Pass the diatom, please: Can Antarctic krill and salps share food?

In my last science post, I introduced you to Euphausia superba (Antarctic krill) and Salpa thompsoni (salps) and I explained how, under changing ocean temperatures and sea-ice conditions, the two might be competing for space. Before I move on to what this competition could mean for the whales, fish, penguins, seals and seabirds that eat krill, I just want to touch on one other thing that salps and krill might fight over.

FOOD. GLORIOUS FOOD.

Personally, I don’t share food. Ever. In Year 12, I remember having a deep discussion about this with a like-minded classmate. She said, “I put friends before myself, but I put food before my friends.” Slightly contradictory, but still, it made way too much sense to me. So, before you ever ask, I will not break you off a piece. I will not give you a bite. I will pull my plate in close to my heart and only, and I mean ONLY, when someone whispers to me, “Paige, this is a shared platter,” will I surrender any.

What about salps and krill? If salps are moving in next door to Antarctic krill, then they’re going to have to shop at the same supermarket. And they are both going to want to go home with a gut full of phytoplankton. Phytoplankton are often called the “grass of the sea.” They don’t technically “eat” anything, instead, they take some water and carbon dioxide and mix it in with a few nutrients. Then, when the sun is out, they convert this water and carbon dioxide into oxygen (used by us humans) and energy (to be taken up by whatever goes on to eat them). Phytoplankton kick off the food chain and are pretty amazing. Some even say they are more amazing than krill, but let’s not fuel the fire.

Krill are, of course, fussy when it comes to the food they eat. They have long legs with setae (hairs) that form a feeding basket, allowing them to take in only the finest quality phytoplankton: big, juicy diatoms. When I say “big,” I’m still talking pretty small (very roughly about 2-200 microns). Salps, on the other hand, typically eat phytoplankton that are even smaller than diatoms—like dinoflagellates. Now, one reason behind this is that salps are typically found in the more northern parts of the Southern Ocean. Northern waters=lots of smaller phytoplankton like dino’s, southern waters=lots of bigger phytoplankton like diatoms. But, if salps are creeping on down into krill territory, then they’re going to have another menu choice and they might end up feeding on both dinoflagellates AND diatoms.

This poses the question …

“Will Salpa thompsoni remove diatoms from the water column, making them unavailable to Euphausia superba?”

Or, the way I like to word it is … “For Salpa thompsoni, does it really ‘all taste the same?'”

Two methods I’m using to answer this question are …

Gut content analysis: This involves removing the guts of salps and krill, smooshing them, and looking at them under a microscope. From here, we can identify the phytoplankton species that are in the guts and look for any overlap across krill and salps. I collected krill and salps in the Indian Sector of the Southern Ocean during January and February 2016, and a very clever student called Abigail Smith with far better eyes than me did the hard yards hunched over the microscope.

Flow cytometry: This involves removing salp and krill guts and passing them under lasers that pick up the different sizes of the phytoplankton in the guts. Again, I’ve used my 2016 samples, so I can match these up nicely with the gut content analysis results.

I know I’m giving you lots of “what ifs” and very few answers. I promise you I don’t just sit around and draw cartoons all day (although, I have spent a lot of time drawing cartoons lately and I better ease up). I am coming up with some answers to these questions. And I wish I could tell you. But, in science, when we find out something new, we write it up and send our work to a scientific journal. Just like a record company distributes an album, a scientific journal distributes a scientific story. And they do lay some claim to it. My first salp story is currently being considered by a journal, and once this is published, then I can direct you all to the article and start sharing some answers to the big salp questions. For now, a cartoon is all I can offer you. krill_salps_food-01

2 thoughts on “Pass the diatom, please: Can Antarctic krill and salps share food?”

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