Hello, Dendronotus

How do larvae disperse in the Arctic Ocean? It's a big question. Over the past several years, I have been on a quest to find the larvae of seafloor invertebrates in the high Arctic. It started with an experiment in 2017. As some of you might remember, I was invited to complete a long-term experiment that year. A lander that had been deployed in 1999 was brought to the surface, and I analyzed the colonists on the lander to understand how communities form in the Arctic deep sea. I found surprisingly little. After 2 decades, only about a quarter of the local species had recruited to the lander, and most individuals were too small to see with the naked eye. I concluded that Arctic deep-sea communities take much longer than 2 decades to develop. 

There are several possible explanations for the slow rate of colonization in the Arctic deep sea. Maybe the adults aren't producing any young. That would be pretty interesting, honestly, because it would mean the Arctic deep-sea basin is different from most of the world ocean, where species reproduce either on an annual or a continual basis. 

Or maybe reproduction happens sporadically. In Antarctica, there are some species that only recruit when there are very specific environmental conditions. Maybe the same is true up here in the high North, and we just missed the window. 

Maybe the offspring - the larvae - aren't surviving long enough to settle on the seafloor. Maybe our experimental panels didn't have the right coating of microbes to induce larval settlement. Maybe the larvae are settling, but they're getting eaten shortly afterward. 

Each of these explanations could be true, partially true, or true in combination with other factors. Since 2017, I have tried my best to test them all. 

A Dendronotus sp. larva, photographed at 100x using
a compound microscope (transmitted light)

A major goal for my team on the current Polarstern expedition is to try and collect as many larvae as we can. We will then identify each individual to figure out which species are where, how the distribution of each species relates to the oceanography of the region, and what that means for larval dispersal in a changing Arctic Ocean. 

We reached our first station yesterday. Using a small net deployed by hand over the side of the ship, we collected two plankton samples and immediately examined our catch under the microscope. To my delight, there were sea slug larvae in the water! The nudibranch Dendronotus sp. is a common species in coastal waters from Norway up to Svalbard. We pulled four of the clear, egg-shaped larvae from our sample and preserved them for later analysis. 

It's very exciting to have our first samples already! I think this will be a productive expedition with lots of beautiful larvae!