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.

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!

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.

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.

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.

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:

• Ashley, Kevin.  Anthopleura xanthogrammica – Animal Diversity Web
• Encyclopedia of Life. 2010. “Anthopleura xanthogrammica” – EOL.org.
• Giant Green Anemone Factsheet – Monterey Bay Aquarium
• Habecker, Bullis, et al. “GIANT GREEN ANEMONES CONSUME SEABIRD NESTLINGS ON THE OREGON COAST.” Marine Ornithology 42 (2014): 1-2.  (PDF available here)
• Ricketts, E., J. Calvin, J. Hedgpeth, D. Phillips. 1992. Between Pacific Tides: Fifth Edition.
• White, B. 2004. Anthopleura xanthogrammica – Walla Walla University