Feeding Experiments

Photos from UAB in Antarctica group on Flickr, related to this post
By Craig
Posted on 04/09/07

Feeding assays are another important part of my research in Antarctica.  Without them, it is impossible to determine if algae are unpalatable to herbivores.  Dissecting amphipods and looking at the guts may provide valuable information about species preference and ingestion, but do not actually confirm that ungrazed algae cannot be grazed upon. 

Along with animal feeding preferences (which may include nutritional content), absence of a particular alga in the gut may also indicate a lack of accessibility or simply inconvenience (would you cross town to get a burger if you lived right next door to Fatburger?).  

A feeding assay is an effective experiment to discern if amphipods prefer certain algae over others.  It would be the equivalent of offering me a dinner choice of chocolate chip cookies (the soft, gooey kind) or liver and onions.  I am going to choose the cookies seven days a weak (twice on Sunday). 

To conduct a feeding assay you first have to gather a horde of amphipods.  This is the easy part.  Our data show that particular species of algae harbor many more amphipods than others.  Grab a Gigartina skottsbergii off the bottom and you get a total of 10 amphipods, grab a Desmarestia menziesii and you could increase the total amphipod count by 3000. 

I find it amazing how my favorite alga changes daily depending on whether my job for the day is to collect or dissect all the amphipods!  I have also noticed that my professors daily algal preference change for the exact opposite reasons.  “Hey, are you almost done processing the amphipods in that Desmarestia because today I managed to get you one that was really big” (and they wonder why I cry).

Once you have collected a bunch of amphipods, you have to sort them and separate them by species.  This entails grabbing a large pipette for siphoning, picking a particular species, and playing the marine science version of “Where’s Waldo?”  Not easy when Waldo and all his cohorts are constantly MOVING!!! 

However, starting with some abundant species like Gondogeneia antarctica, Prostebbingia gracilis, or Metaleptamphous pectinatus (Cracker-Jack amphipod…still love that name) reduces the frustration of tracking down the less abundant species.  Soon, you have a plentiful amount of amphipods (300-400) for a variety of different species.

Now, the food.  Start by grabbing specimens of algae that you think are potentially unpalatable and one you know your amphipods will eat.  Currently, I am testing a group of filamentous red algae against Palmaria decipiens (the supposed chocolate cake of the Antarctic amphipod world).

Once you have selected your algae, divide them up into equally sized pieces, weigh them, and record your their initial weights.  Next, gather a bunch of plastic bottles and fill them with seawater.  Add one piece of your pre-weighed algae along with a set number of amphipods (all the same species) to each bottle.  For instance, one set of my bottles each contains a piece of Palmaria and ten Gondogeneia antarctica.  Once all your bottles are sealed, put them into a flowing seawater table to ensure that there is no temperature shock.

When the testing period is up, gather your bottles and remove each algae piece.  Take special care of the amphipods as they are reusable, allowing you to skip time consuming steps 1 and 2!  Weigh each of the algal pieces again and record the final weight.  Now you can compare these weights to your initial weights and see how they have changed. After several experiments using different species, you can compare notes and determine which algae amphipods heavily consume versus algae that are unpalatable!

Follow these simple instructions and you are ready to set up your own Antarctic feeding assay.  WAIT, you do not have access to Antarctic macroalgae or…even….amphipods (sniff).  Oh well, gather all the neighborhood children and offer them either chocolate chip cookies or liver for dinner.  Get back to me on the results.

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