopleura xanthogrammica
Anthopleura xanthogrammica. Source:

While young seabirds are obviously on the menu, these anemones seem to subsist mainly on a diet of mussels, crabs, sea urchins, and small fish which they capture with the help of their cnidocytes (stinging cells which are present in all Cnidarians) arranged along each tentacle.

Giant Green Anemone (Anthopleura xanthogrammica) and Pink-Tipped
Giant Green Anemone (Anthopleura xanthogrammica) and Pink-Tipped Green Anemone (Anthopleura elegantissima. Source:

When exposed to adequate levels of sunlight, A. xanthogrammic will also derive some of its nutritional intake from the activity of endosymbiotic zoochorellae (green algae) or zooxanthellae which live within the tissues around the gut.  While these green/blue-green endosymbionts may partly contribute to A. xanthogrammic’s characteristic green ensemble, much of the color is the result of its inherent pigmentation.

Giant green anemone (Anthopleura xanthogrammica), Fitzgerald Marine Reserve, San Mateo County, CA.
Giant green anemone (Anthopleura xanthogrammica), Fitzgerald Marine Reserve, San Mateo County, CA. Source:

It should also be mentioned that A. xanthogrammic is a long-lived anemone with at least one recorded instance of a captive specimen surviving for 80 years!  Of course, life can still be cut short.  A. xanthogrammic still has its predators to worry about and its greatest threats include the seastar Dermasterias imbricata (Leather Star) and the snail Opalia borealis (Boreal Wentletrap), among others.

So there it is…a fascinating green anemone chock full of endosymbionts ready to opportunistically tackle whatever may come its way.  Good thing they’re sedentary!

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Further Reading:

2 thoughts on “Invertebrate of the Week #6 – Giant Green Sea Anemone (Anthopleura xanthogrammica) Leave a comment

  1. That’s amazing! Have you come across any information on how this anemone catches e.g. mussels, and how it deals with their hard shells?

    • Hi! It’s my understanding that the anemone feeds on mussels that have detached from the mussel bed and fallen within reach. As far as how it deals with the shell, I don’t know for sure but it is likely that the detached mussels are weakened, dead, or otherwise damaged at the time they are captured. This would allow the anemone some means of opening or penetrating the shell and accessing the mussels interior. If the mussel is alive and intact, it would be very difficult for the anemone to make much progress and it might just give up. For instance, a study on anemone diet published in the Marine Ecology Progress Series, demonstrated that the anemone Actinia equina would often ingest intact live mussels only to egest (i.e. vomit) the mussel unharmed after a period of about 1-2 hours if it couldn’t get into the shell.

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