Horizons

Kathleen M. Wong

Starry, Starry Eyes

Microscopic lenses have been found on the arms and body of a brittlestar similar to this one. Photo by Gerald and Buff Corsi, California Academy of Sciences.

The Japanese describe the concept of total awareness as having eyes all over the body. If that’s true, then the waters of the Caribbean Sea harbor the Zen masters of the oceans.

A brittlestar known as Ophiocoma wendtii arms itself with thousands of microscopic lenses when it constructs its skeleton. The lenses are so distortion-free that engineers hope to copy them someday to build better telecommunications networks and optical computers.

“If you listed the qualities you wanted to incorporate into an optical device to make it perfect, the organism went through this list and did everything,” says Joanna Aizenberg, a researcher at Bell Laboratories in New Jersey who reported the lens discovery in the journal Nature. The scientists suspect that the thousands of little lenses on the brittlestar’s arms might even function as one large compound eye similar to those of flies and other insects.

The discovery of O. wendtii’s lenses began with the simple observation that, come nightfall, its color changes from a drab reddish-brown to dramatic stripes of gray and black. The habit mystified its discoverer, Gordon Hendler, now a marine biologist at the Natural History Museum of Los Angeles County and co-author of the study. No brittlestar nor any of its echinoderm relatives had ever been reported to sense light and darkness before, much less alter its appearance in response.

Hendler determined that the color change is controlled by cells in the brittlestar’s skin that expand during the daytime and contract each night. These chromatophores are clustered around translucent bumps covering the upper surface of the animal’s arms. Under a microscope, the bumps resolved into crystalline protrusions of the skeletal plates protecting the upper portion of the brittlestar’s five arms.

“I took a closer look at them, and realized you could see through these bumps into the rest of the skeleton. I thought these things actually looked like lenses,” Hendler says. His experiments ruled out camouflage as a reason for the color change, supporting the idea that the brittlestar uses the pigment like sunglasses—to modulate the amount of light passing through its skeleton.

When Hendler posed the problem to Aizenberg, who studies how echinoderms and other animals form ornately patterned skeletons, she immediately became intrigued.

Her first task was to determine whether the bumps functioned like lenses. “The experiment I’ve done was what every kid does when trying to burn a piece of paper with a magnifying glass,” Aizenberg says—but with a high-tech twist. She placed pieces of brittlestar skeletons atop photosensitive plates used to make semiconductors, then exposed the assembly to low-intensity light. The light would burn patterns into the plates only if the skeleton bumps collected and focused the light like lenses.

When Aizenberg examined the plates, she found bright dots burned into the masking material. Further experiments showed the brittlestar’s bumps were able to focus light about 100 times better than currently manufactured plastic microlenses. The focused light was most intense at the same distance where the brittlestar’s nerve bundles would have been located beneath its skin, suggesting that these nerves are geared to respond to light.

What makes O. wendtii’s optical achievement possible is the very substance of its skeleton, calcium carbonate. These tiny windows on the world consist of individual calcite crystals about one-twentieth of a millimeter across. The upper surface of each lens is shaped like a dome, while the lower surface roughly resembles the tip of a fat turnip. “When we calculate the optimal profile of a lens that wouldn’t suffer from spherical aberration, it would be this same strange shape,” Aizenberg says.

By applying a carefully orchestrated system of enzymes to create the intricate crystalline shapes that make up its bones, O. wendtii can grow each lens crystals with the calcium carobonate molecules so perfectly aligned that they don’t split light waves into multiple images. It is this crystal formation process that engineers want to recreate in manufacturing plants.

Perhaps the brittlestar really is wiser than we know.

Umbrella Species Concept Leaks

An electra buckmoth laying eggs on California buckwheat. The moth is now seldom found alongside the gnatcatcher, suggesting the bird isn't such a good indicator of coastal sage scrub health after all.

