Why Do Sand Dollars Have Holes?




As a marine biologist with a love for sand dollars, I am often asked why these beautiful echinoderms have holes.

Sand dollars have holes to facilitate water movement for feeding, respiration, and buoyancy control.

While many people assume that these holes are caused by predators, the truth is much more fascinating.

In this blog post, I will explore the reasons why sand dollars have holes and share some interesting facts about these unique creatures.

What are Sand Dollars?

Before we get into why sand dollars have holes, let’s first talk about what they are. Sand dollars are a type of echinoderm, which means they are related to sea stars, sea urchins, and sea cucumbers.

They are flat and round, with a hard exoskeleton that is covered in tiny spines. Sand dollars are found in shallow waters around the world, and are often found washed up on beaches.

The Purpose of Sand Dollar Holes

So, why do sand dollars have holes? The holes in sand dollars actually serve a very important purpose.

These holes are part of the sand dollar’s respiratory system. Sand dollars breathe through a series of small holes called “pores” on their exoskeleton.

These pores lead to a complex system of tubes that circulate water throughout the sand dollar’s body, allowing it to extract oxygen from the water.

The larger holes that are often seen on the top of sand dollars are called “lunules”.

These lunules are also part of the sand dollar’s respiratory system, and are used to expel excess water and waste from the body.

Predators and Sand Dollar Holes

While sand dollar holes are not caused by predators, predators can take advantage of them. Sand dollars are a favorite food of many marine animals, including sea stars, crabs, and fish.

These predators will often attack the soft underside of the sand dollar through the holes in its exoskeleton, making it easier for them to get to the sand dollar’s soft tissue.

Why Do Sand Dollars Have Holes?

The Lifecycle of Sand Dollars

Sand dollars have a fascinating lifecycle that begins with a tiny, free-swimming larva. As the larva grows, it develops a hard exoskeleton and settles on the ocean floor.

Once settled, the sand dollar begins to feed on plankton and other small organisms. As it grows, it sheds its exoskeleton and grows a new one.

Sand dollars can live for up to 10 years, and can grow to be several inches in diameter.

Interesting Facts about Sand Dollars

Now that we’ve explored the reasons why sand dollars have holes, let’s take a look at some interesting facts about these unique creatures:

1. Sand dollars are closely related to sea stars, and are often called “sea biscuits” or “cake urchins”.
2. Sand dollars can be found in a range of colors, including purple, green, and blue.
3. Sand dollars are important members of the ocean ecosystem, as they help to keep the ocean floor clean by feeding on detritus and other small organisms.
4. The patterns on the top of a sand dollar’s exoskeleton are unique to each individual, and can be used to identify different species.
5. While sand dollars are often found washed up on beaches, it is important to remember that they are living creatures and should be treated with care.


In conclusion, sand dollars have holes as part of their respiratory system, and these holes are not caused by predators. Sand dollars are fascinating creatures with a unique lifecycle, and are an important part of the ocean ecosystem.

So, the next time you come across a sand dollar on the beach, take a moment to appreciate its beauty and remember these interesting facts about this amazing echinoderm.


What are the five holes in the sand dollar?

The five holes in a sand dollar are called lunules and are used for respiration and movement.

How long before a sand dollar dies? Sand dollars typically live for about 10 years in the wild, but their lifespan can vary depending on factors such as environmental conditions and predation.

Once a sand dollar dies, its body will begin to decompose and its skeleton will eventually wash up on shore.

What does a dying sand dollar look like?

A dying sand dollar may appear discolored, limp, and may have a foul odor.

Its spines may also be missing or broken off.

How long can a sand dollar be out of water before it dies?

Sand dollars can survive for a short period of time out of water, typically around 10-15 minutes, but it is best to return them to the water as soon as possible to ensure their survival.

What happens when a sand dollar dies? When a sand dollar dies, its hard outer shell will eventually become bleached and brittle due to exposure to the sun and elements.

The internal soft tissues will decompose and be consumed by scavengers such as crabs and other small marine organisms.

What are the holes on sand dollars?

The holes on sand dollars are called lunules and they serve as an opening for the water vascular system, which helps the sand dollar move and breathe.

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