Coral larvae — they grow up so fast
The next morning, the embryos have developed into larvae. At first, they are round and move only a bit from side to side, but then they grow up FAST. By the afternoon, they have truly learned to swim. The spheres have elongated into pear shapes like you see here, and they begin to swim across the water surface. They use small hairs called cilia to propel themselves in the water, and they’re pretty fast! We’ve measured them swimming a few millimeters — the equivalent of a few body lengths — in a second. Not bad for something so tiny. They need to be able to swim so that they can search a coral reef for exactly the perfect place to attach. Given that they usually spend their entire lives in that same place, this is a critical decision to make and these larvae have amazing sensory abilities to help them make the right call. As such, this is where we do the most research. We’ve studied how corals sense and react to different surfaces, sounds, light levels, colors, bacterial species, algae — even other coral species. It turns out coral larvae have very strong “opinions” about where they want to live. And human-induced threats like fertilizers, sewage, and temperature change all stress them out and reduce their ability to settle and survive. By studying their settlement choices and survival rates in lab experiments, we can help make sure the right settlement cues and habitats still exist for them out in nature. For some species, we can even return a few precious settlers to the reef to try to jumpstart the next generation.
Photo: Kristen Marhaver.
The star coral settle into their first homes
Here, about a dozen star coral larvae all chose to settle on the underside of a piece of pink coralline algae that we collected from the reef. The corals have now gone through the full settlement process including attachment, metamorphosis (growing their tentacles, mouth, and digestive system), and are beginning to grow their skeletons (the small white cups). They are still only 1-2 millimeters wide and very fragile. They often prefer to settle somewhere protected so they aren’t crushed by mobile animals like crabs. The delicate nature of the process, and the many ways human activity interrupts it, explain why coral settlement in the Caribbean is so much lower than it used to be.
Photo: Kristen Marhaver.
Prime real estate — if you’re a star coral
Encouragingly, though, we’ve recently seen corals settle in some surprising places on a few reefs in Curaçao. For instance, a few new star coral colonies started their lives on a concrete block that sunk in a storm. The species only grows about a half an inch a year, so this colony is probably three or four years old. We’ve found that they really prefer vertical surfaces — perhaps because they seem to be safer places, with fewer animals walking around, and where there’s less sediment and sand piling up. From these tiny humble beginnings, a star coral can build a massive and impressive structure.
Photo: Kristen Marhaver.
The beauty of a healthy coral reef metropolis
This is me diving on the north shore of Curaçao; the star coral here could be over 500 years old so even with my fins, I’m still not big enough to be the scale bar! Each coral colony tries to grow vertically toward the sun while it competes for space horizontally with other corals. You can see here how different colonies have grown up right next to one another. A healthy coral reef is like the center of a huge city — there’s almost no free space and competition for a place to live is fierce! Some of the reefs on Curaçao have suffered a lot in the past years, but the reefs furthest away from humans, and some of our luckiest reefs in town, still have some beautiful stands of coral.
Photo: Mark Vermeij
We have some of the best remaining reefs in the Caribbean, but we won’t be able to keep them alive just by luck. Fortunately for the corals and the humans who depend on them, the government of Curaçao has started taking some bold steps to protect and nurture its coral reefs. What we learn in our research at CARMABI can be applied not only on the island but across the Caribbean, and even to other coral species in other oceans. By studying how corals reproduce, we can help give coral babies — the reefs of the future — their very best chance at surviving in our human world.