Sea Creature Sees Without Eyes

 

A relative of starfish does not have eyes, but can still see. That information comes from scientists who studied sea creatures in the coral reefs of the Caribbean and Gulf¹ of Mexico.

The researchers reported this month that the starfish relative -- called the red brittle² star -- is only the second creature known to be able to see without having eyes. This ability is called extraocular vision.

The other creature said to have extraocular vision is a kind of sea urchin³.

Brittle stars, with five arms extending from a central disk, are part of a group of marine⁴ life called echinoderms. They have a nervous system but no brain.

The red brittle star measures up to about 35 centimeters wide, from the end of one arm to the other. It lives in bright and complex environments. Because of the possibility of being eaten by fish, the creature hides during daylight hours.

The red brittle star possesses extraocular vision as a result of light-sensing cells, called photoreceptors. These photoreceptors cover its body and chromatophores, the cells responsible for pigment⁵ or coloring.

During daytime, the chromatophores narrow the field of light being detected, making each photoreceptor like the pixel of a computer image. When combined with other pixels, the image becomes complete.

The visual system does not work at night, when the chromatophores contract in size.

Laboratory experiments suggested that the brittle stars have very simple vision. Placed in a circular environment, for example, they moved toward walls that were white with a black bar, suggestive of a daytime hiding place.

Lauren Sumner-Rooney is a research fellow at the Oxford⁶ University Museum of Natural History. She led the study, which appeared in the publication Current Biology.

"If our conclusions about the chromatophores are correct, this is a beautiful example of innovation in evolution," she said.

Sumner-Rooney added, "It's such an alien concept for us, as very visually driven animals, to conceive of how an animal might see its habitat without eyes, but now we know of two examples."

Words in This Story

coral reef – n. a rocky, underwater area formed by the growth of coral

extraocular – n. taking place outside the eye

species – n. a group of living organisms made up of similar individuals

detect – v. to identify or discover

pixel - n. any one of the very small dots that together form the picture on a television screen or computer monitor

contract – v. to decrease in size; shrink

innovation - n. the act or process of introducing new ideas, devices, or methods

evolution – n. the development of something, from a simple to a more complex form

conceive - v. to think of or create (something) in the mind

1 gulf   

n.海湾；深渊，鸿沟；分歧，隔阂

参考例句：

• The gulf between the two leaders cannot be bridged.两位领导人之间的鸿沟难以跨越。
• There is a gulf between the two cities.这两座城市间有个海湾。

2 brittle   

adj.易碎的；脆弱的；冷淡的；(声音)尖利的

参考例句：

• The pond was covered in a brittle layer of ice.池塘覆盖了一层易碎的冰。
• She gave a brittle laugh.她冷淡地笑了笑。

3 urchin   

n.顽童；海胆

参考例句：

• You should sheer off the urchin.你应该躲避这顽童。
• He is a most wicked urchin.他是个非常调皮的顽童。

4 marine   

adj.海的；海生的；航海的；海事的；n.水兵

参考例句：

• Marine creatures are those which live in the sea. 海洋生物是生存在海里的生物。
• When the war broke out,he volunteered for the Marine Corps.战争爆发时，他自愿参加了海军陆战队。

5 pigment   

n.天然色素，干粉颜料

参考例句：

• The Romans used natural pigments on their fabrics and walls.古罗马人在织物和墙壁上使用天然颜料。
• Who thought he might know what the skin pigment phenomenon meant.他自认为可能知道皮肤色素出现这种现象到底是怎么回事。

6 Oxford   

n.牛津（英国城市）

参考例句：

• At present he has become a Professor of Chemistry at Oxford.他现在已是牛津大学的化学教授了。
• This is where the road to Oxford joins the road to London.这是去牛津的路与去伦敦的路的汇合处。