Is there something inside a sand dollar?

Yes, there is something inside a sand dollar. Sand dollars are a type of sea urchin and related to starfish. They have a hard, flattened, circular body that contains their inner organs.

Table of Contents

What is a sand dollar?

A sand dollar is a species of flattened, burrowing sea urchin belonging to the order Clypeasteroida. There are approximately 200 extant species of sand dollars, which are found in oceans all over the world. They live on sandy sea floors, usually just beneath the surface of the sand. Their distinctive flattened, circular shape allows them to burrow effectively.

The top surface of a sand dollar is covered with spines, while the bottom is covered with tiny tube feet that are used for movement and feeding. Their mouth is located on the bottom surface in the center. On the top surface, sand dollars have a flower-like pattern that consists of five paired rows of pores. These pores are part of their water vascular system and are used for gas exchange and waste removal.

Sand dollar anatomy

Although sand dollars appear quite simple and plain on the outside, they have a complex internal anatomy related to other echinoderms like sea stars and sea urchins. Their inner organs are contained within their hard, calcified endoskeleton.

Key features of sand dollar anatomy include:

Mouth – Located on the bottom (oral) surface in the center.

Anus – Located on the top (aboral) surface off-center.
Gut – Runs from the mouth through the center of the body and loops back to the anus.
Water vascular system – A network of fluid-filled canals used for gas exchange, waste removal, and locomotion.

Tube feet – Tiny flexible tubes extending from the water vascular system used for movement and feeding.
Spines – Cover the top surface and are used for protection and sensation.
Lantern – Five small calcareous “teeth” used to scrape food into the mouth.

Additionally, sand dollars have all of the standard organs of a echinoderm including a nervous system, digestive system, reproductive system, and respiratory system.

Inside the endoskeleton

The endoskeleton of a sand dollar is made up of calcium carbonate plates fused together. This protective plating has openings and slits where the internal organs are located. On the oral surface, there are five paired slots known as lunules where tube feet emerge for feeding and gas exchange.

If you were to crack open a sand dollar’s endoskeleton, here is what you would find inside:

Gut – The digestive system runs in a loop from the mouth through the center of the body and back to the anus.
Gonads – The reproductive organs. Sand dollars have five gonads that branch off the digestive tract.
Ampullae – Bulb-like sacs connected to the water vascular system that help pump fluid through the canals.

Hearts – Sand dollars have two hearts on either side of the digestive tract that pump blood.
Muscles – There are various muscles inside a sand dollar for locomotion.
Nerves – The nervous system consists of a nerve ring around the mouth with radial nerves extending outward.

So in summary, inside a sand dollar you would find all the major organ systems including digestive, reproductive, circulatory, muscular, and nervous systems. The bulk of the interior space is taken up by the looped digestive tract and the five gonads where eggs or sperm are produced.

Unique features

Sand dollars have some unique anatomical features compared to other echinoderms:

Flattened body – Their thin, flattened shape allows them to burrow into the sand.

Spines – The spines on their upper surface are very short and fine, unlike the long spines of sea urchins.
Five-part symmetry – Their body exhibits five-fold radial symmetry rather than bilateral symmetry.
Modified tube feet – Their oral tube feet are covered with cilia for collecting food particles.

Lantern – They have a specialized complex of five teeth for scraping food instead of the single tooth of sea urchins.
No pedicellariae – Sand dollars lack the tiny grasping pedicellariae found on sea urchins.

These adaptations allow sand dollars to live buried in the sandy or muddy seabed where they filter feed. Their mouth and food grooves assist them in eating organic matter from the sand.

Sand dollar skeleton

The skeleton of a sand dollar is known as a test. It is composed of calcium carbonate in the form of calcite microcrystals. This material is very similar to the calcium carbonate shells of mollusks or the skeletons of coral.

The test is covered by a thin skin-like membrane called the peristome. On top of the peristome are the spines that cover the aboral surface. The spines are moveable and provide protection.

Key parts of the sand dollar test include:

Petals – The flower-like pattern on the top surface formed by pores.
Ambulacral areas – The five broad avenues between the petals where the tube feet emerge.
Interambulacral areas – The five zones at the edges of the petals.

Lunules – The slit-like pores near the edge of the oral surface where the tube feet protrude for feeding.
Apical disc – A circle on the aboral surface containing the anus, genital pores, and madreporite (part of the water vascular system).

The complex shape and structure of the test with all its openings allows sand dollars to thrive in their sandy ocean habitat.

Sand dollar habitat and ecology

Sand dollars are found in oceans all over the world. They live on sandy or muddy seabeds from the intertidal zone down to depths exceeding 6,000 feet. Most species live partially buried in the sediment.

Some key facts about sand dollar ecology and habitat include:

Feeding – Sand dollars filter organic particles from the sand using their tube feet. Their oral tube feet use cilia to pass food to the mouth.

Burrowing – They use specialized tube feet to burrow themselves just under the surface of soft sediments.
Habitat – Sand dollars live wherever there are clean sands or muds on the continental shelf. Different species prefer different sediment types.
Range – They are found everywhere from tropical waters to polar oceans if suitable sediment is present.

Migrations – Some species migrate seasonally to deeper waters to escape extreme temperatures.
Predators – Crabs, fish, sea stars, and other echinoderms prey on sand dollars.

