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Home › Sea Life › Marine › Invertebrates › Echinoderms › Sea Urchins › Sand Dollar

Fun and Interesting Facts about Sand Dollars

[Sand Dollar Phylum: Echinodermata] [Classification: Echinoidea] [Order: Clypeasteroida]

The early forms of Clypeasteroida (flattened, disc-shaped, burrowing sea urchins) first appeared around 60 million years ago.

Check out these curious facts about sand dollars (echinoids), including where they come from, what they eat, and how "sea cookies" reproduce.

Sand Dollar Global Distribution and Habitat

Common sand dollars are found widespread in the intertidal zones of oceanic waters in both hemispheres.

Apart from some extreme depths and polar regions, sand dollars are abundant in many areas, but especially:

• Atlantic coast of Africa
• North and South America
• Japan (Hokkaido, Kyushu, and Okinawa)
• The Indian and Pacific Oceans

Like most of the sea urchin species, sand dollar habitats tend to be coastal seagrass beds, kelp forests, or rocky substrates with moderate wave action.

These neritic zones situated slightly below the surface - and even down to lower depths approaching ninety (90) metres (300 feet) - are where herbivores graze on different types of algae and feed on suspended food particles like detritus and plankton.

Interesting Fact: Even though the scientific name of the common sand dollar is Echinarachnius parma, the origin comes from its resemblance to the old Spanish dollar (piece of eight) which was silver in colour and measured 1.5 inches (38 millimetres).

Sand Dollar Characteristics

The adults are typical of most echinoids (from the phylum Echinodermata). They exhibit radial symmetry with five (5) rows of pores (tube feet) in the center of the endoskeleton, measuring between five and ten (10) centimetres in diameter (up to 4 inches).

However, when a sand dollar dies, it loses its spines and leaves a smooth, sun-bleached and coin-shaped skeleton, often found washed up on sandy beaches.

So, their flattened appearance also created several other vernacular names used in different regions, including the sea cookie, snapper biscuit, cake urchin, and sand cake.

Sand Dollar Anatomy

The internal anatomy of a common sand dollar is made of rigid calcium carbonate skeleton (called a "test") - which is typical of all Clypeasteroida members. Thus, sand dollars are marine invertebrates that do not have a backbone (vertebral column).

The mouth opening is located at the central point of the star-shaped grooves on the underside (oral surface) of the sand dollar.

These petal shaped perforations contain podia, which are important for gas exchange, and maneuvering food along the grooves towards its mouth.

When the sand dollar is living, the skeleton consists of five radial calcium plates covered with coloured velvet-textured tube feet and spines, often blue, green, or purple.

But wait - there's more:

Sand dollars use their spines to slowly crawl over the sandy ocean floor.

Tiny hairs (called "cilia") cover the skin and spines and when they die, dead sand dollars lose their textured skin and hairy spines resulting in a sun-bleached white body form.

The sand dollar mouth and anus are located at the center on the bottom and at the rear of its body. The anus of most sea urchins is located at the top (epibenthos).

Yet, the sand dollar anatomy has adapted so they can burrow beneath the sand (endobenthos) even though the anus is also underneath.

Fun Fact: The colony formation of sand dollars can be extremely thick, made up with dense aggregations (called dense beds) of hundreds per square metre.

Types of Sand Dollar Species

What Do Sand Dollars Eat?

Even though the sea urchin (Echinoidea) is mainly a herbivorous grazer, they supplement their diet with other food sources, depending on their habitat.

Sand dollars are mostly deposit and suspension feeders, meaning they consume tiny organic particles that settle in the sand or drift with prevailing water movements, such as:

• Diatoms (microscopic algae with silica shells)
• Detritus (tiny bits of decomposed plants and animals)
• Microorganisms and organic film coating sand grains
• Plankton (microscopic algae and larvae)

How Do Sand Dollars Reproduce?

After reaching sexual maturity (between 2 and 4 years old), the reproduction strategies of sand dollars takes place through the release of gametes (specialised male or female cells) into the water.

Gametes of sand dollars unite with opposite sexed cells in a process known as sexual reproduction - with external fertilisation.

Key points...

Nektonic larvae can swim independently of the water currents during their metamorphosis stages. In fact, a recent biological study found that sand dollar larvae use asexual cloning as a self-defense mechanism.

As they mature, the skeleton (test) starts to develop and the sand dollar becomes one of the benthos organisms, living in (or near) marine sedimentary seabeds.

Fun Fact: Scientists can determine the age of a sand dollar by counting the number of growth rings displayed on its test (flattened shell).

Sand Dollar Predators and Threats

Having reached full maturity, sand dollars only have a few natural predators in the wild. Nonetheless, they are a welcome source of nutrients for some large scavenger fish, such as:

• Flatfishes (e.g., flounders)
• Triggerfish
• Wrasse fish

All that being said, some of the worst human-induced threats to the long term survival of sea urchins include:

• Climate change (resulting in varying degrees of coral bleaching).
• Coastal habitat degradation and destruction.
• Underwater garbage pollution.
• Overfishing (particularly in countries where ocean urchins are considered as being a culinary delicacy, such as Europe and Japan).

Interesting Fact: The International Union for Conservation of Nature (IUCN) does not see any specific risk for the population status of sand dollars (classification Echinoidea).

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