Scientists at the University of British Columbia have identified an eye-like structure in a rare, single-celled marine plankton from the family Warnowiaceae. It is more common that multicellular organisms gain morphological complexity, but this miniature human-like ‘eye’ indicates a significant advancement from the marine plankton’s single-celled competitors.
Members of the Warnowiaceae family, called warnowiids, are found off the coast of Japan but are very rare. It took Greg Gavelis, a zoology Ph.D. student at the University of British Columbia, over a year to find them.
“You’d be lucky if you ever saw more than five in a single Petri dish,” Gavelis said.
Another challenge faced in the attempts to study the warnowiid is its ability to disintegrate once taken out of seawater. To combat this roadblock, Gavelis placed the organism in plastic resin, arresting it in place and assuring it stayed intact. His team used electron microscope snapshots of the creature to create a 3-D model.
The eye-like ‘ocelloid’ is produced from repurposed cellular organelles creating complex structures similar to a lens, cornea, iris and retina. The ‘cornea’ is made up of mitochondria — an organelle specialized for respiratory needs and energy production — linked together to form a dome-like layer around the ‘lens.’ The dome shape, like our own eyes, concentrates light on the retina. The ‘retina’ is made up of a web of plastids enclosed in a single membrane. Plastids play an important role in photosynthesis.
“They’re like drops of oil in a lava lamp,” says Gavelis. “The degree of specialization in this one structure just boggled my mind.”
Warnowiids are predatory creatures, so it is peculiar that they have so many structures that are used for photosynthesis. The team of scientists theorizes that a warnowiid ancestor consumed algae and kept some of the photosynthesizing structures, in this case, plastids, for its own use. When the warnowiid finally became predatory, it recycled the plastids into the ‘retina’ of the light-sensing organ.
So, how do they use their eye and why? This is still a puzzling question for the scientists. Although the eye may have structures resembling a human eye, its purpose is entirely different than that of a human eye. The researchers believe that the eye is used for hunting; instead of projecting images, the lens increases light sensitivity allowing warnowiids to see their prey when light bounces off of them. Also, the structure could send chemical messengers to other parts of the cell to provide a sense of direction. This is an important mechanism as most of the warnowiid’s prey is transparent.
The scientists’ research has implications beyond understanding the elusive warnowiids. The evolution of the organism possessing a light-detecting structure to an increasingly complex eye structure can provide a starting point for understanding the evolution of animal eyes.
“Critics of evolution often talk about how there are no transitional forms of eyes,” says Gavelis, “But they’re alive and well in this case.”