Sea anemones are captivating marine invertebrates. Belonging to the phylum Cnidaria, these relatives of jellyfish and corals are voracious predators that anchor themselves to the seafloor or rocks, often leading to the misconception that they are plants. A striking feature is their astonishing palette of colors, which spans the entire visible spectrum. This intense color variation is far from random, serving as a biological blueprint that reveals the animal’s species, health, and its relationship with its environment.
The Immediate Answer: A Spectrum of Species
The simplest answer to the question of sea anemone color is that they display a dazzling array of hues, determined largely by their species and habitat. Shallow-water anemones in tropical reefs are frequently the most vivid, showcasing fiery reds, bright oranges, rose pinks, and spectacular purples. For instance, the Magnificent Sea Anemone (Heteractis magnifica) can exhibit columns in shades of blue, green, and white. The Bubble-Tip Anemone (Entacmaea quadricolor) often appears light brown with reddish tips.
In contrast, other species exhibit more subtle shades or striking single colors. The Giant Plumose Anemone (Metridium giganteum), found in colder waters, is commonly white, orange, or yellow, lacking the complex patterns of its tropical cousins. Deep-sea anemones, which live in perpetual darkness, often lack any pigment and appear translucent or pale white. However, species like the Burrowing Green Anemone (Anthopleura artemisia) demonstrate a strong green coloration, which is a species-specific trait.
The Biological Source of Coloration
The vibrant and varied coloration of sea anemones stems from two distinct biological mechanisms: one extrinsic and one intrinsic. The most common source of color, especially in tropical, shallow-water species, is the extrinsic relationship with microscopic algae called zooxanthellae. These single-celled dinoflagellates of the genus Symbiodinium live within the anemone’s internal tissues, engaging in a mutually beneficial symbiosis.
The zooxanthellae photosynthesize, providing the host anemone with a significant portion of its nutritional energy. In return, the anemone offers the algae protection and carbon dioxide. The algae contain photosynthetic pigments, such as chlorophyll, which impart earthy colors like brown, gold, or olive-green to the anemone’s tissues. This is the primary reason why many reef-dwelling anemones have a brownish or greenish base color.
The second source of color is the anemone itself, which produces various pigments and fluorescent proteins in its own cells. These intrinsic pigments create spectacular colors, such as neon green, brilliant blue, pink, and purple. The anemone’s tissues create chromoproteins, which are proteins that absorb and reflect specific wavelengths of light. These fluorescent proteins also serve a protective function, acting as a kind of sunscreen by shielding the symbiotic algae from damaging ultraviolet radiation.
Color Change and Environmental Influence
The color of a sea anemone is a dynamic indicator of its health and surrounding environment. The most dramatic change is bleaching, which occurs when the animal is under stress. Elevated sea surface temperatures, high light intensity, or changes in salinity trigger the anemone to expel its symbiotic zooxanthellae. Since the algae impart brown and gold hues, their expulsion leaves the anemone’s translucent body visible, making the animal appear white or pale. Bleaching signifies severe stress and the loss of the animal’s primary food source, which can lead to death if conditions persist.
Conversely, light exposure influences the concentration of the anemone’s intrinsic host pigments. Anemones living in brighter, shallower waters produce more fluorescent proteins to screen their symbiotic algae from UV light. A healthy anemone gradually adjusts the concentration of these internal pigments as it adapts to changes in water depth or cloud cover. This allows for subtle color shifts as they maintain the balance between maximizing photosynthesis and minimizing sun damage.