Chapter 1: The Mystique of Coral Fluorescence

Even if you've never attended a rave, the imagery associated with one is likely familiar. A simple search for "rave" reveals dark settings illuminated by vibrant neon lights in a range of colors and patterns.

When you take an ultraviolet flashlight underwater at night on a coral reef, a strikingly similar spectacle unfolds.

Researchers have long recognized that corals exhibit fluorescence, yet the reasons behind this phenomenon remained largely unclear. A recent investigation has shed light on the idea that this fluorescence aids corals in their feeding processes.

As a budding marine biologist during a scientific diving course, I had the chance to witness this remarkable occurrence firsthand. In such a course, you learn to balance the essential skills of SCUBA diving with scientific exploration beneath the waves. While it may sound dramatic, mastering diving techniques to the point they become second nature is crucial. Regularly checking your air supply and dive computer, along with maintaining proper buoyancy, is essential to ensure you can work effectively without harming the reef.

During one memorable night, we were focused on diving in darkness while studying coral fluorescence. Initially, we began with standard waterproof flashlights to familiarize ourselves with the reef. Once oriented near a coral formation, we extinguished our lights, plunging into the depths of night.

Our instructor brought out her waterproof ultraviolet flashlight, which emitted a softer purple glow compared to our bright yellow beams. She aimed it at a nearby coral, which instantly transformed into a stunning shade of lime green. After allowing us to appreciate this display, she moved the light to another coral, revealing a burnt orange hue.

Experiencing this spectacle up close was astonishing; with enough ultraviolet lights, my peers and I could have orchestrated our very own underwater rave.

Photo by Francesco Ungaro on Unsplash

The fluorescence in corals results from a compound known as green fluorescent protein (GFP). This protein absorbs ultraviolet light and re-emits it at a lower wavelength, creating the vivid neon colors we observe. While fluorescence occurs throughout the day, it is most visible in the absence of surrounding light.

Various hypotheses have been proposed by scientists regarding the purpose of coral fluorescence. Research has explored whether this characteristic serves as photoprotection (essentially coral sunscreen), enhances photosynthesis, acts as antioxidants, offers defense against herbivores, or attracts symbiotic algae. Some studies suggest that GFP may help broaden the light spectrum available for photosynthesis, although this remains a contentious topic.

Stony corals are a fascinating blend of animal, plant, and mineral elements. The visible coral structure resembles a collection of small anemones interconnected, representing the animal component. Inside, they host tiny symbiotic algae capable of photosynthesis—the plant aspect—which supplies nourishment to the coral. Additionally, as corals grow, they produce a calcium carbonate skeleton beneath them, constituting the mineral component.

Depending on their environment, corals can derive their energy from either photosynthesis or by consuming plankton.

Photo by Joan Li on Unsplash

In the deeper waters, ranging from 30 to 150 meters, evidence supporting the various hypotheses explaining coral fluorescence is lacking. At these depths, corals receive minimal light and tend to rely more on consuming plankton than on photosynthesis. Thus, it is possible that their fluorescence relates to their dietary needs.

The recent study published in June 2022 investigated whether plankton are drawn to the fluorescence emitted by corals. Researchers used a small brine shrimp species, Artemia salina, and discovered that these shrimp were attracted to fluorescent targets, while other larvae, such as fish, showed no preference.

These findings lend credence to the theory that corals may attract planktonic prey to their mouths, using a warm glow as bait for a midnight snack. The authors of the study have termed this the "light trap hypothesis." Given that plankton are generally less abundant near the seafloor than in the upper water column, corals, which grow from the bottom upward, require additional assistance to secure their food.

In conclusion, it appears that corals aren't perpetually hosting an underwater rave without us; rather, they're employing their fluorescence to catch dinner.