Amazing Nature: Sea stars: the fish that are not fish

Sea stars

Sea stars are once again the stars of science education in the Ocean Science Discovery Lab as Kodiak’s fifth-grade students learn about how to conduct a scientific experiment, measure and record data, make graphs and discuss their results.

I have trained myself to call them “sea stars” although for years I thought their name was “starfish.” On an email list of marine educators that I watch closely, there has recently been an ongoing and extensive discussion about the use of these terms.

The name change from “starfish” to “sea star” was driven by the reasoning that starfish are not fish, and thus, should not have the misleading term as part of their name. Some educators claim that people confuse them with fish, if they are called “-fish.” Henceforth, starfish became sea stars and jellyfish became sea jellies.

However, old habits die slowly and we now face a situation where the same animals are sometimes called by one name, and sometimes by another, causing further confusion. Many of those on the email debate welcomed the muddle, arguing that it is a great opportunity to educate about the differences between the respective animal classes.

While I agree that we should call things by their proper names and education should avoid misleading those we seek to educate, this debate about names of sea creatures lends itself to some funny extensions.

Will we change the name cuttlefish to sea cuttle (or sea cuddle), and will we look for the hooves on a sea horse? What about the sea dragon (which, by the way is different from the dragon fish)? There is a lion fish, which is a fish, and there is a sea lion which is not a lion! You could have some fun finding more examples of this nature.

This year, Kodiak students are presented with a number of different sea star species when they come to the Ocean Science Discovery Lab for their science field trip. Among them, is the blood star, so named for the bright red color of some specimens, though blood stars can come in orange, pink and shades of red.

One student, sitting in front of another species, a mottled sea star, said wistfully, “I bet I can make this one a blood star, too.” I smiled at him and shook my head. Then I went on to explain that even if that sea star did become the victim of some brute violence, (which of course would not be tolerated, because we teach respect for all sea creatures), it would still not become a bloody sea star, for sea stars do not have blood.

The main function of blood in other animals is to transport oxygen and nutrients to the various organs. In sea stars, the same function is conducted by channeling seawater through their vascular system.

The lack of blood is just one of many amazing sea star features. As invertebrates, sea stars have no backbone. In fact, there is no bone in their bodies at all. Instead, they have spicules of bony material embedded in their skin to provide the necessary rigidity.

Another organ they are lacking is a brain, though there is a nerve ring going around the central disc with an extension into each arm. If one of these arms is lost, it can grow back given enough time. However, it is a myth that you can cut a sea star in two and it will live on as two individuals.

Each sea star has a small light dot on the upper side, called a madrepore. This is the water intake and is essential for the body functions of the animal. In other words, only the part of the body that has the madrepore can grow back arms. Sometimes, when one arm was bitten off, the cells along the wounded edge end up creating two new arms, which is why sometimes we can find sea stars with fewer or more than the usual number of arms.

The blood star usually has five arms, and so do the mottled sea stars, the red-banded, the flat-bottom, ochre and leather sea stars. However, there are also some common sea stars with more than five arms. Among those are the rose star, which usually has 11 arms, the sunstar, and the sunflower sea star. The latter is common and conspicuous on Kodiak beaches, growing to impressive size and growing more arms the older it gets.

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