No eyes, no problem for color-sensing coral larvae

Staghorn coral larvae don’t have eyes. Yet shifting the color of underwater light can reverse their usual preferences for spots to settle.

Horizontal surfaces bathed in blue-green light attract more larvae of Acropora millepora coral than normal, says behavioral ecologist Marie Strader of the University of Texas at Austin. And vertical surfaces lit through red filters attract fewer than normal larvae, she and her colleagues report October 7 in Royal Society Open Science.

Such subtleties of color could serve as one of the cues that larvae use to pick a good place to settle down and start calcifying, Strader proposes. “It’s the most important decision they make in their lives,” she says.

A typical coral starts life as a fertilized egg adrift in the ocean. “It looks like a little jelly bean,” Strader says. Dot-beans develop some measure of control over which way they waft, and researchers have discovered that light, texture or attractive chemical traces such as a taste of coralline algae attract larvae toward certain spots.

Research on light’s effects has focused more on the intensity than on hue, Strader says. For staghorn coral, dim is good. “They like to hide,” she says, but “they don’t want to be completely in the dark.” The color of that light matters, too, a few studies have demonstrated. Some species tested responded differently to bluish or white light. A 2011 paper even reported that more coral larvae settled on red surfaces than on other colors. Gradations of color might give the discerning larvae a clue about finding their favored depth, researchers have speculated. Seawater absorbs red wavelengths fast, so rosier light generally indicates shallow water.

Strader and her colleagues have done a series of experiments exploring how colors may act as cues for depth or even orientation of habitat. She based one experiment on the idea (confirmed by measurements) that exposed upper surfaces of something horizontal tend to be redder than a vertical surface. Nearby vertical surfaces get their light in large part by a long, slanted path through the water that saps red, making the location look more blue-green.

Working on the Great Barrier Reef, Strader and her colleagues set out terra-cotta H-shaped frames in clear plastic boxes. The top of the flat crosspiece of the H had slightly redder natural illumination than the bluer and greener uprights on either end. In some boxes, filters for theatrical lighting reversed those trends, reddening the sides and greening and bluing the tops.

Larvae placed inside the boxes mostly attached themselves in the shadows underneath the horizontal part of the H. But in boxes without color-swapping filters, those larvae that did not tuck themselves in the shadows were more likely to settle on the vertical surface than the horizontal one. (An exposed horizontal surface might prove especially vulnerable to predators.).  In boxes where filters flipped the colors, larvae reversed their usual preference and settled on the horizontal surfaces.

Just how the larvae detect those colors remains a puzzle. A. millepora larvae fluoresce strongly red or green, often both, and Strader was originally trying to test whether the fluorescent substances were involved in perceiving colors in the environment. It’s unlikely, she has concluded. But the simple jelly bean larvae have abundant other light-reactive proteins to test.