The hunt for baby corals

When I first came to Palau in 2018, I had a very idealized view of corals. I expected them to behave exactly how I wanted them to – just like every other benthic invertebrate. At the time, I had been very successful studying recruitment in temperate and polar environments. I just stuck some panels underwater and watched what grew. It worked every time – something was bound to recruit.

A terra cotta tile deployed on a reef. Photo by Kharis Schrage.

Not so with corals. First of all, the larvae are extremely picky about where they settle. They need a porous, rocky surface with a thick, healthy biofilm and plenty of crustose coralline algae. Second, there’s just not that many coral larvae that actually make it to settlement. Most corals have ridiculous fecundity – they spawn hundreds of thousands, even millions of eggs at a time – but if you look at a coral reef, there are not that many young recruits. The small ones are few and far between.

Hanny and I deployed some tiles in Palau in 2018 in the hopes of catching coral recruits, but we didn’t get much – only a handful of individuals. We conducted plankton tows, too, with the goal of collecting larvae, and those samples didn’t yield much either. I think we had a total of 18 individuals. Ever since then, I’ve been on a hunt for baby corals. I want to catch their larvae and newly-settled juveniles so I can study the patterns of connectivity between their populations. But how do I find the young individuals I’m looking for?

Crustose coralline algae on a piece of dead coral skeleton.

This trip, I tried out a couple different strategies. First, I deployed panels again, but I’m leaving them for six months this time. I figure they’ll have plenty of time to accumulate a biofilm and crustose coralline algae before the coral spawning season next spring, so they’ll be attractive habitats for settling coral larvae. Second, I collected plankton samples again. I didn’t actually find any coral larvae this time (that’s not surprising, since we’re not in a spawning season right now), but I found plenty of other cool stuff.

The third thing I’m trying this trip is collecting pieces of dead coral skeleton. I realized I was spending a lot of time and effort trying to imitate the substrata that corals settle on. Why not give up the imitation game and just go straight to the real thing? I collected coral rubble at two different sites to check for small juveniles under the microscope. I didn’t actually find any coral juveniles, but I found a veritable ecosystem of crustose coralline algae, foraminiferans, brittle stars, and sponges. Coral rubble is fascinating to look at under the microscope!

A small coral recruit on a piece of rubble.
Photo by Kharis Schrage.

Here’s the thing, though. The more we swam around the reefs, the more I started noticing small corals, about the size of a silver dollar. I think my brain adapted to the task – I was looking for small corals, so eventually, I got better at noticing them. The major spawning season was about 6 months ago, and corals of the size I was seeing would be about 6 months old. Those were the juveniles I was looking for! They were on coral rubble, just like I expected, but they were very spread out. I must not have found any juveniles on the pieces of rubble I collected just because there are so few juvenile corals on the reef – the probabilities were against me.

The good news is that now I have a new, potentially effective way to find juvenile corals. I can just swim around the reef and collect the pieces of rubble that have juveniles on them. When we’re back here next spring, I’ll use my new strategy to collect samples and hopefully find some coral larvae too. I’m looking forward to collecting the samples!