Sizing Up the Food Chain
February 10, 2011 
5 Comments
The microscope room on the Palmer is just big enough for a slide preparation table and two bulky microscopes. It’s dark inside, and the microscope tabletops are designed to rock back and forth on an air cushion to counteract the ship’s rolling. As they do this, they make a deep sighing that sounds like Darth Vader is behind you.
Staring through a moving microscope and looking at a watery slide on a rocking ship in the dark is a prescription for seasickness. But the biologists on this voyage spend many hours per day in here counting cells, and I haven’t heard them complaining yet.
At one point Dr. Angelicque White sat me down and showed me some beautiful diatoms, including one that I called ‘grass skirt’ because it had a fringe of long spines around its middle. (Turns out it’s called Corethron). Ever since, I’ve wanted to look at some more examples of the microscopic citizens of the Ross Sea.
Today I got my chance as we dipped an 8-foot-long plankton net over the side of the Palmer. I looked at less than a tablespoon of that water, and still the microscope revealed a parade of plants and animals. We found at least four different levels of the food chain, each one a bit larger than the next. Read on through the slideshow and travel up the food chain with us:
-
- Here’s a look at some phytoplankton you’ve already met on this expedition. There’s Corethron—the capsule-shaped diatoms with the long spines radiating from them. The dark specks are Phaeocystis cells, each about 5 microns across (see Feb. 3 post). Also in view are three other diatoms, rectangular Fragilariopsis, spiky Chaetoceros, and small, round Coscinodiscus. Each of these organisms contains chloroplasts and conducts photosynthesis to make food. They’re the first step in the Antarctic food chain.
-
- We know that 5 microns (the size of the Phaeocystis in the last slide) is small, but how small is it, really? In this less-magnified view, the big Corethron capsules from the last slide are now the little specks and rods in the background. The two large objects are a marine snail larva (left) and a kind of zooplankton called a copepod (center). Its head is pointed downward and its two long appendages stick out like oars on a rowboat. Copepods eat both phytoplankton and other tiny animals. Farther right is a ball about the size of the copepod—that’s a colony of hundreds of gooey Phaeocystis cells.
-
- After keeping our plankton net in the water for 15 minutes, we had slightly less than a gallon of water. It contained hundreds of these tiny snails. They’re called veligers, a name that applies to snail larvae that can swim, Dr. Scott Fay told me. They float around at the surface filtering food such as phytoplankton from the water. To give you an idea of these creatures’ size, the ‘1994’ is printed on the edge of a dime.
-
- Next across our microscope’s field of view was an even bigger creature. It’s a ctenophore, or comb jelly (named for the comb-like lines of waving hairs along its sides). If you look closely you’ll see it has eaten one of the marine snails. There’s also a spherical colony of Phaeocystis along the jelly’s bottom left edge, to give you a sense of how far we’ve come since the second slide.
-
- In a drop of water about the size of the tip of a magic marker we found these two creatures. At right is a copepod like the one in the second slide in this post. The big pink koala-bear-shaped creature is another kind of zooplankton called an amphipod. It has a shrimplike tail tucked up under its body. ‘My, what beautiful eyes you have,’ Dr. White said, when she saw it. She’s used to seeing these kinds of amphipods in tropical waters, where they are weak swimmers that hitch rides on jellyfish. Amphipods eat both phytoplankton and small zooplankton such as copepods.
-
- We’re working our way up to the world that makes sense to us—the one we can see and feel. In the middle is that same amphipod with its tail extended. Curled around it is an Antarctic krill. It’s one of the most abundant animals in the Southern Ocean and the source of food for many fish, seals, penguins, and whales. This one is about as big as a large piece of macaroni. Scientists have estimated that several hundred million tons of krill live in the Southern Ocean during summer. The animals use those fine hairs on their legs to filter phytoplankton (including diatoms like Corethron) and other creatures including copepods from the water.
-
- Now that we’ve seen a little of what’s living in the water, perhaps we can return to our question of what Snow Petrels eat (see Feb 1 post). Remember when we said we wanted to devise a way to answer this question using only materials we have on the ship? Well this is what we meant—petrel poop. I scraped up a few fresh samples from the bow and brought them to the microscope room. (Believe it or not, this is a tried-and-true method scientists use to explore what animals eat.)
-
- From watching petrels fly low over the waves, I thought they might be plucking food such as large krill or small fish that they saw. So I was surprised when I got the poop under the microscope. It certainly looked like krill—full of shrimplike pink meat and shredded bits of shell. But I didn’t expect the pieces to be so small. That big white fleck is a grain of sand (probably from the bird’s gizzard, where it would have been used to grind up the food). After seeing this slide, I think maybe the petrels find patches of smaller creatures and drink them down with a little seawater instead of picking them out one by one.
-
- Early in the expedition Chris spotted this crabeater seal swimming around the ship, and he’s been saving the photo ever since. This is the most abundant seal in the world, numbering as many as 40 million. Despite their name, these seals eat mostly krill. In fact, their back teeth are highly specialized to fit together and create a sieve that’s just the right size to strain krill from the water.
-
- A minke whale surfaces in the ice channel we visited on Jan. 30. Minkes are relatively small whales—they weigh 10 tons and are about 32 feet long. When you see their fins, keep in mind that the fin is about 2/3 of the way along the whale. At right you can see a little wave where the whale’s tail just broke the surface, so the whale’s head is about twice that distance to the left of the fin. Minke whales strain krill through fibrous plates of baleen in their mouths. An estimated 250,000 minke whales eat between 8 and 20 million tons of krill each year.
There’s one more photograph we needed for this post, but we just couldn’t get it: a picture of you. Humans sit at the top of the food chain—we eat pretty much anything we want, and nothing else eats us. So even if you’ve never been to Antarctica, what you eat for dinner can weigh on the health of its ecosystems.
As alien and distant as the microscopic creatures in this post look, they’re just a couple of stops away from us on the food chain. Small fish and krill from the Southern Ocean are caught in great numbers to feed to farmed fish and shrimp. It takes many, many of these small creatures to end up with a few giant shrimp.
Even here in the remote Ross Sea, vessels are venturing farther south each year to catch Antarctic toothfish, which is sold as Chilean seabass. These large fish eat other fish that have fed on krill, so even they are only a few steps removed from the creatures in this slideshow.
Many thanks to Dr. Angelicque White and Dr. Scott Fay for help with the net sampling, petrel poop sampling and filtering, and microscope setup. You can find more information about where seafood comes from through the Monterey Bay Aquarium’s Seafood Watch program.
Read more in the following posts:
5 Responses to “Sizing Up the Food Chain”