In conservation circles, the reasoning sometimes goes like this: Save the territory of the Florida panther, and preserve its Everglades neighbors to boot. This idea of designing ecosystem preserves around the habitat of an “umbrella” species has mushroomed in popularity in recent years. Ecologists see it as a handy shortcut for deciding which land parcels to preserve, and how well the conservation program is working. But now a study of a rare California ecosystem suggests that the idea of a single umbrella species protecting all the plants and animals around it might not be so watertight after all.

When Congress declared the California gnatcatcher (Polioptila californica californica) a threatened species in 1993, it seemed the ideal animal to help ecologists and developers compromise over some of the rarest habitat and most expensive real estate in the United States (see California Wild, Summer 1993).

The tiny blue-gray bird nests exclusively on plants that grow in coastal sage scrub, which only occcurs in a narrow strip along the shorelines of southern California and northern Mexico. To prevent angry clashes between conservationists and developers coveting the sage scrub’s ocean views, the state set up a plan whereby developers would set asidepatches of scrub populated by gnatcatchers for conservation, and build on the rest. The plan was hailed as a model for future conservation efforts.

But entomologist Dan Rubinoff of the University of California at Berkeley says that saving the gnatcatcher might not have been such a great way to protect smaller and more vulnerable coastal scrub sage residents. Rubinoff surveyed 50 remaining patches of coastal sage scrub in San Diego County ranging in size from 1.3 to 335 hectares. He checked each patch for the presence of gnatcatchers, as well as three sensitive insects known to live only on California buckwheat within the sage scrub. If the gnatcatcher had been a successful umbrella species for the coastal sage scrub ecosystem, the insects should have been found wherever the bird is.

What Rubinoff actually found was deeply disturbing. Although the gnatcatcher was present on all but two patches, the Electra buckmoth (Hemileuca electra), Bernardino blue butterfly (Euphilotes bernardino) and Mormon metalmark butterfly (Apodemia mormo) appeared far less often. The Mormon metalmark proved the most versatile insect, living on 76 percent of the sites. The Bernardino blue appeared in 66 percent of sites, while the buckmoth lived in only 22 percent of the patches. It suggests that the prescence of the gnatcatcher did not reflect habitat health. The best indicator, Rubinoff reported in the journal Conservation Biology, wasn’t the presence of the gnatcatcher, but the size of the habitat patches. “The paradigm is to save vertebrates, but if it doesn’t also work for the insects, that’s trouble,” he says. “It’s scary because we don’t have any idea how often it’s failing. My worry is, it’s failing on a regular basis, and that means the loss of biodiversity,” Rubinoff says.

Insects play a critical role in ecosystems, pollinating plants, enriching the soil, serving as food, and recyling nutrients throughout the system. Their disappearance in a habitat is a warning that its finely woven web has begun to unravel. As it is, Rubinoff chose winged insects better equipped than most for traveling from patch to patch. It suggests the gnatcatcher’s failure as an umbrella species bodes ill for scores of less mobile animals shoehorned into other similar conservation plans.

The problem is that those responsible for writing these plans are working in the dark. “We want to protect all species, but don’t have enough information about how an ecosystem works,” says conservation biologist Sandy Andelman of the University of California at Santa Barbara. With animals disappearing at an alarming rate, the impulse is to save now, and study later.

Andelman, Rubinoff, and other scientists are now realizing there’s a better way to approach habitat conservation. They propose studying a region’s ecological relationships in great detail, and using that information to select a wide variety of umbrella species to monitor. By that measure, managing coastal sage scrub patches to preserve not only the gnatcatcher but also other rare and sensitive species such as the buckmoth, the orange-throated whiptail lizard, and the San Diego thorn mint might do a better job at shepherding more species into the next century.

“We don’t have the resources to monitor all of the species all of the time,” admits Dan Simberloff, a conservation biologist with the University of Tennessee, “but we don’t have to. We just have to do it once, at the beginning, to figure out what is an indicator of what.”

“It’s not impossible,” Simberloff says. “We just have to try.”

Kathleen M. Wong is Senior Editor of California Wild.