Skip to Content

What happens when a sand dollar turns green?


Sand dollars are a type of sea urchin that have a round, flattened shape that resembles a coin. They are found in shallow waters along sandy or muddy bottoms in temperate and tropical seas worldwide. Sand dollars typically have a white or tan coloration, but sometimes they can turn different colors like green, purple, or pink. There are a few possible reasons why a sand dollar might become green.

Algal Growth

One common cause of green coloration in sand dollars is algal growth. Small microalgae can colonize the rigid upper surface of a sand dollar, staining it green. This algal growth is often just a superficial discoloration and does not affect the health of the sand dollar. The algae take advantage of the sand dollar’s solid, stable structure to anchor themselves in environments with strong wave action or currents. Species of green algae like Halimeda or Codium may be responsible for the green tint.

Over time, a thick layer of algae can build up and obscure the original patterning and coloration of the sand dollar’s shell. Algal growth can occur when the sand dollar remains in one place for an extended period. A healthy sand dollar will shed and replace its outer skin over time, helping remove algal buildup. Occasional algal growth is harmless, but an excessive accumulation may indicate the sand dollar is compromised and unable to properly maintain its exoskeleton.


In some cases, a sand dollar’s green color comes from its diet. Sand dollars are omnivores and will feed on various types of algae, including green marine algae. As they ingest large quantities of the algae, the green pigments accumulate in their body and can tint the outer skeleton greenish. This effect is only temporary, and the green hue will fade as the sand dollar processes and digests the algal meal. However, in areas with extensive green algae growth, the sand dollar may maintain a persistent green tint due to constant consumption.


The green coloration may also serve as camouflage for the sand dollar. Their shape and color allow sand dollars to blend in against sandy marine floors covered in algae and aquatic vegetation. By taking on green hues, they become less visible to predators. The color change happens slowly over time rather than instantly, suggesting it is not a voluntary response. Sand dollars do not seem to have much control over their color, but the natural greening can still improve their survival chances in the right environment.

Respiratory Distress

In more serious cases, an abnormal green tint in a sand dollar may indicate respiratory issues. Sand dollars breathe through delicate respiratory structures called papulae, which are the tiny hairy projections covering the upper surface. The papulae can only function properly when kept clean and free of debris. An accumulation of sediment, organic matter, or excessive algal overgrowth can suffocate and close the papulae. Clogged papulae restrict oxygen intake, stressing the sand dollar. The lack of oxygen can cause a greenish tinge due to increased anaerobic respiration. A healthy sand dollar maintains clear, open papulae and even coloring.

Bacterial or Fungal Infection

Sudden green spots or a slimy green sheen on a sand dollar may signal a bacterial or fungal infection. Marine microorganisms opportunistically attack weakened sand dollars with compromised immunity. Bacteria, fungi, and other microbes can form infectious biofilms that damage external tissues and internal organs. By the time signs of infection appear, the sand dollar is usually seriously ill. These infections often spread quickly and can be fatal if not treated promptly with antibiotics in an aquarium setting.

Exposure to Toxins

Pollution and toxins in ocean water can also turn sand dollars green. Exposure to certain heavy metals like copper and lead, or chemicals like antifoulant boat paints, can react with proteins in the body and alter shell coloration. The green hue results from disrupted metabolic processes and physiology. Toxicity will usually coincide with other symptoms of poor health like lesions, abnormal spines, and odd behavior. Improving water quality and removing pollutants is critical for recovery.

Checking Sand Dollar Health

When you find a green sand dollar, examine it closely to evaluate its condition. Look for any lesions, missing spines, cracks, or abrasions on the shell. Check whether the fine papulae appear intact and unobstructed. Gently turn the sand dollar over to check the oral side. The mouth, anus, and five ambulacral grooves radiating from the center should be clear and undamaged. Any odd texture, fuzziness, or mucus may indicate microbial infection. Assess if the green color seems superficial or deeper inside the body. finally, consider any recent changes in the environment like pollution, temperature swings, or storms that could cause stress.

Treating Green Sand Dollars

If the green color is mild with no other signs of illness, the sand dollar may naturally recover with no intervention. Providing optimal water quality and nutrition can support its health. Remove excess algae or debris resting on its surface so the papulae can function. If the color is unnaturally dark or accompanied by lesions, quickly quarantine the sand dollar in a separate recovery tank and treat with antibiotics. Address any husbandry issues and improve general care to avoid future problems. With early treatment, mildly affected sand dollars can often make a full recovery.

Preventing Green Coloration

Stop greening before it starts by maintaining pristine aquarium conditions. Perform regular water changes, filter maintenance, and testing to remove pollutants. Avoid overfeeding and remove uneaten debris to keep the water clean. Lightly brush the sand dollar’s surface with a soft toothbrush weekly to clear any algal buildup. Ensure healthy nutritional variety in their diet. Quarantine new specimens for 30-60 days to prevent introducing pathogens. Stable, stress-free environments will keep sand dollars vibrant and their original natural hue.


A sand dollar turning green is usually harmless in the short term but may indicate issues with its habitat or health in some cases. Algal growth, diet, camouflage, and minor environmental stress can all temporarily green a sand dollar. More concerning causes requiring medical intervention include respiratory distress, infections, toxicity, and serious trauma. Examine the sand dollar closely and improve care conditions. With attentive husbandry, green sand dollars can often make a full recovery and regain their lovely white coloration and patterning. The green warning signs can help catch problems early and get sand dollars the treatment they need.