Slide Sea Star

Banded Sea Star © Stephani Pyron

SEA STARS AND SAND DOLLARS

Nothing says “the beach” more than an image of a Sea Star (commonly called a starfish) or a Sand Dollar (a type of Sea Urchin). They bring to mind white sandy beaches and relaxing walks along the surf. Beachgoers are very familiar with these images, but know very little about the live creatures.

 

Sand dollars and sea stars are in a group of invertebrates called echinoderms that means spiny skin. They are the only invertebrates with an internal skeleton and internal cavity, which allows for the development of internal organs. Much of this cavity is filled with the reproductive organs – and they can produce a LOT of offspring. Most species have males and females, but reproduce through external fertilization. They will gather in large groups and mass release gametes into the water. Some sea stars can produce as many as six million fertilized eggs during one spawning event.

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Sand Dollar and Jellyfish © Diane Furrow

Echinoderms are so much more than the skeleton some take home as a souvenir. Live sand dollars are not white and are covered in flexible bristles. Sea stars are amazing in action. They are carnivores and hunt slow moving or sessile marine creatures. They can grab prey using their tube feet, pull the shell open and invert their stomach inside of the prey’s shell to consume it. A process called evisceration. It is really something to see.

 

Sea urchins use their tube feet to move across rocks grazing on algae with a series of five teeth that extend from the mouth called an Aristotle’s Lantern.  However, the Aristotle’s Lantern in a sand dollar is greatly reduced and does not extend from their mouths. They typically feed on organic material in the sand.

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Sand Dollar and Sea Urchin © Diane Furrow

Echinoderms have pentamerous radial symmetry. Meaning there are five sections of the body that radiate out from the central disk. Most sea stars, though not all, have five arms. Sea urchins have five arms but roll them into a ball and cover that with long spines. Sand dollars are just sea urchins that have been flattened like a pancake and have very short spines. Echinoderms can regenerate body parts. If you remove an arm from a sea star, it can grow a new arm back. If it is removed with enough of the central disk material, that arm could regenerate into a new sea star. Sea stars, sand dollars, and sea urchins do not survive long out of water, turning into brittle skeletons in no time.

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Royal Sea Star © Diane Furrow

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