Have you ever strolled along a sun-kissed beach and stumbled upon shells with a flower pattern washed ashore? They are sand dollars – captivating marine creatures. Prepare to be amazed by the wonders of these seemingly simple creatures, as we dive deep into their lives and unearth the hidden facts of these species!
Class: Echinoidea
Order: Clypeasteroida
1. They’re cousins to Sea Stars and Sea Urchins
Sand dollars, alternatively referred to as “sea cakes” or “sea cookies,” are echinoderms that dwell in the ocean, belonging to the echinoid class, which encompasses spiny-skinned creatures. They share comparable anatomy with sea stars, sea lilies, sea urchins, and sea cucumbers, establishing a close relationship among these marine species.
These marine bottom-dwellers display diminutive disc-shaped bodies, generally ranging from 2 to 5 inches (5 to 12.7 cm) in length. The biggest species ever found measured an impressive 6.3 inches (16 cm) across.
Sand dollars often look like white “seashells” on the beach, but in reality, these are their skeletons. This rigid outer covering, called a test, consists of five symmetrical plates made of calcium bicarbonate. Interestingly, even in death, this test tends to remain undamaged and remain connected to their bodies. The shell is covered with small, hair-like structures called cilia. These spines help the species move faster when they traverse the ocean floor in search of food.
On the shell, you’ll see a flower-like pattern, comprising five sets of gas and water-processing holes. These tiny openings, known as lunules, function as pressure drainage channels. They effectively prevent the sand dollar from being swept away by waves.
These echinoderms exhibit a wide range of colors, from rich reddish-brown to vibrant shades of purple, blue, or green. As the species ages and dies, the cilia fall off, and the shell undergoes bleaching due to sun exposure and other environmental factors, turning into a white or pale gray appearance. This is one of the ways to tell if a sand dollar is alive or dead.
In the center of the shell, you’ll see their mouth. On the upper surface of the test, there is a pattern resembling five petals, each of which extends five sets of tube feet. These sea creatures use these feet for respiration purposes.
There are about 250 different types of sand dollars, some of them are:
– Keyhole sand dollar: possesses a distinctive keyhole-shaped feature in its endoskeleton and velvety skin. The species inhabit the coastal regions of the US and Brazil.
– Common sand dollar: has a purplish brown color and lives in the Northwest Atlantic and the North Pacific regions.
– Eccentric sand dollar: is also called the western or Pacific sand dollar. The creature exhibits a range of colors, including brown, gray, black, or various shades of purple. Its natural habitat lies within the northeast Pacific, spanning from Alaska to Baja California.
– Japanese sea biscuit stands as one of the largest species, commonly inhabiting Japan.
2. Sand dollars Habitat
Sand dollars are widely found across the ocean waters of the Northern Hemisphere, ranging from the North Pacific to the eastern North Atlantic oceans, and they are particularly abundant in the Caribbean Sea. The majority of sand dollars inhabit seawater environments, while specific species may also thrive in estuarine habitats characterized by unique chemical properties.
Discovered at various depths, spanning from just below the intertidal zone to depths surpassing 7,000 feet, these sand cakes live across different coastal regions. Their preferred habitats are sandy and muddy areas within the intertidal zone, encompassing the foreshore and seashore regions. In these places, they use their spines to burrow into the sand to find food and shelter.
Essential to their survival, these sea creatures depend on water. They can briefly survive out of water for a few minutes.
3. Diet
Sand dollars are omnivores with a diverse diet that comprises crustacean larvae, debris, microscopic algae, aquatic insects, copepods, and diatoms.
In tranquil waters, these fascinating animals position themselves upright with one end buried in the sand, ready to seize passing prey. However, when the waters turn turbulent, they adapt by lying flat and completely burying themselves if necessary. To remain in place, they can apply some methods, such as cultivating denser skeletons or ingesting sand to increase their weight. In this flattened or buried state, they skillfully employ their spines and cilia on their lower surfaces to capture and eat debris from beneath them.
These species will use their spines covered with cilia to move food through the sand and across their prickly body surfaces to reach their mouth. This mouth is located on the ventral side of its body, while its anus is situated on the dorsal side.
Referred to as Aristotle’s lantern, the sand dollar’s mouth consists of five jaws, each equipped with five sections of small teeth. When the food finally arrives at their mouth, they employ these specialized jaws to chew it. Fascinatingly, it can take them 15 minutes to grind their food before swallowing. The whole digestion process can last up to 2 days.
Once a sand dollar perishes and dehydrates, these teeth disengage, taking on a striking resemblance to diminutive, white birds commonly known as doves.
4. They have a great defense mechanism
Due to their hard skeletons and limited edible parts, sand dollars boast a relatively small number of predators. These include sea stars, ocean pout, starry flounders, crabs, California sheepshead, and even humans.
