When making observations of the tiny flatworms I study, I seldom paused to consider that they might also be looking back at me. However, when sampling in estuaries near Woods Hole, MA, I recently encountered several animals whose visual system proved impossible to ignore. On each eye, normally an inconspicuous black spot, I found a tiny spherical lens perfectly situated above their light-sensitive cells! These animals turned out to belong to a group of minuscule predatory flatworms that have an organ on their head (the acorn-looking thing) which they can shoot out rapidly to subdue their Lilliputian prey. The animal you see above is probably Toia ycia, which gets to be about half a millimeter long as an adult. We’re not sure if Toia can see in the same sense that your dog or goldfish can – I’d doubt it personally. But it’s likely that their eyes are more powerful than those of most other flatworms, which at best distinguish light from dark and can approximate the direction of the light source.
I’m apparently not the only person that’s found himself interested in these eyes—zoologists have been using electron microscopes to peer deep inside their cells for years. These investigations revealed quite a surprise. Most of us think of mitochondria as the “powerhouse” of the cell, as we learned in high school; some may remember learning about their origins as symbiotic bacteria. In these flatworms, however, these enslaved microbes serve another purpose: by accumulating refractive proteins, packing together, and becoming enlarged, these mitochondria have become lenses that focus ambient light onto the light-sensitive cells.
Toia and its ilk aren’t the only flatworms that do this: mitochondrial lenses seem to be a feature of quite a few distantly related flatworms. Some of these worms are free living, such as the beach predator Ptychopera westbladi, or the photosynthetic Dalyellia viridis. Others are more or less parasitic, as for example, Urastoma cyprinae, a pest of the commercial oyster, or the fish-gill parasite Entobdella soleae, a distant relative of the too-familiar tapeworm and liver fluke.
What are we to think of this spotty distribution of mitochondrial eyes in the flatworm tree of life? Maybe the most recent common ancestor of these organisms had an eye with a mitochondrial lens, and this feature was lost or unrecognisably modified in most descendant lineages. Another possibility, however, is that this represents a convergence – unrelated lineages of flatworms may have all found a way to build lenses with mitochondria, just as dolphins, sharks, and ichthyosaurs all independently became streamlined for drag reduction in the water. Without knowing exactly how these flatworm eyes develop, and in particular the cellular signals these animals use to guide their mitochondria to differentiate into lenses, it is difficult to distinguish between one or multiple origins of the mitochondrial lens.
Other organisms, though, have clearly discovered their own ways of making lenses with endosymbionts. Some dinoflagellates, single-celled photosynthesizers of the shallow ocean, have found a way to make lenses of their photosynthetic plastids, endosymbionts that were engulfed independently of mitochondria. Yet another clear de novo reinvention of the lens has occurred in the acoel Proporus venosus, a type of animal that was once considered a flatworm, but which has recently been shown to be as distantly related to to Toia it is to you or I. Below is a video I made of a Proporus venosus individual I captured in Sardinia.