Stars of the Sea

A guest post from Lisbeth Bornhofft
Lisbeth is a Senior Aquarium Educator who has also contributed to the Rescue Blog. You may have seen her around the building answering visitors' questions or offering up an interesting bit of information about a particular animal or exhibit. Here she shares her knowledge with readers of the Exhibits Blog!

If you spend some time at the Edge of the Sea exhibit, you can learn some fascinating facts about the most popular echinoderm and one of the ocean's most recognizable inhabitants: the sea star.

Sea star in Acadia National Park, ME, photo credit: Aldaron via Wikimedia Commons

For example, an Aquarium educator might teach you how sea stars circulate salt water throughout their body and use it to propel their tube feet, which helps them get around. They also have clusters of nerves called eyespots that help them detect light and dark.

Tip of sea star arm with eyespot and sensory tube feet

Some sea stars eat mollusks, like clams and mussels. Others dine on a variety of foods like microalgae, other sea stars or even small fish! (Learn about other cool sea star eating habits here and here.) Their mouths are located on their bellies. Many species are able to start digesting their food before it goes into their bodies by everting their stomachs!

Northern sea star with stomach everted, photo: Lisbeth Bornhofft

During a busy week recently, our visitors asked some interesting questions that I thought I'd share with you.

I used to see so many sea stars when I was little and now I can never find them. 
It seems there are fewer sea stars than there were 50 years ago, but the truth is that sea star populations have not been studied enough for us to know how their numbers have changed and what has impacted them. Sea stars are subject to all the usual threats, such as chemical pollution, coastal development and warming oceans, plus over-collection as ornamental keepsakes. Since water is pumped directly into their bodies via the water vascular system, sea stars have little or no ability to filter pollutants and toxins out of the water. This makes them highly susceptible to damage from pollution and contaminants.

I have a sea star on my Christmas tree. It was already dead when I brought it home from the beach.
Most the sea stars that you see in the wild are probably alive. Dead sea stars disintegrate quickly unless left high and dry. Even an active sea star will pull in its tube feet when disturbed and may appear to be dead.

Do sea stars have a skeleton?
Sea stars are invertebrates, but they do have a supportive structure (endoskeleton) composed of calcium carbonate components, known as ossicles.

That sea star has fungus on it.
Sea stars breathe through their skin, which is covered with small, finger-like papula for the exchange of gases. When these "skin gills" are extended, the sea star might look fuzzy, but it's not fungus.

Check out these close-up clips showing the surface of a sea star!

Can sea stars pinch?
Well, that all depends on how big you are! They can certainly pinch small creatures that crawl on top of them. Sea stars have small pincer-like structures called pedicellariae that cover the upper (aboral) surface. They use these pincers for protection, but also to help remove debris (detritus) that has fallen from above.

A magnified image of a tiny caprella skeleton shrimp on Forbes sea star, photo: Lisbeth Bornhofft

Are sea stars poisonous? 
None of the sea stars at the Aquarium are dangerous to humans, however many species of sea stars in the wild do contain toxins. People of some cultures have been known to eat sea stars, especially the gonads, but there are also reports that dogs and cats have become violently ill or died after consuming sea stars. Of the approximately 2,000 types of sea stars, the crown-of-thorns may be the only sea star that is venomous. Filled with toxin, the spines can break off and penetrate the skin causing a sharp burning pain, swelling and numbness.

Crown-of-thorns sea star photographed by Aquarium researcher Randi Rotjan. Read about her research expedition to the Red Sea on the Global Explorers Blog.

Why do they curl their arms up?
At our exhibit, this behavior is commonly thought to be a sign of stress. When we see that, we usually give the animal a little time-out so it can rest. In the wild, however, a sea star can move to a different mussel or oyster bed by curling its arms to release its hold and drift with the tide.

How do sea stars hold on?
Contrary to popular belief, the gripping action is a function of adhesive chemicals rather than suction. A second adhesive secretion breaks the bonds and allows the tube feet to be released. Tube feet consist of internal bulbs called ampullae and external podia, (feet). The ampulla forces water into the foot, which expands to contact the rock or other substrate.

The undersides of a Northern sea star, photo: Lisbeth Bornhofft

How can they live without a brain?
Lots of species don't have brains and have been successful on our planet (like jellies, for example)! Echinoderms like the sea star have somewhat complex nervous systems, but don't have a true centralized brain.

Do sea stars have eggs?
Yes, and sperm, too. Sea stars usually reproduce by broadcast-spawning: They release their gametes into the water where they are fertilized by gametes from the opposite sex. Sea stars probably use environmental signals as a cue to gather in groups when they are ready to spawn. Some species can also reproduce asexually by regeneration. A detached arm can sometimes develop into another sea star.

Look at the baby sea star!
A "baby" sea star is so small that you would need a microscope to see it. Besides, you would probably not even recognize it as a sea star—the larval form is a completely different shape and is free-swimming.

Are there freshwater sea stars?
Sea stars do not to have mechanisms for osmoregulation, that is, they are not able to adjust the concentration of salt in their bodies. They keep their body fluids at the same concentration as the surrounding water. Although some species can tolerate brackish water, the lack of osmoregulation explains why sea stars are not found in fresh water.

Do you have other sea stars at the Aquarium?
There are sea stars (Asteroidea) or brittle stars (Ophiuroidea) in almost every gallery of the Aquarium! Ask someone with an Aquarium name tag to point them out to you.

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