Sea anemones, marine invertebrates related to jellyfish and corals, display a spectacular and complex range of hues. Their coloration is a dynamic biological signature reflecting genetics, diet, and environmental conditions. Anemones are soft-bodied polyps that anchor themselves to surfaces, extending tentacles to capture prey. Understanding their colors requires looking at the microscopic life and specialized proteins within their tissues.
The Vast Spectrum of Anemone Coloration
Sea anemones showcase a remarkable palette of colors that varies significantly by species and habitat. Many species exhibit subdued shades of olive, gray, and brown, while others are vibrant with intense pigmentation. Common colors include various greens and browns, often seen in intertidal species exposed to sunlight. Vivid colors like deep purples, blues, reds, pinks, and bright oranges are less common, particularly in species favored by marine aquarists. The anemone’s environment, such as water depth and available light, heavily influences the expression of its pigments.
Symbiotic Algae: The Source of Brown and Green Hues
The most prevalent colors—browns, olive greens, and muted shades—originate from millions of single-celled organisms living within the anemone’s tissues. These microalgae, known as zooxanthellae, are dinoflagellates that form a mutualistic relationship with the host. The algae use photosynthesis to convert sunlight into energy, producing sugars and organic compounds. The anemone uses these compounds for up to 90% of its nutritional needs.
The photosynthetic pigments within the algae, such as chlorophyll, impart the characteristic brown and greenish tint to the anemone’s tissues. In return, the anemone provides the algae with a protected habitat and essential compounds like carbon dioxide. The density of these algae directly affects the intensity of the coloration; a high concentration results in a darker, richer brown or green. This symbiotic arrangement is a defining feature of many anemone species in sunlit, shallow waters.
Host Pigments and the Phenomenon of Fluorescence
The anemone’s own contribution to its color comes from specialized molecules called fluorescent proteins (FPs), which create vibrant colors like neon greens, blues, and reds. These proteins absorb light at one wavelength, such as blue or ultraviolet light, and then re-emit it at a longer, visible wavelength, a process known as fluorescence. This biological light show is controlled by the anemone’s genetics and is distinct from the colors produced by the symbiotic algae.
These fluorescent pigments serve several biological functions. They act as a form of sunscreen, absorbing intense, potentially damaging UV light and dissipating it as safer, longer-wavelength light. The proteins also function as potent antioxidants, helping to protect the anemone’s cells from oxidative stress caused by intense light. Research suggests these pigments may help regulate the light spectrum reaching the symbiotic algae, potentially enhancing photosynthesis in lower-light conditions.
Why Anemones Change Color
Anemone coloration is not static but can change dramatically in response to environmental stress, a phenomenon known as bleaching. Bleaching occurs when the anemone expels its symbiotic algae (zooxanthellae), usually due to stressors like elevated water temperature, pollution, or excessive light intensity. Without the algae’s photosynthetic pigments, the anemone’s tissues become translucent, appearing white or pale.
A bleached anemone has lost its primary food source and is highly vulnerable, linking color change directly to the health of the marine ecosystem. If the stress is temporary, the anemone may recover and reacquire the algae, regaining its brown or green coloration over time. Subtle color shifts can also occur as the anemone adjusts its internal pigment levels, producing more fluorescent proteins for photoprotection when exposed to increased sunlight.