18 April, 2000

Amphipod Bioassay

Question 59: What baleen whales are found in the Southern Ocean?

Today is windy and overcast. We are inside for the day and I am setting up another amphipod assay. Almost a month ago our science group took a boat trip to Laggard Island to collect algae and amphipods in the intertidal pools. After several weeks of experimenting with different ways to feed the amphipods, Chuck Amsler and Jim McClintock have found a procedure that works well. Over the last week Chuck and I have been running two bioassays almost every day. We will continue on this schedule until just before we leave Palmer Station.

A bioassay is a test using living organisms to find out more about the properties and chemical make-up of a substance. In this case we are using amphipods' feeding habits to tell us more about the chemicals in different types of algae. The amphipods, small (1 cm) crustacean invertebrates, are herbivorous (they eat algae). Unlike the fish (Notothenia coriiceps) and starfish (Odentaster sp.) bioassays, this is a preference bioassay rather than an acceptance/rejection one. This means that the amphipods are offered both options at the same time and we compare the amounts consumed.

We store the amphipods in large jars with screened openings that float in the saltwater aquarium tanks. When I set up an assay, I have to be careful at all times to keep the temperature of the water they are in from warming up. They are kept on ice if they are out of the saltwater tank. For organisms that are adapted to living in the extremely cold temperatures found here, an increase of five degrees in their water can be fatal. Within the Southern Ocean, water seldom ranges more than four degrees from high temperature to low temperature. We mix powdered extract from the macroalgae with alginate that then hardens into a gel (a texture like firm jello) in a petri dish. We also use the same type of alginate to make a control gel that has no extract added to it. The alginate is what is called a feeding stimulant, something the amphipods would normally be willing to eat. After the plates solidify, we use a tissue corer to cut each gel into 20+ small round discs. Dye is added to the experimental alginate so we can tell the difference.

After I dump all the amphipods in one storage jar into a shallow tray, I use a large glass pipette (calibrated tube with a squeeze bulb on one end) to put 20 amphipods into each of 10 small jars. They move around so quickly that they are difficult to count in large groups, so I usually try to draw five of them into the pipette at a time. Once I have 20 in each jar, I fill the amphipod jars to the top with salt water and fill 10 identical jars without amphipods with salt water also.

Before the two discs (control and experimental) are added to each jar they have to be weighed and the starting weight recorded. After adding the discs, all 20 of the jars are returned to the large saltwater tanks to sit for 12 hours. After 12 hours, I collect the jars, put them on ice and take them to the lab. There I retrieve all the alginate discs and record their ending weight. The discs in the 10 jars without amphipods are controls. Alginate discs absorb water during the experiment, changing their weight. We need to have an accurate weight for the amount of disc eaten by the amphipods and to get that we need to be able to control for how much weight the discs gain from the water.

It is often clear from just looking at the chewed-on discs whether the amphipods preferred the control disc or the disc with the algae extract added to it. In only two of the assays so far have the amphipods had no preference between the two. We try to keep an eye on the jars every couple of hours after the halfway point as some groups of amphipods eat faster than others. If all of the discs are consumed, there will be nothing to measure and no data at the end of the 12-hour assay.

This preference information gives us a general idea of which algae have secondary metabolites that function as feeding deterrents. There are many functions that secondary metabolites can perform, and protecting the algae from grazing is only one possibility.

Answer 58: Baleen whales are whales who have plates made of keratin (the same protein found in fingernails) hanging from their upper jaws in two rows along the sides of their mouths. These plates are edged with fringe-like bristles and act as a filter to strain plankton from seawater.