Warnowiids- the floating eyeballs

Imagine gazing down a microscope to find a tiny eye peering at you through the lens. This is what scientist found after analysing Japanese seawater.

This incredible unicellular organism has evolved to almost mimic a human eye, equipped with a lens, cornea and retina equivalent structure, called an ocelloid.

The warnowiid left, and right an illustration of how the single-celled creature has fashioned an eye with basic components.

CREDIT: GREGORY GAVELIS UNIVERSITY OF BRITISH COLUMBIA

The cornea of this organism is made of mitochondria which are interlocked to form a layer wrapping the lens. This sheet is curved, and it’s purpose is to focus the incoming light onto the retina type structure.

This retina is the light sensitive part of the cell. After analysing the biochemical structure of this retina, scientists found structures called plastids. Plastids are a collective name for organelles involved with manufacture or storage of food in plants. A generic example would be the chloroplast: The structure responsible for photosynthesis, thus synthesising energy storage compounds. Other plastids include chromoplasts and leucoplasts, involved with storage of compounds such as carotenoids and production of starch or oil respectively. Plastids are surrounded by a double membrane. These membranes today are fused together, forming an interconnected parallel network sensitive to light, akin to the polarizing filters on cameras and sunglasses.

The plastids found in these warnowiids contained algal DNA, leading the scientist to believe that at some point during the organism’s evolutionary path, some algae were incorporated into the cell (symbiosis), and their photosynthetic equipment was adopted. Once photosynthesis was no longer essential, the plastids function was amended. Now, they are used primarily as a light sensory organ.

Light micrograph of a warnowiid dinoflagellate: double arrowhead points to the ocelloid, and arrow to the posterior cell extension. Scale bar – 10 μm. Image credit: Mona Hoppenrath et al., doi: 10.1186/1471-2148-9-116.

Scientists are not entirely sure about the function of this ocelloid- some suspect that they are used as a hunting weapon. As the warnowiids swim, they are seen to ‘spiral’ through the water. The eye could detect light that has reflected off other unicellular prey, focusing the light to increase the sensitivity (unlike the human eye which project an image onto the retina).

These occelloids are so bizarre that initially scientist mistook the structure for part of an engulfed prey!

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