To defend, they come up with a remarkable defense strategy against predators. Their undersides are equipped with specialized spines that act as an anchor, enabling them to firmly embed themselves in the sand or mud. When faced with a predator’s threat, the species use its spines to shift and bury itself even deeper. This creates a formidable barrier that hinders the predator’s access and provides a measure of protection against potential predation.
As larvae, sand dollars protect themselves in another interesting way. They clone themselves to increase their likelihood of survival and divert attention from the threat.
Through these mechanisms of defense and adaptation, the species manage to thrive in their marine environments despite some lurking dangers.
5. They like to gather together
Sand dollars possess distinctive characteristics and behaviors. Despite the vastness of the oceans available to them, they favor living in densely populated colonies, with up to 625 individuals inhabiting a single square yard.
Due to their inflexible skeleton and small spines, sand dollars face a considerable challenge when attempting to flip themselves over while upside down. Righting themselves can be a laborious process, taking as long as half an hour. For these creatures, this struggle is particularly arduous, and failure to regain their proper position can lead to death. Thus, it is crucial to handle them with care and ensure they are placed in the correct position after handling.
These species lack a central brain, eyes, and heart, but they boast a distributed nervous system that extends throughout their bodies. This unique system enables them to perceive and react to their surroundings effectively. This means these ocean dollars can feel pain.
Moreover, their lack of conventional eyes is compensated by light-sensitive cells that enable them to perceive changes in light levels. This adaptive feature aids in recognizing potential threats and triggers a response to safeguard themselves by burrowing deeper into the sand.
Similar to the cousin starfish, these sea cakes can regenerate their lost limbs. In the event of predation or injury, they can replace new ones in a few months. During this remarkable process, the new limb develops the distinctive five-fold symmetry characteristic and ultimately regains full functionality.
The sea creatures are not dangerous, without teeth, they can’t bite you. However, their soft spines could potentially cause minor scrapes. When handled, they will release echinochrome, a harmless yellowish stain on your hand. This substance indicates their stress levels.
6. Reproduction
These species engage in sexual reproduction. During this process, both males and females release their gametes into the water, a phenomenon referred to as external fertilization. The role of salinity in the fertilization of their eggs is crucial. Research has indicated that a specific level of salinity is necessary for the successful completion of their reproduction.
Incredible numbers of over 350,000 eggs are annually released each year. These eggs undergo fertilization and are enveloped in a protective jelly, ultimately transforming into minuscule larvae that float along ocean currents. Throughout their growth process, these nektonic larvae encounter numerous obstacles, including the potential risks posed by predators.
After a few months floating, the larvae firmly attach themselves to the ocean floor and undergo a transformative metamorphosis, entering their juvenile phase. During this stage, the young ones energetically gather nourishment from the seafloor. In regions with robust currents, they might even consume sand to anchor themselves down. As they continue to grow, these juveniles gradually migrate from subtidal zones to open beach regions. Even the tiniest individuals predominantly are located at higher elevations along the beach.
Although adults solely engage in sexual reproduction, the larvae possess the remarkable capacity to reproduce asexually. They can clone themselves under ideal circumstances or when confronted by a predator.
7. You Can Tell their Age by Its Rings
Sand dollars demonstrate a striking resemblance to trees when it comes to aging. Similar to the growth rings on a tree stump that represents each year of growth, these animals also develop growth rings on their shell or test.
The number of the rings corresponds to their body size. The bigger size sand dollars have, the older they are. These particular species have a relatively long lifespan, ranging from 6 to 10 years.
8. Legend and symbolism
Sand dollars are believed to symbolize the story of Christ. On the top of the shell, there’s a star-shaped symbol resembling the Star of Bethlehem and an Easter lily outline, representing Christ’s birth and resurrection. The five holes on the shell symbolize the wounds of Christ – four for the nails on his hands and feet, and one for the spear wound. If you break a sand dollar, you’ll see their five dove-shaped teeth, symbolizing concepts of peace and goodwill.
Among the captivating myths surrounding sand dollars, one delightful legend suggests that they were lost coins from mermaids or from the people of the mythical city of Atlantis.
The species can represent strength, wealth, good luck, change, renewal, enlightenment, and awareness.
9. Threats
According to the IUCN, sand dollars are categorized as Near Threatened. These species are facing a multitude of threats that decrease their population and threaten their survival. One of them is fishing practices like bottom trawling, which have the potential to disrupt their habitat and cause significant harm to their existence.
Moreover, the issue of ocean acidification poses a significant concern, as it directly affects the development of their test, rendering them more susceptible to harm. The acid oceans also make their reproduction less successful. Additionally, climate change contributes to the alteration of habitats available for these endangered creatures.
References:
- Thoughtco.com
- Treehugger.com
