Just keep swimming

"Just keep swimming, just keep swimming"

- the movie Finding Nemo

There's this concept in invertebrate biology called "delayed metamorphosis." It refers to a larva remaining in the water column for longer than its peers and only settling to the seafloor when it's older. Delaying metamorphosis can cause a larva to disperse farther away from its parents and maybe even find a new reef, but it can also have some carryover effects. Previous research in ascidians has shown that larvae that delayed metamorphosis had slower growth, higher mortality, and lower reproductive output later in life. I guess it's analogous to a scientist with a severe case of wanderlust who travels the world and never finds the time to raise children (not that I'm referring to anyone specifically...).

As far as I know, nobody's looked at delayed metamorphosis in corals. It's actually kind of crazy to me that nobody has (and I'm prepared to be corrected), since it could have such a profound effect on success later in life. There are rumors of coral larvae dispersing for months and reaching new atolls or islands, but if they do that, are there carryover effects? 

Installing the tiles while freediving with Cas. I have got some
high-class marine biology fashion going on! Photo by
Matthew-James Bennett.

To answer that question, I'm setting up an experiment in Palau. I took eggs and sperm from all the parents I could, mixed them all together to get a diverse pool of larvae, and then cultured them in plastic bins at the research station, PICRC. I was planning to have some larvae settle as soon as they could (my control) and keep some larvae in the bins longer - forcing them to keep swimming. When my collaborator and I originally wrote the proposal for this project, we said we'd keep the delayed metamorphosis larvae swimming for 21 days because that's the retention time of water in one of Palau's semi-enclosed lagoons. 

My larvae lasted two. That's right - two days. I checked on them two days after settling the control larvae, and they were already rebelling. My larvae did not want to keep swimming. In fact, for as notoriously picky as coral larvae are about where they settle, these little buggers had found one millimeter-sized ridge at the bottom of my plastic bins and decided that spot was good enough. 

Thankfully, not all the larvae settled in the bins, but when I saw that some had, I acted fast. I set up settlement tiles for the rest that were still swimming. They only ended up delaying metamorphosis by two days, but that is just going to have to be good enough.

This experiment is off and running!

With my team's help, I outplanted the control tiles on a rack near PICRC yesterday. I'll leave them for a couple days and then come back to count how many have survived. The days right after settlement are the most critical for invertebrates - some studies have found up to 99% mortality in the first 24 hours. If there are carryover effects of delayed metamorphosis, I expect to find higher mortality and slower growth in the larvae that kept swimming for those extra two days. 

I hope I find interesting patterns! If nothing else, the fact that my larvae started settling in the bins so quickly speaks to their potential behavior in the field. Maybe they don't ever move away from home. It's an observation, and I'm excited to have my first experiment started!