In an article/letter published in the Journal "Nature" on July 1st, 2015(paywalled), Gregory S. Gavelis, Shiho Hayakawa, Richard A. White III, Takashi Gojobori, Curtis A. Suttle, Patrick J. Keeling & Brian S. Leander explain that they found a single celled eye in the already complex unicellular Warnowiid Dinoflagellate.
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Warnowiid Dinofalgellates are plankton have been previously been studied for their highly elaborate ultrastructural systems: pistons, nematocysts, and ocelloids in a single celled eukaryote. (free full text)
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| Single celled organism with an EYE (Ocelloid) |
This Dinoflagellate Warnowiid is a single-celled sea orgaism that has fashioned itself an eye with a basic lens, cornea and retina. That is incredibly complex for a single cell.
In fact, the ‘ocelloid’ within the planktonic predator looks so much like a complex eye that it was originally mistaken for the eye of an animal that the plankton had eaten.
The warnowiid is found off the coast of Canada and Japan, but only in tiny numbers. Researchers first stumbled across one more than a century ago, but studying the creature proved difficult because it rapidly disintegrates after being taken out of seawater.
To catch his own warnowiid, Gregory S. Gavelis searched seawater samples under a microscope for a year. When he eventually found one he froze it in plastic resin, preserving it like a fly trapped in amber. He then made a 3-D model by taking snapshots of the warnowiid under an electron microscope – and was stunned by what he saw.
While other single-celled creatures can detect light using “eyespots” – simple structures that allow an organism to tell dark from light – the warnowiid seemed to have repurposed its internal organelles to form what resembled the lens, cornea, iris and retina of a complex eye.
The “cornea”, for example – the transparent outer layer at the front of the eye – was made of mitochondria, the bits of the cell normally responsible for energy production. The mitochondria in the warnowiid interlocked to form a sheet-like layer around the lens, curved to concentrate incoming light on to the “retina”.
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| (Left : Electomicrograph Right :Schematic ) of the Eye of the Warnowiid |
The light-sensing “retina” at the back of the eye was made up of plastids, structures normally involved in photosynthesis. The team extracted genetic material from inside the plastids and found it rich in algal DNA. The team suspects that at some point during evolution, a warnowiid ancestor gobbled up some algae and adopted its photosynthetic equipment. When the organism later abandoned photosynthesis for the predatory life, it repurposed its light-capturing plastids into a light-sensing organ.
What is the Warnowiid Eye used for ?
It is not clear how warnowiids use their “eye”, but the authors believe they’re used to hunt. Warnowiids spiral through the water as they swim and Gavelis thinks the eye lets them see flashes of light as it bounces off the single-celled organisms that are their prey. The lens likely concentrates incoming light to increase sensitivity – rather than projecting an image as our own eye does.
