Geocoral

Have you ever gone geocaching? I went once with a friend when I was living in Germany, and it was fun. You find the coordinates for a "cache" in an online database and then go hunting for it using nothing but a GPS and your wits. The one we found was in a public park in Bremerhaven, and unfortunately it was near a creek that I ended up stepping in, filling my boots in the process. The cache can be anything. Ours was a small waterproof case with a log inside of all the geocachers who had found it. 

I was reminded of geocaching this week because my team was dealing with very precise locations. Not for caches, but for corals. 

Maikani and I on the boat (with our driver, Kaiton) after a day 
of coral sampling and georeferencing. Photo by Cas Grupstra.

You see, we're trying to determine whether different lineages of Porites lobata corals are reproductively isolated. In order for corals to cross-fertilize, their gametes have to meet. We know that different lineages co-occur at our study sites, but what if that's not the spatial scale that matters? What if there is more subtle variation within our sites? If one side of a bay is populated by one lineage and the other side of the bay has another lineage, technically they're in the same bay, but their eggs and sperm might never find each other.

This question is especially interesting, considering that Porites lobata is a gonochoric broadcast spawner. In order to reproduce, the corals release their gametes straight into the water column. Hermaphroditic corals package their eggs and sperm together in these mucus bundles, so the sperm are carried to the surface by the floaty, fatty eggs. Even if different coral lineages occur at different depths, their gametes will all meet at the surface. Gonochores don't work that way. Half the corals (the males) are only releasing sperm, which have no buoyant fats in them (sperm are basically just DNA with a tail) and will never make it to the surface on their own. The sperm are distributed throughout the water column. That means the eggs should be distributed throughout the water column, too. In fact, Porites lobata eggs don't rocket to the surface like some species; they hang out at all depths from where their mom is up to the top. 

What that means is that gonochoric species are much more likely to have lineages segregated by depth. A really deep mama coral will release her eggs, but they might never rise up to the level where a daddy coral of a different lineage is releasing his sperm. It's not just location that matters for our reproductive isolation question - depth is important too. 

A 3-dimensional problem requires a 3-dimensional solution. We headed out to each of our study sites armed with coral sampling gear, a GPS, and a slate. Two divers below the surface chose corals to sample, took small tissue chips with a hammer and chisel, and recorded the depths. Meanwhile, a snorkeler swam directly above them and used the GPS to record the exact coordinates where the sampled coral was located. That way, we got latitude, longitude and depth - the 3D location - for each individual coral we sampled. After some genetic analysis back in the lab, we'll know which lineage each of them belongs to, and we can figure out whether our lineages are unevenly distributed across our study sites. 

There were definitely some quirks to the sampling that made it interesting. The biggest challenge was making sure that the divers and the snorkeler were using the same sample numbers and didn't get off from one another. Before each dive, we would agree on the serial number of the sample we were starting on, and then we would each advance one number per coral. Sounds easy, but sometimes the snorkeler could forget what number we were on, accidentally repeat a number, or skip one entirely. Every 5 or 10 samples, I would roll on my back, look up at the snorkeler, and use hand signals to convey what serial number I had just collected. If they misread my signal or were on a different number, it would lead to a really confusing conversation conveyed almost entirely using shadow puppets. That's right - when you're the diver underwater looking up at a snorkeler in bright sunlight, all you see is their shadow. I had to ask the surface person to make their hand signals to the side of their body rather than under their own belly so I could actually see them. No wonder so many marine animals are countershaded!

In the end, we got all the samples we needed - 180 in total. It will be a ton of work back in the lab, but it will be worth it to answer our scientific question!