Guiding Lights: How Newborn Corals Find a Home

By Pedro Cardoso

As dawn breaks, a lizard crawls up a rock in order to bask in the sun and heat itself before starting its daily activities. A caterpillar slinks up a tree branch to reach the green leaves it will graze upon. Dolphins surf breaking waves and quickly catch their prey. These examples illustrate how animals have developed multiple types of locomotion, and how it is essential for their lives. With structures such as wings, fins, legs, and cilia, animals depend on their movement to escape predators, migrate and find food, mates, and suitable homes. It’s clear that moving is critical for their survival, however that begs the question, how can animals that are immobile for most of their lives, such as corals, cope with staying in one place? 

One possible answer to this question is that they need to find a particularly suitable place before settling. All coral (as well as sea fans and anemones) have a mobile larval life phase, in which they can disperse to different locations before finding the right spot to install themselves and grow as immobile adults¹. This brings another intriguing question: exactly what signals can coral larvae detect to know where to settle? 

Substrates for Coral Settlement

To find places to install themselves, coral larvae rely on chemical and light cues to find out where they are more likely to survive, or in other words, they need to smell and look for these locations¹. Biological cues commonly related to coral settlement are the presence of microbial mats, and certain species of CCA (crustose coralline algae), on which many species of coral prefer to settle¹. CCAs are hard rock-like calcareous red algae, commonly found in reefs and responsible for much of the ecosystem’s calcification. While some species of CCA could prove detrimental to juvenile coral survival by dislodging these recruits or removing them by sloughing off their own outer layers, their presence can signal larvae to reefs, and tends to indicate the absence of other sources of juvenile coral mortality, such as turf algae and sediment. Microbial mats are multi-layered sheets of microorganisms, mainly composed of bacteria or archae. Such mats could represent places that are consistently submerged for microbes to be able to grow consistently, which would mean that the spot where they are growing would be beneficial for settlement².

Chemical Cues

One of the ways coral larvae have been demonstrated to find patches on reefs dominated by CCA is by detecting their chemical cues (or smells). A study by Sneed and collaborators in 2014 was one of the pioneers to demonstrate what specific chemicals coral larvae were sniffing for². By isolating and culturing bacteria from CCA, these researchers were able to find a specific bacterium capable of producing a chemical which they also demonstrated to be capable of inducing larvae settlement in the absence of CCA.

Light Cues

Porites astreoides spat attached to red/fluorescent orange cable tie, image by Mason et al. 2011

In conjunction with such chemical detections, coral larvae have also been shown to prefer certain surfaces by light detection. Coral larvae generally settle on spots with light intensities matching places where their populations usually grow, meaning corals adjusted to shallower waters will tend to settle on places with brighter lights compared to larvae from populations adapted to deeper environments³. Other than the light intensity, coral larvae have intriguingly also shown the capacity to choose surfaces depending on their color.  A study by Dr. Mason and colleagues in 2011 was the first to demonstrate the ability of coral larvae to select red surfaces for settlement⁴. In this study, the authors suggested that this kind of selection would favor corals to settle on CCA, since this type of algae have red pigments which would possibly be detected by the larvae. The question of how coral larvae could actually “see” light differences however still remained speculative at the time, since these organisms don’t have eyes to detect their guiding lights.

This matter did not remain a mystery for long. In 2012, a project led by Dr. Mason was able to prove the presence of opsins and G proteins, light-sensitive substances responsible for sight in animals, in the larvae of the Caribbean elkhorn coral⁵. While the presence of these substances doesn’t actually mean the larvae have proper eyes, these molecules are commonly found in the retina and light receptors of many animals, and provide a way in which coral larvae might be able to detect different light wavelengths (colors).

Even though we now know of some of the cues responsible for the settlement of coral larvae, there are likely many others that are detected by larvae at the same time to be discovered. In recent years, multiple new genes related to opsins in corals have been described^6,7, as well as different strains of bacteria capable of inducing larval settlement^8,9. Other than giving us a better idea of how these animals cope with their environment, getting to know these cues would make scientists understand better how coral larvae survival might be affected by current changes to reef ecosystems, and help conservation projects based on coral reproduction find better solutions for larvae settlement in reef restoration efforts.

References:

1. Price, Nichole. “Habitat selection, facilitation, and biotic settlement cues affect distribution and performance of coral recruits in French Polynesia.” Oecologia 163, no. 3 (2010): 747-758.

2. Sneed, Jennifer M., Koty H. Sharp, Kimberly B. Ritchie, and Valerie J. Paul. “The chemical cue tetrabromopyrrole from a biofilm bacterium induces settlement of multiple Caribbean corals.” Proceedings of the Royal Society B: Biological Sciences 281, no. 1786 (2014): 20133086.

3. Strader, Marie E., Sarah W. Davies, and Mikhail V. Matz. “Differential responses of coral larvae to the colour of ambient light guide them to suitable settlement microhabitat.” Royal Society open science 2, no. 10 (2015): 150358.

4. Mason, Benjamin, Matthew Beard, and M. W. Miller. “Coral larvae settle at a higher frequency on red surfaces.” Coral Reefs 30 (2011): 667-676.

5. Mason, Benjamin, Michael Schmale, Patrick Gibbs, Margaret W. Miller, Qiang Wang, Konstantin Levay, Valery Shestopalov, and Vladlen Z. Slepak. “Evidence for multiple phototransduction pathways in a reef-building coral.” PLoS One 7, no. 12 (2012): e50371.

6. Mason, Benjamin M., Mitsumasa Koyanagi, Tomohiro Sugihara, Makoto Iwasaki, Vladlen Slepak, David J. Miller, Yusuke Sakai, and Akihisa Terakita. “Multiple opsins in a reef-building coral, Acropora millepora.” Scientific Reports 13, no. 1 (2023): 1628.

7. Shi, Zongyan, Ee Suan Tan, and Akihiro Takemura. “Day–night expression patterns of opsin genes in the coral Acropora digitifera under natural and LED light conditions.” Coral Reefs 43, no. 5 (2024): 1535-1547.

8. Sharp, Koty H., J. M. Sneed, K. B. Ritchie, L. Mcdaniel, and Valerie J. Paul. “Induction of larval settlement in the reef coral Porites astreoides by a cultivated marine Roseobacter strain.” The Biological Bulletin 228, no. 2 (2015): 98-107.

9. Jorissen, Hendrikje, Pierre E. Galand, Isabelle Bonnard, Sonora Meiling, Delphine Raviglione, Anne-Leila Meistertzheim, Laetitia Hédouin, Bernard Banaigs, Claude E. Payri, and Maggy M. Nugues. “Coral larval settlement preferences linked to crustose coralline algae with distinct chemical and microbial signatures.” Scientific Reports 11, no. 1 (2021): 14610.