The marine creature uses crystalline microscopic lenses to escape predators
Merit Sloin
Update - 23/08/2001
Pictured: Kochvon Yam. His "vision" system is the first and only one of its kind discovered so far in an animal that exists today on Earth
A sophisticated optical system consisting of crystalline microscopic lenses, discovered by Israeli and American researchers in a certain species of starfish, allows these creatures to notice an approaching predator ahead of time. An optical structure with the same level of sophistication was developed only a few years ago and is used in micro-optics. The discovery is published today in the journal Nature
The beginning of the research was a series of experiments carried out by Professors Leah Eddy and Steve Weiner from the Department of Structural Biology at the Weizmann Institute - with Dr. Jenna Eisenberg (who was then a research student) - in collaboration with Gordon Handler from the Natural History Museum in Los Angeles. The two Israeli researchers work Together for many years on deciphering the mechanisms used by marine animals to build their exoskeleton, calcium crystals Carbonates called calcite are the main component of the skeleton. The skeletal materials are built on top of a substrate of proteins and other molecules, which supervise the deposition of calcite and serve as a kind of template in which the crystals are formed.
"About ten years ago, when we were in California, we met Gordon Handler," Addi says. "Handler told us about a certain species of starfish (Ophiocoma wendtii) in which he discovered a great sensitivity to light, which allows them to detect light changes caused by an approaching predator and run away." The sea stars live among the rocks, they are equipped with five long arms (about ten centimeters in arm length) that stick out and grab the food. When a potential predator passes by, it casts a shadow on the arms, which the starfish quickly recognize and react to.
Handler suspected that tiny bumps located on the surface of the arms function as lenses, which transmit light from the environment to the starfish's nervous system. This suspicion was strengthened after he realized that nerve tubers were found under the bumps. It is possible, he thought, that within the calcite crystals that make up the bumps there is a mechanism that focuses the light on the nerves.
Eddy, Weiner and Eisenberg began to investigate the phenomenon. "We discovered that each bump is a calcite crystal, whose geometry indicates a structure of many lenses that focus the light. If you know the nature of the lens and the material it is made of, you can calculate where it will focus the light," says Addi. According to these measures, Eisenberg found that the geometric structure of the lenses means that the position of their focal plane corresponds to the position of the nerves passing under the lens. But is this really what happens in nature?
The answer was given after about seven years. Eisenberg removed calcite lenses from the exoskeleton of a sea star, installed them in a kind of mask under which he placed a light-sensitive material, and found that the focus plane of the lens (where it concentrates the light at a rate of 50 times) is exactly at the depth where the nerve tubercles are found.
The sea starfish's "vision" system is the first and only one of its kind that has been discovered so far in an animal that currently exists on Earth. A similar system was probably available to the trilobites, a series of marine animals that lived about 350 million years ago. "You can't really call it eyes, because we have no proof that starfish process visual data. We also don't know if they see an image. They are obviously sensitive to light," Weiner says.
In an accompanying article in Nature, the physicist Roy Sembles from the University of Exeter in England writes that such a precise structure and function of crystalline microscopic lenses requires a very precise control system, which supervises the growth of the calcite crystals until the formation of the lens. According to him, humans managed to develop similar microscopic lens arrays, at the same level of sophistication, only a few years ago. "Time after time," he writes, "we find that nature has been ahead of our technological developments in its sophistication."
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