Introduction
Sand dollars are interesting and beautiful sea animals that don’t have a front, back, head, or tail. They belong to the same group phylum (echinoderm) as starfish & sea urchins.
They are flat and round with star-shaped patterns on their hard shells.
Sand dollars live on the sandy sea floor, usually in groups, and they play an important part in the ocean’s ecosystem.
Life Cycle of a Sand Dollar From Larval to Adult Sand Dollar
Interestingly, the life cycle of sand dollars has four stages:
Egg, larva, juvenile, and adult. Each stage lives in a different habitat. Their birth is very different from humans.
Sand dollars survive for about 6 to 10 years and die either from natural causes (often leaving their white skeletons on the seashore), predators like pink starfish and snails, or strong water currents.
Marine biologists can determine their age by counting the growth rings on their skeletons.
During our research at Mote Marine Laboratory & Aquarium in Sarasota, Florida, we observed their fertilization process, which took about half an hour, under a microscope during a live virtual event.
This experience made us realize how amazing these creatures are as they go through a complex process and eventually play an important role in ecosystems.
What is a Sand Dollar? Is It Real Currrency Coins?
Sand dollars, also called snapper biscuits, belong to a group of animals called Echinodermata and have over 250 species.
They are related to sea stars and sea urchins. These animals live at the bottom of the ocean and eat tiny algae and bacteria. They live in groups called “beds.” Sand dollars are not poisonous to humans nor harm humans. They are very innocent creatures.
Animal type:
Invertebrates (animals without backbones)
Habitat:
They live on the sandy ocean floor in the Pacific, Atlantic, and Arctic Oceans, in shallow water about 90 meters (300 feet) deep.
Size:
Up to 3.5 inches (9 to 10 cm) across.
Diet:
They eat small creatures like bacteria, Phytoplankton, crustacean larvae, tiny copepods, bits of dead plants and animals, diatoms, and algae, and dead stuff from the water by filtering it out.
Range:
They are found from Alaska to Baja California in waters as deep as 130 feet (40 meters).
Favorite places
Sandy lagoons and shallow areas of the open ocean.
Where they are found:
Along the east coast of North America,
British Columbia,
Japan.
Relatives:
Sea urchins, sea stars, sea cucumbers (all part of the Echinodermata group).
Description of What is Inside a Sand Dollar?
Back in 2018, I was working on a super cool research project about marine life with the California Academy of Sciences. One day, while walking on Morro Bay Beach in Cambria, California to see california starfish, my 10 year old son found some washed-up sand dollars.
He was so excited and curious ! He picked them up, shook them, and they made some funny rattling sound!
Have you ever wondered why a sand dollar makes that sound? Let me explain!
Sand dollars have a hard shell called a test, made from calcium carbonate (which is the same stuff found in chalk!).
This shell protects their soft, squishy insides from predators. It’s like wearing armor! Pretty smart, huh?
Inside the sand dollar, there’s a tiny mouth called Aristotle’s lantern (fancy name, right?).
This mouth has five little teeth that help them chew up food like small plants and tiny sea creatures. Even though they can chew their food in for 15 minutes, it takes them a whopping
two days to digest it.
Another fun fact: sand dollars have 50 hard bones (called skeletal elements) and 60 muscles inside. Besides, inside the dollar has ring nerve, digestive organs especially also have canals filled with water that help them move and eat.
When a sand dollar dies, all the soft parts inside dry up, leaving only the hard shell behind. That rattling sound my son heard? It was because the dried-up sand dollar skeleton was shaking around.
Overview of The Sand Dollar Diagram
Did you know that sand dollars are like the cousins of starfish and sea urchins?
A live sand dollar is kind of purple, and scientists found out something cool about them — over time, they became flat, not round, because of evolution!
A breakdown of what makes sand dollars special?
- sand dollars are totally flat and don’t have any arms or a central disc like starfish.
2. Their bodies are covered in tiny spines and hair-like cilia. These help them walk, dig, and even breathe.
3. We did an experiment at Whitney Lab for Biosciences Florida, by placing them in loose sand, they started shuffling around, hunting for tiny plants and organisms to eat. So they are sand lovers.
4. Sand dollars have two sides – the oral(mouth side) and aboral (opposite of the mouth) surfaces.
5. On the oral side, there’s a mouth in the center, and five petal-shaped grooves that help with their movements.
6. On the edges of these grooves are little tube feet that help them walk and move food around
7. On the top (aboral side), there’s a special plate in the middle that helps with water flow, along with five pairs of tiny plates.
8. They even have pedicellariae little jaw-like structures with three parts, kind of like tiny pincers.
9. Inside sand dollars have a cool structure called Aristotle’s lantern, which helps them grind up food!
10. Their anus isn’t in the center, it’s actually off to one side, located on the bottom.
11. Their tube feet don’t just help them move. they also act like gills to help them breathe.
12. Sand dollars have separate sexes, and when they reproduce, the baby sand dollars go through a larval stage called the echinopluteus.
