The stud

A Porites lobata coral about to release gametes.

It was 7:30 pm, and we were on spawn watch. We had gotten eggs and sperm from our coral colonies the night before, so we watched our potential parents with eager anticipation. A second night of spawning was pretty much expected. Who would be the first to release their gametes? 

I should have known which coral it would be. It's always the same one. He goes early, and he goes big. Coral number thirteen, our stud. 

What's cool about Thirteen is that he belongs to a distinct genetic lineage with high thermal tolerance. He and his brethren basically never bleach. They live inside these semi-enclosed lagoons in Palau's Rock Islands where the water heats up during the middle of the day. So when a bleaching event happens, to them, it's just an average day. Thirteen thrives in the heat. And he produces a lot of sperm. 

Maikani collecting sperm from a coral.

One of our goals for this trip is to figure out whether the different genetic groups (we call them "lineages") are actually different species of corals. This whole time, we've nominally been studying Porites lobata. If you dive on any coral reef in the Pacific, it's the big yellow mounding coral. But here's the thing: Porites lobata's lineages are all pretty different from one another. Their DNA is different enough, in fact, that we've started to wonder whether they're actually different species. When you have a lot of individuals that look pretty much the same but are super genetically different, we call them "cryptic species." Could Porites lobata actually be a complex of cryptic species? 

The best way to find out is to test for reproductive isolation. In other words, can individuals from different lineages cross-fertilize? If they can't, it's pretty strong evidence that the lineages are actually cryptic species. [Interestingly, hybridization between different species is common enough in corals that if the lineages can cross-fertilize, it's not solid evidence that they're the same species. There's definitely a debate to be had here about what even constitutes a species when closely-related ones can cross-fertilize, but that's a topic for another day. Between hybridization and cryptic lineages, corals just basically do not respect human-imposed species boundaries. It makes our research a little messy but super interesting!]

Alright, so let's talk about what cross-fertilization looks like in the field. Basically, you take eggs from a female of one lineage and sperm from a male of another lineage, and you mix them in the same dish. We used these plastic 6-well plates for our fertilization bins and mesh cell strainers to isolate the gametes. You want to mix the gametes at a high concentration for about 45 minutes, and then you want to separate them (if you don't, multiple sperm will try to fertilize each egg, and the embryos won't develop correctly). Matthew started each cross by adding eggs and sperm to the well-plates, and Maikani would stop the crosses by washing the sperm away. The mesh on the cell strainers was larger than a sperm but smaller than an egg, so if you lift the strainer out of its well, only the eggs remain. After a few rinses, you dump the eggs in a larger cup of water so they're at lower density. Stopping a cross takes only a minute or two, but you have to do it at the right time. 

Fertilized embryos of Porites lobata.

You see where this story is going, right? Anyone who's met me before knows that I have a flair for organization. Naturally, I played the role of timekeeper. I didn't even volunteer for the role - I claimed it. 

An onlooker would have wondered what we were doing. Notebook in hand, I paced back and forth through the tank room, checking every 10 minutes for spawning corals. Over and over, I checked my watch, noted times when crosses started, and calculated when they should stop. I called 5-minute warnings to Maikani so she could label her dishes and get ready. The whole thing was a finely orchestrated dance. Seriously, we nailed it. 

And the data look incredibly interesting, Our stud, Coral Thirteen, was able to fertilize eggs from some females but not others. We're not sure which lineages those females belong to yet, but we'll find out this summer after a round of sequencing. I am so excited to figure out whether our corals are cryptic species! We've got an incredible dataset taking shape!