Isotopes

Recently, I've found myself wondering a lot about how larvae get nutrition. I know, it's a common dilemma. Every person has at one point in their life sat on a park bench, gazed off at the shoreline, and wondered how they could best tell if a larva was planktotrophic or lecithotrophic. The experience is almost universal. 

One of the larvae I collected for my 
stable isotope test.

There is a way to tell what organisms are eating - several very good ways, in fact. You can trace specific molecules that organisms have to acquire from their diet. Or you can look at the ratios of isotopes like carbon-13 to tell how high up in the food web your species is. There are a range of options ranging from the general to the specific - but rarely are they used on larvae. 

Part of the reason might be that larvae are too small. You need a large amount of tissue for some of the specific analyses, which larvae just don't have. They're tiny. But for a general analysis using stable isotopes, larvae should provide enough tissue. In theory, it should work. 

This week, I'm collecting samples to try and answer a very specific question: can carbon and nitrogen stable isotopes be used to tell whether a larva is feeding in the environment (planktotrophic) or relying on yolk from its mother (lecithotrophic)? Animals that are herbivores tend to have low ratios of heavy isotopes like C-13 and N-15, while predators accumulate higher ratios of C-13 and N-15 through a process called bioaccumulation. My theory is that lecithotrophic larvae should have isotope ratios that match their mothers (because all their nutrition is derived from mom), while planktotrophic larvae might start out matching their mamas but should come to match the food they're eating in the environment over time. 

A crab larva included in my isotope test. 

It sounds all great in theory, but I don't know if it will actually work. As a test, I collected larvae from right here in Woods Hole. I took my plankton net down to the dock and towed it back and forth in the water, then checked out my catch under the microscope in my lab. It was honestly really nice to spend hours at a time looking through the microscope at larvae - plankton samples are seriously my happy place. For some species, I already know or could look up whether their larvae are planktotrophic or lecithotrophic. For others, I'll have to wait until the results come back to find out. I also collected adults of the same species as the larvae I collected (their mamas) and phytoplankton they could be eating for comparison.

If I'm lucky, the results should come back and show a clear split between species that are feeding in the water column and species that are not. We'll see if my theory holds!