How Are Sand Dollars Formed?
In 2023,At Mote Marine Laboratory & Aquarium in Florida, we got to watch sand dollars as they made baby sand dollars, a process that took just about an hour. Let me tell you how it works.
Female sand dollars release their eggs into the water, while the males release sperm.
When the sperm meets the eggs, they fertilise, and the baby starts as tiny larvae with two little arms.
These larvae float around in the ocean for a few weeks, eating tiny organisms, before they settle down on the ocean floor. Interestingly , if the larvae feel threatened by predators (like fish), they can make clones of themselves.
This means they split in two and make copies of themselves to escape. When they do this, they become smaller and harder for predators to spot.
Young sand dollars don’t like to stay on the ocean floor for long. Once they grow a little, they go through a big change called metamorphosis. During this change, they start looking like adult sand dollars.
An adult sand dollar has a flat, hard skeleton called a “test,” which is covered with tiny, soft spines. These spines help the sand dollar move and bury itself in the sand. As sand dollars grow, their skeletons harden, and a cool five-petal design forms on top of their bodies.
They live on the sandy or muddy ocean floor, eating plankton and tiny food particles using their tube feet and spines.
Sand Dollar Reproduction?
Sand dollars reproduce through a sexual reproduction process called “broadcast spawning.” This behaviour is used by many sea animals like corals and some species of fish.
It is a very interesting process. Groups of female sand dollars release their eggs into the water, while males release their sperm into the water column.
When observed experimentally, billions of gametes (sperm and eggs) enter the water. But sadly, very few survive to adulthood.
During this time, eggs are fertilised by sperm immediately, helping them avoid predators on the seafloor. Since sand dollars live in groups, large numbers.
More than 625 sand dollars can occupy one square yard (0.85 sq m).
While this is a rough estimate, I have seen large carpets of them on the seafloor here in South Florida. This abundance makes broadcast spawning very interesting and successful.
It occurs seasonally or during certain events, but many factors such as water temperature, light intensity, and the phase of the moon make this process more complex.
Marine biologists have observed that in Mexico, Dendraster excentricus experiences a peak in reproductive activity during spring and summer.
The Life Cycle of a Sand Dollar Starts from Gametes to Adult Sand Dollar
The life cycle of a sand dollar begins when the reproductive cell (egg) of a female sand dollar is fertilised by male sperm in a suitable water environment.
Larvae
Spermatozoa initiate fertilisation, and the embryo develops. The transformation of the embryo into a larva is called the pluteus stage.
At the Whitney Lab for Bioscience, we observed that the full larval development of *Dendraster excentricus* takes 21-23 days from fertilisation to metamorphosis.
The larva, initially developing two arms, drifts freely in the water for about two weeks.
Sometimes, the larva splits itself into two clones, which allows it to escape predators due to its tiny size. The pluteus then develops four, six, and later eight arms.
The larval stage is critical for the sand dollar’s growth, and the duration of this stage depends on the species. It takes about six weeks to reach a size of 0.03 inches (0.0762 cm). The early life of a sand dollar is full of challenges.
They cannot defend themselves or hide in the sand, making them easy prey for crabs, fish, or octopuses, or susceptible to being washed away by strong water currents. At this stage, they use different strategies to escape predators, such as avoiding settling on the sea floor.
Juvenile
When the larvae grow their 8 arms, they settle on the ocean floor and go through metamorphosis, which takes about 1 hour and 30 minutes.
After that, the juvenile starts developing. At first, they don’t have a skeleton, gut, mouth, or anus. These parts start forming after 7 days, and then they can begin feeding.
They crush and collect sand grains, but how they do this depends on the species’ size and spine development. As they continue growing, they gradually become adults.
Adult Sand Dollars
When sand dollars grow to adulthood, reaching 3 to 4 inches (7.62 to 10.16 cm), they can live for 6 to 10 years.There are around 250 species of sand dollars found worldwide, especially in tropical coastal regions.
Their preferred habitats are the warm waters of the Pacific, Atlantic, and Indian Oceans. They are not found in Europe or Antarctica. You can check local websites to learn about the species present in your area.
Sand dollars prefer sandy and muddy ocean floors, mostly in low subtidal zones.
Their dark color helps them camouflage on the sandy bottom. Some species of sand dollars like clypeaster genus have been found nearly 1000 fit 304.8m below the ocean surface.
Final Stage of Life Cycle of a Sand Dollar:
The life of a sand dollar is full of challenges, from the larval stage to adulthood. Their enemies include sea star, strong water waves currents, which can cause trouble for them and even lead to death.
