Every species needs a backup strategy when food is difficult to find. For sea anemones, Plan B is their symbiotic relationship with tiny algae living under their skin. University of California, Irvine biologists have published findings describing how anemones control this remarkable interaction. Their discovery provides new insights into ways organisms form associations that make them more successful than if they lived by themselves. The team’s research appears in the Proceedings of the Royal Society B.

Sea anemones typically feed on mussels, shrimp, squid and other prey, but if this food isn’t available, they can obtain sugar from the photosynthesizing algae that live inside them. In exchange, the algae receive the nitrogen they need from the sea anemones. However, there is a potential downside to the relationship, because the anemones suffer if the algae become too numerous.

To examine how the creatures manage the interaction, the UCI team waded into tidepools on the central California coast. The scientists studied sea anemones from two species dwelling in the region’s waters, experimentally adding or removing food, including mussels, squid, and whatever else the anemones had eaten.

“We found when sea anemones have lots of food, they don’t need as much energy from photosynthesis, and therefore host fewer algae,” said Matthew Bracken, UCI professor of ecology and evolutionary biology.  “When less food is available and the sea anemones require more energy from photosynthesis, the algae proliferate. The mutualism between sea anemones and their algae is flexible and depends on how effective the anemones are at capturing food.” He noted the anemones control the number of algae they contain by spitting them out if they aren’t needed.

Samuel Bedgood, a graduate student in ecology and evolutionary biology and the paper’s first author, said: “This flexibility may help explain why the sea anemones are so good at surviving periods of low food abundance. The ability of organisms to form close associations that allow both to be more successful has puzzled biologists, especially in terms of how these fragile relationships are maintained.”

The team’s discovery with California coastal anemones offers insights into the workings of coral reefs, which rely on algal symbionts to live in tropical waters where nutrients are scarce. The findings also help understand how ecosystems can respond to environmental stresses such as increased ocean temperature. The researchers are now interested in examining the relationship between sea anemones and the marine animals that share their habitat.

 “During high tide, the anemones pull in water and during low tide, they cool themselves by expelling it in small amounts,” Professor Bracken said. “This creates a moist patch that helps snails and other small mollusks survive the harsh, low-tide conditions. It is another example of the partnerships that benefit the individuals involved.”

The researchers conducted their study at the Kenneth S. Norris Rancho Marine Reserve in Cambria, California, part of the University of California Natural Reserve System.

Funding for the research was provided by UC Irvine Oceans, Changing Environments, Arts, and Nearshore Societies, known as UCI OCEANS, and the National Science Foundation.

About the University of California, Irvine: Founded in 1965, UCI is the youngest member of the prestigious Association of American Universities. The campus has produced three Nobel laureates and is known for its academic achievement, premier research, innovation and anteater mascot. Led by Chancellor Howard Gillman, UCI has more than 36,000 students and offers 222 degree programs. It’s located in one of the world’s safest and most economically vibrant communities and is Orange County’s second-largest employer, contributing $5 billion annually to the local economy. For more on UCI, visit www.uci.edu.