How sea anemones feel the pinch of climate change

The climate crisis throws many things into disarray — in tidepools, a tiny yet crucial relationship is strained.

If you’ve visited the rocky California coast at low tide, you’ve probably seen, stepped on, and stuck fingers into carpets of aggregating sea anemones, or sea flowers. These elegant and colorful masses of tentacles, ever-present in the intertidal, close up during low tide to present as dark green blobs.

But as the ocean warms due to the climate crisis, these sea creatures may be at risk. For many inhabitants, the ocean is an increasingly stressful place as the climate crisis tips countless ecological relationships toward instability. Already, corals are declining fast, and the algae that partner with corals are abandoning their homes due to the stress of a changing ocean. It’s a worrying shift that’s attracted attention from scientists and ocean lovers worldwide, but the same process — with different players — may happen in California’s tide pools.

The aggregating anemone, Anthopleura elegantissima. (Photo by Dan Hossfeld)

Aggregating sea anemones are relatives of jellyfish and coral, and use stinging cells to immobilize prey or defend themselves. These anemones share another similarity with coral species — a bond with microscopic algal cells that grow, live and reproduce entirely within their own anemone cells. The algal cells soak in energy from the sun, providing the host anemone with nutrition in return for safekeeping. And, like coral, anemones can lose this valuable relationship when conditions worsen.

Coral reefs, harbors of incredible ocean diversity, are in a steep decline worldwide. These reefs, which make up about one percent of the earth’s surface, account for a quarter of all ocean species. In 2018, researchers determined that more than half of the Great Barrier Reef coral is dead, and just last month, government scientists downgraded the overall health of the reef to “very poor”. This issue is global and dire, and many researchers are devoting their entire careers to help discover solutions.

Because anemones and corals are so closely related — and have the same kind of partnership with algae — we can study one to understand the other. Aggregating anemones in California aren’t yet dying because of climate change. However, that doesn’t mean these fragile partnerships are safe now or in the future. In the lab of Professor of Biology Sarah Cohen at the Estuary and Ocean Science Center at San Francisco State University, we’re looking into forces that disrupt the relationship between algae and anemones, a phenomenon also known as bleaching.

Algae in a group (the brownish dots) viewed under a microscope. (Photo by Bing Huey)

Bleaching can be caused by anything stressful for algae: extreme heat, cold, lots of freshwater input from storms, or other factors. My research in the Cohen lab and in collaboration with Dr. Lorraine Ling in Stanford’s Department of Genetics found the most influential effect on bleaching is extreme heat during low tides. In other words, while we humans grapple with how to prepare for more and more extreme heat days for our own safety, tidepool creatures are also facing increased stress. Unlike humans, intertidal dwellers are unlikely to get access to community cooling centers.

Dan Hossfeld, the author, near some tidepools. (Photo courtesy of Nicholas Simon)

Through our research, we found a way to test the overall recent stress from the environment and read the severity of bleaching in anemone colonies. We are currently monitoring local tidepools to observe how sea anemones will adapt to the changing climate.

Anemone communities operate at a different time scale than we do, appearing beautifully slow and deliberate, yet living longer than many humans. When the tide recedes, we can visit the intertidal world where organisms regularly experience exposure to the above-water elements. When the tide advances and anemone colonies are submerged, they regain control as a voracious predator that eats microscopic critters, snails, mussels, limpets, crabs, and fish (and larger relatives are known to consume the odd seabird occasionally).

Aggregations of anemones live in close quarters, and often take over entire boulders in the intertidal zone. These are photographed at low tide, when they are exposed to the elements and withdrawn inwards to protect themselves from exposure. (Photo by Dan Hossfeld)

Sea anemones are sophisticated. They make colonies of clones, assign jobs to different individuals within the community, protect their reproducing colony-mates, and even wage organized war on encroaching threats. Their lives are sustained nutritionally by algae, and ongoing research has found that their natural life strategies will be jeopardized more and more as bleaching increases.

California tidepools have entranced visitors for years, from our indigenous predecessors to John Steinbeck, who famously wrote “It is advisable to look from the tide pool to the stars and then back to the tide pool again”.

Yes, tidepools contain multitudes. Further appreciation comes from knowing bits and pieces about the many worlds under your feet, especially those that aren’t always the obvious charismatic attention-grabbers.

This story was written by Dan Hossfeld, who received his Master’s degree from SF State in 2019 for the studies referenced above. He is a California Sea Grant Fellow working on climate adaptation for California’s coastal ecosystems and communities. Dr. Sarah Cohen’s lab at the Estuary and Ocean Science Center, Biology Department, SF State continues to investigate intertidal ecosystems and how they fare in the changing environment – if you are interested in learning more, volunteering, or joining the lab, contact her at This story is part of an ongoing series about the bay and sea around us through the researchers at the EOS.

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