Protection and Predators
Earlier, I mentioned that sand dollars use different strategies to escape from predators. Although their internal body parts are protected by a calcified shell called a “test,” many sea animals still prey on them.
Seagulls:
One of their biggest threats is seagulls. These birds pick up sand dollars like biscuits, dash them against hard objects like rocks, and eat their internal organs.
Marine Predators:
In the ocean, sand dollar predators include snails, pink sea stars, starry flounders, California sheephead, crabs, octopuses, and haddock.
Challenges of Strong Currents
Strong water currents and tides are also major challenges for sand dollars. They live on sandy or muddy sea floors in shallow waters. However, strong currents often wash them ashore, where they die.
Please be careful not to pick up live sand dollars from the water. If they are purple, and their spines move when touched, they are still alive.
Be attentive: Killing a live sand dollar in South Carolina can result in a fine of up to $500.
From Shore to Silver
When strong currents bring sand dollars to shore, the lack of water causes them to dry out in the sun.
Sand dollars can’t survive out of water for more than two minutes. The sun’s heat turns them silver-white, making them look like coins, which is why they are called sand dollars.
As they dry, their spines fall off, and their internal organs die, leaving behind a dried skeletal structure with teeth. These skeletons are what we usually find on the beach.
My Personal Experience
As a marine enthusiast, I find many sand dollars around the Charleston Bay area. If I see they’re alive, I immediately return them to deeper waters so they can continue their life. If they are dead, I take photographs and sometimes bring them home.
Preserving Dead Sand Dollars
Be cautious: dried sand dollars are fragile. I place them in diluted bleach for 15 minutes, rinse them, and let them dry. This way, they become beautiful ornaments.
But remember, if they are alive, don’t take them out of the water or disturb them.
They play an important role in maintaining the balance of microorganisms in their ecosystem, and they provide calcium carbonate, which other organisms use to build their shells.
Also Read:
- Decomposers Of The Ocean? 06 Important Types Scientifically Explained
- Can Starfish Swim? 5 Must-Know Amazing Facts (Biologically Explained)
- The Lobster Mouth Incident: 01 Inspiring Tale Of Bravery And Bites
Frequently Asked Questions
Sand dollars have a very unique life cycle, this FAQ section covers common questions about their reproduction process, development stages, identity of living and dead , diet etc.
What Do Sand Dollars Consume During Their Lifecycle?
Sand dollars eat tiny creatures, bacteria, and bits of dead material from the sand & sea floor. They use their spines to filter food from the water.
What Content Can Be Found Inside a Sand Dollar?
Inside a sand dollar, organs like a stomach,intestine, mouth, nerves, muscles,reproductive tissues a complex system of water filled canals and teeth are found. These organs help it eat, move, and sense its surroundings.
How Does The Reproduction Cycle of Sand Dollars Occur?
Sand dollars reproduce by releasing sperm and eggs into the water. Fertilization is external . The fertilized eggs become larvae that float in the ocean before settling and growing into adult sand dollars.
What Are The Stages of Development For a Sand Dollar?
The life cycle of sand dollars has four stages: first egg, then larva, and grow into juvenile, and adult. Each stage lives in a different habitat. Their birth is very different from humans.
Sand dollars survive for about 6 to 10 years
Can You Identify Living Sand Dollars Versus Dead Ones?
If you touch a purple or reddish sand dollar you will feel velvety and hair like structure cilia movement. If you see white solid sand dollar then its died, it has empty shell if shaken rastling sound produced called test.
What Are Some Intriguing Facts About Sand Dollars?
Sand dollars are relatives of sea stars , sea cucumbers and sea urchins. It has 250 species. Live 6 to 10 years old. Its larva can spit and clone. Their dried skeleton or tests are collected as ornaments.
How Long Can a Sand Dollar Stay Alive Out of Water?
Sand dollars can only survive out of water for a 2,3 minutes. They need constant access to sea water. If you find a live sand dollar on the beach, don’t bother it, leave it gently into water.
Conclusion:
The life cycle of a sand dollar has four stages: egg, larva, juvenile, and adult. They help clean the ocean and are food for other sea animals. Their white shells wash up on beaches, but live sand dollars should st/ay in the water. They live for 6 to 10 years and are important to the ocean ecosystem.
References :
- Gonad index and larval development of the sand dollar Dendraster excentricus (Echinodermata; Echinoidea) in Baja California, Mexico
- Embryonic, Larval, and Juvenile Development of the Sea Biscuit Clypeaster subdepressus (Echinodermata: Clypeasteroida)
- Sand Dollar Larvae Use Cloning To ‘Make Change,’ Confound Predators
- Analysing the evolution of form and function in sand dollars
- Phylogeny, ancestral ranges and reclassification of sand dollars
