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Headline: Susan's Daily Journal

It Looks Delicious, But Will a Starfish Eat It?
22 January 2002

Chris Petrie is a graduate student at the University of South Florida. He is also one of the divers who has been doing research here at Palmer Station.

Chris is investigating the chemical defenses of sponges. He wants to see if they make chemicals in their tissues so that other animals won't eat them. Chris is using the starfish as a "test animal". That means he is finding out what the starfish will eat, but he is really interested in finding the chemical that makes a sponge less delicious to other animals. (And, he chose the starfish as a test animal because this starfish will eat almost anything!)

So, first Chris went out and collected a lot of starfish and he put them in a big tank in the aquarium room.

Then Chris went diving to a depth of 100 feet and brought back a big red sponge. He says that the skin of the sponge looks a lot like a football. He fed pieces of the sponge to the starfish to see if they would eat it.

If the starfish didn't eat the sponge, then Chris took the sponge and soaked it in chemicals. He analyzed this fluid to see what chemicals were in the sponge tissues.

Then he made some test pellets out of some of the sponge chemicals, some powdered krill, and alginic acid. (This is made from algae and it is used like gelatin to bind things together. In fact, if you have ice cream in your house, it probably has alginic acid in it, because that helps keep the ice cream ingredients together.)

Then Chris fed the pellets to the starfish, to see if the starfish would eat them. If they did eat the pellets, then he made different pellets with other sponge chemicals in it. If they didn't eat the pellet, then he might have found the chemical that the sponge makes as a defense. It would be a chemical defense against being eaten by some other animal. Chris has found a couple of sponge chemicals that the starfish decline to eat! When Chris gets back to Florida, he will do more analysis on these chemicals!

Here's the math: the starfish live in a circular tank that is 780 centimeters (cm) high, and the water level in the tank is 120 cm down from the top. The diameter of the tank is 2 meters. What is the volume of the tank? (One liter equals 1,000 cubic centimeters.)

Chris Petrie beside the tank containing starfish. This starfish has just been fed a pellet. Chris places the pellet on the underside of one of the starfish arms. Here is another view of the starfish.
Chris and a package of freeze-dried sponges. Yummmm--freeze-dried sponges.

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