Plankton's 'eye' made up of organelles, study suggests

Scientists have cracked open the structure of a one-celled organism's mysterious eye, which they believe it may have evolved to better see its prey.

The single-celled marine plankton, known as a warnowiid dinoflagellate, evolved an ocelloid, which is a small version of a multi-cellular eye.The eye looks like a dark purple spot on the plankton.

• Read the research inNature

"It's an amazingly complex structure for a single-celled organism to have evolved," said Greg Gavelis, lead author of a paper published in Nature on Wednesday and a PhD student at the University of British Columbia, in a statement.

"It contains a collection of sub-cellular organelles that look very much like the lens, cornea, iris and retina of multicellular eyes found in humans and other large animals."

Due to its complicated nature, the eye was initially mistaken for one of the plankton's consumed prey when it was first seen and described by marine biologists in the 1920s.

It's still unclear how the plankton uses its ocelloid.

The plankton hunts other dinoflagellates,many of which are transparent. The ocelloid may help the plankton detect its prey, the Nature paper authors say, by allowing them to notice shifts in light as it passes through the prey. It's possible the ocelloidthen informs the plankton which direction to hunt in.

This theory has yet to be tested.

The researchers obtained the hard-to-find warnowiid dinoflagellates off the coasts of British Columbia and Japan. They then sequenced their DNA before examining them with powerful microscopes capable of assembling three-dimensional images of structures that are smaller than cells.

Those images led them to believe that the ocelloid's retina-like part is composed of cellular components called plastids, while its equivalent of a cornea is made of other organelles called mitochondria.

Symbiosis

Both plastids and mitochondria are thought to have once been independent unicellular organisms red algae and bacteria, respectively. At some point more than a billion years ago, they entered into the more complex cells of other single-celled organisms in a process called endosymbiosis, the theory goes.

"As such, the ocelloid is a chimaeric structure, incorporating organelles with different endosymbiotic histories," the scientists write.

They say their findings show that plastids and mitochondria, known mainly as power plants within the cell that help generate energy, can have other roles as well and "be building blocks for greater structural complexity."