Sand dollars, a type of flattened sea urchin, are common inhabitants of sandy coastal areas worldwide. These disc-shaped marine invertebrates often wash ashore, their bleached exoskeletons familiar finds for beachcombers. Living sand dollars are covered in tiny spines and range in color from reddish-brown to purple. Understanding their presence in marine ecosystems involves exploring their life cycles and the factors influencing their time in the wild.
Understanding Sand Dollar Lifespan
Sand dollars typically live for several years in their natural habitats. The average lifespan for most species ranges from six to ten years. Some sand dollar species, under ideal environmental conditions, may even live for up to fifteen years.
These creatures often live in dense groups on the seafloor, with hundreds inhabiting a single square yard. They spend their lives burrowing into the sand, feeding on microscopic particles and algae.
Factors Affecting Longevity
Several environmental and biological factors influence how long a sand dollar survives in the wild. Predation is a natural challenge, with sea stars, crabs, and certain fish like cod, flounder, and sheephead preying on them. Gulls also pose a threat, dropping them onto hard surfaces.
Habitat quality significantly impacts sand dollar longevity. They thrive in sandy or muddy sediments where food, such as detritus, algae, and plankton, is abundant. Stable water quality, including appropriate temperature, salinity, and oxygen levels, is important for their health and survival. Fluctuations in these conditions can cause stress, affecting their metabolism and overall well-being.
Environmental stressors, such as strong storms and significant wave action, can dislodge sand dollars from their buried positions, exposing them to predators or washing them ashore. Pollution from chemical runoff, plastic debris, and oil spills can contaminate their habitats and food sources, directly harming them and disrupting their reproductive systems. Ocean acidification, caused by increased carbon dioxide absorption, weakens their calcium carbonate skeletons, making them more vulnerable.
Human impact, including unregulated bottom trawling, coastal development, and the collection of live specimens, also poses a substantial threat to sand dollar populations and their habitats.
How Scientists Determine Age
Scientists estimate the age of sand dollars by examining growth rings on their skeletal plates, known as tests. Similar to the rings found in trees, these rings are formed by the deposition of calcium carbonate as the sand dollar grows. Researchers use microscopes to count these rings, which typically represent a year of growth.
Counting these growth rings can present challenges. Factors like food availability, water temperature, and salinity can influence the clarity and distinctness of the rings. Periods of rapid growth may result in wider, less defined rings, while stressful conditions can lead to thinner, less visible bands. Wear, breakage, or irregular growth patterns can also make accurate counting difficult. While size can offer a rough indication of age, it is less precise than analyzing growth rings due to variations in growth rates.