22 March, 2000

Odentaster Bioassay

Question 32: How deep do penguins dive?

Over many dives we have collected not only macroalgae and sponges to examine for secondary metabolites (chemicals not actively used in life-sustaining processes) but also quite a few starfish. One species that is abundant is Odentaster sp., a predator who eats mostly invertebrates. Odentaster is one of the organisms that we use for bioassays.

In general, an assay is a test used to analyze a substance to find out the nature and proportion of the ingredients. In a bioassay, we use a predatory organism (in this case, a starfish) to make the test. We offer it food pellets treated with extracts or parts of organisms we suspect use chemicals to defend themselves and see whether the star accepts (eats) or rejects them.

Starfish use their tube feet to perceive chemicals as well as to move themselves and other things around. It is especially easy to offer test food pellets to Odentasters because of the way they climb the wall of their tank to the surface of the water and lay back, exposing the underside of their "legs." The food pellet is placed on the exposed tube feet. The time it takes the star to walk the food to its "mouth" or to drop the food is recorded and used as a measurement of how preferred or distasteful the sample is. Brachiopods are shelled bivalve-like (not a mollusc) invertebrates that attach to the substrate on a short stalk called a pedicle. They are 1-6 cm long. They use two lophophores (long tentacles covered with cilia) to create currents that pull their food (diatoms, other plankton, and colloidal material) into their digestive tract. Two years ago, Jim McClintock found that the tissue of a brachiopod was distasteful to Odentasters.

This time, Andy Mahon is looking for which specific type of tissue (reproductive, digestive, lophophore, pedicle etc.) has the chemicals getting that response. Since secondary metabolites are expensive (energy consuming) for organisms to make, Andy thinks that they will be only in certain tissue rather than all tissue. One of the uses of secondary metabolites is as a feeding deterrent (defense). They can also have structural roles since they tend to be large compounds.

To make a meaningful test, he offers the same type of pellets or tissue samples to several starfish. These repetitions are called replicates. If he offered the treated pellet to only a single starfish, there would be no way to screen out possible errors and no way to generalize about Odentaster preference. That individual starfish might not feel like eating, it may not be a good representative of Odentasters, or it might be clumsy and drop the pellet accidentally. However, if Andy finds that nine out of ten Odentasters reject that type of pellet, he has more confidence that Odentasters in general don't like the chemical extract it contains. The more replicates he uses, the less likely it is that random error is affecting his findings.

Andy also offers the same group of replicate starfish a control pellet exactly the same as the test pellet except without the chemical extract. This helps him tell the difference between a pellet rejected because of its content and a pellet rejected because a star is not eating. For Odentaster, the pellet base is a mush made out of krill. Mixing it with other compounds and dropping it into a calcium chloride solution allows Andy to make solid pellets that hold together.

Answer 31: In March there is the smallest amount of ice cover; in September there is the maximum amount.

Dropping pellet mixture into calcium chloride solution with pipette.

Cluster of brachiopods attached to a rock.

Finished bioassay pellets.