Isotopes: part 2

Most of the time, I refrain from sharing actual data on this blog. It's an unofficial, informal science communication tool, but still, I don't want to risk a scientific journal thinking I've already "published" my work by sharing it online. But I'm going to make an exception today because I want to show you something I'm working on. These data are not publishable by themselves, so I'm not risking any future misunderstanding with a journal. Getting to see the actual data and peek inside my head will help you understand how scientists interpret the world. And plus, the experiment is pretty cool. 

Earlier this summer, I collected larvae at two sites around Woods Hole to see if I could tell what they're eating. I thought maybe the well-established technique of stable isotope analysis could help me differentiate larvae that were feeding (we call them "planktotrophic") versus larvae that are relying on yolk from their mothers (we call them "lecithotrophic"). I grabbed individuals of species that I knew were lecithotrophic and planktotrophic, and I worked with a technician at WHOI to prep them for biochemical analysis. 

Stable isotope data for Bugula neritina

The data came back, and I finally had a chance to look at them. I think it worked. Let's look at a lecithotrophic species first - the bryozoan Bugula neritina. It's a purple branching species that is really common on docks. The larvae are these cute purple blobs, and they rely on yolk from their mothers for energy. In theory, they should have the exact same isotopic signatures as their mamas because they're not eating anything else. 

Take a look at the graph to the right. The x-axis is the isotopic enrichment of carbon, and the y-axis is the isotopic enrichment of nitrogen. Each dot represents one sample. Generally speaking, the more isotopic enrichment you have (closer your dot is to the upper right corner of the graph), the higher your trophic level - you're a predator. The lower enrichment you have (the closer your dot is to the bottom left corner of the graph), the lower your trophic level - you're a primary consumer, an herbivore. My Bugula dots are in a straight line. So what does that mean? 

The level of nitrogen isotope enrichment for adults and larvae was all relatively the same. That's good. It's actually what I was expecting to see. The larvae are getting all of their nutrition as yolk from mom, so they're in the same trophic level. The carbon numbers are a little weird, to be honest. I don't know why larvae have lower carbon enrichment than their parents. I need to do some reading on larval metabolism, I guess. One thing I do know is that a lot of researchers pay more attention to nitrogen than carbon because nitrogen enrichment tends to change more between trophic levels. So at least the important element shows what I expected. 

Now let's look at a species with feeding larvae, a planktotroph. For this, I chose the snail Crepidula fornicata. It's a super common species in New England, and I've talked about it on the blog many times. The disadvantage for Crepidula is that the adults are filter feeders, so the things they're eating are very similar to the things their larvae are likely to be eating. My theory was that planktotrophic species should show different isotopic signatures between adults and larvae because they belong to different trophic levels, but I wasn't sure if it would work for Crepidula. In an ideal scenario, you'd want a species with adults that eat different things than the larvae can eat so there's a big difference in their isotopic signatures. (For the record, I also tried collecting a predatory snail but didn't get enough individuals.)

Stable isotope data for Crepidula fornicata

Here's the graph for Crepidula. The x-axis and y-axis are the same as above, and each dot represents one sample. I added some shaded bars so you can see the difference between trophic levels. Dots in the blue region are at a lower trophic level, and dots in the gray region are at a higher trophic level. 

The first thing you'll notice is that once again, the dots are more or less in a line. Adults have the highest carbon isotope enrichment, and larvae have lower enrichment. The same weird pattern is showing up again, so that makes me think it's a real phenomenon and not some random anomaly. I really need to do some reading to figure out why that is. 

I'm more concerned with the nitrogen isotope enrichment here, because nitrogen usually shows a stronger difference between trophic levels. Most of the dots are in the gray region - they're all at the same trophic level, eating the same things. Crepidula diets include particulate organic matter, some phytoplankton, and detritus - whatever small particles they can find in the water column. Adults and larvae are all eating the same things. 

But check out those two dots in the blue region. The nitrogen values are different enough that individuals in those samples have to be eating different things. I was a little confused by those two samples until I came across a study earlier today that found Crepidula larvae can eat bacteria. Maybe individuals in those samples are eating more bacteria, so their isotopic signatures are different. 

Overall, I think my experiment was a success. The data are a little hard to interpret, especially because stable isotope analysis is a new skill for me. The more I read, talk to other scientists, and think about my graphs, the more they'll begin to make sense. I am excited to see some differences between the lecithotrophic and the planktotrophic species, because that gives me a good idea of what to look for in future studies. If I see a wide range of carbon values between adults and larvae, it doesn't necessarily mean anything, but variation in nitrogen values might mean the larvae are planktotrophic and eating different things than their parents. 

This has been a long post, but I hope it gives you some insight into how I think as a scientist. I have a question, try an experiment, look at the results, and try to draw logical conclusions. If I find something that makes zero sense, it might mean I missed something, I was wrong, or I don't have enough information. And that, my friends, is the most exciting part of the scientific process.