Sand dollars play an important ecological role helping oxygenate sediments, recycling nutrients, and serving as prey for other species.

Behavior and movement

Sand dollars display a variety of interesting behaviors and types of movement:

Burrowing – Using specialized tube feet, they burrow themselves into sediment with only their spines exposed.
Circling – Some species move in a circular pattern while others zig-zag.

Seasonal migration – They migrate offshore to deeper waters during very warm or cold seasons.
Righting reflex – If flipped over, they will quickly right themselves.
Aggregations – Large groups sometimes aggregate together, possibly for spawning.

Avoidance – They use their tube feet to detect and rapidly retract from threats.

Their ability to bury in the sand helps protect them from predators and sudden temperature changes in their environment.

Sand dollar feeding

Sand dollars are suspension feeders, meaning they eat tiny organic particles suspended in the water. Using their specialized tube feet, they pass these particles to their mouth after filtering them from the sand or mud.

Specifically, sand dollars feed using the following methods and structures:

Tube feet – Cilia on their oral tube feet catch organic particles and transport them to the mouth.
Lantern – Their five-part calcareous “lantern” structure scrapes food into the mouth opening.

Burrowing – While buried in sediment, their oral podia extend above the surface to feed.
Water vascular system – Supplies water to the tube feet for capture and transport of particles.
Mucus – Mucus traps organic matter, which is moved by cilia on the oral tube feet.

Sand dollars are not scavengers or hunters. They do not eat large pieces of food. The microalgae, bacteria, protozoa, and detrital matter they consume is passed to their mouth using a constant filtering process.

Reproduction and life cycle

Sand dollars reach sexual maturity around 2 years of age. They reproduce by broadcast spawning, releasing their eggs and sperm freely into the water. After fertilization, the eggs develop into planktonic larvae.

Key facts about sand dollar reproduction include:

Hermaphrodites – They produce both eggs and sperm but usually function as just one sex.
Gonads – Gametes develop in five gonads that branch off the digestive tract.
Spawning – Mass spawning events are triggered by environmental cues like temperature and photoperiod.

Fertilization – Eggs and sperm combine by chance in open water after release.
Larval forms – Fertilized eggs develop into bilateral larvae that eventually metamorphose into juveniles.
Settlement – After 2-4 months, larvae settle to the seabed and adopt the adult radial body form.

This complex life cycle allows sand dollars to produce huge numbers of offspring. Only a tiny fraction will survive to adulthood.

Evolutionary history

Sand dollars belong to the phylum Echinodermata which also includes sea stars, sea urchins, sea cucumbers, and crinoids. They share a common ancestor with other echinoderms dating back to the early Cambrian over 500 million years ago.

Highlights of sand dollar evolution include:

Cambrian – Earliest echinoderm fossils.
Ordovician – Extinction event wiped out two-thirds of marine invertebrates including many echinoderms.
Carboniferous – Modern crinoids appeared.

Triassic – Sea stars and sea urchins diversified.
Jurassic – First flattened sand dollar fossils appeared.
Cretaceous – Rapid evolution produced many new sand dollar genera.

Cenozoic – Further diversification to ~200 modern species.

Today’s sand dollars retain the same basic five-part body plan of their distant ancestors. Over hundreds of millions of years, they adapted this form for a burrowing lifestyle.

Uses and significance for humans

Throughout history, sand dollars have had various uses and significance for humans, including:

Food – Some cultures eat sand dollars. They may be eaten pickled, fermented, dried, or raw.
Folk medicine – Sand dollars were used in folk remedies thought to cure conditions like asthma, ulcers, and cardiovascular disease.
Fertilizer – Dried sand dollars were used to fertilize gardens. Their calcium carbonate offers some benefits.

Crafts – Craftspeople and artists use them to make jewelry, frames, mirrors, Christmas ornaments, etc.
Currency – In the Bahamas, sand dollars were once used as local currency.
Collectibles – Beachgoers commonly collect and preserve them as souvenirs.

Today sand dollars are more often collected for artistic purposes or left untouched to play their role in the ocean ecosystem.

Threats and conservation

Most species of sand dollars still have large, stable populations and are not at risk. However, a few key threats to sand dollars exist:

Habitat degradation from pollution, trawling fishing, dredging, etc.

Harvest for food, medicine, or crafts.
Climate change impacts like ocean acidification and rising temperatures.
Invasive species that may disrupt their habitat or prey on them.

Coastal development that destroys sandy marine ecosystems.

Conservation efforts for sand dollars include protecting marine habitats, establishing no-take reserves, managing fisheries sustainably, enforcing harvest regulations, and reducing pollution. With ongoing stewardship, their unique ecology and evolutionary history can continue.

Conclusion

Although they appear simple, sand dollars have a fascinating internal anatomy and complex life history. Their flattened circular shape disguises an echinoderm with five-fold symmetry, a looped digestive tract, intricate water vascular system, specialized tube feet, and remarkable burrowing capabilities. Inside the porous endoskeleton is everything needed to filter feed, move through sediment, spawn new generations, and thrive in sandy seabeds around the world. Sand dollars offer a glimpse into echinoderm evolution and continue to play an important ecological role in marine ecosystems.