I've mentioned before that when I was a practising research scientist I investigated symbioses between animals and unicellular algae, and my chief experimental organism was the humble green hydra
. It's a freshwater coelenterate (a relative of jellyfish, sea anemones, and corals, which use specialised stinging cells, cnidocytes, to catch prey and to defend themselves and move about) that's easy to culture in the laboratory and reproduces by asexual budding, so in a relatively short time the researcher can grow up a large, cloned population (hydras can also reproduce sexually, but if they are kept in constant conditions it's a rare event). It's also a wonderfully simple animal, with just two cell layers separated by an acellular mesoglea, a mouth ringed by tentacles at one end, and a foot, or pedicule, at the other, which adheres to the substrate. There are relatively few types of cells, a simple nerve net, and that's about it. So it's a useful lab model not only in the investigation of symbioses, but in all kinds of developmental studies, too.
Here's one of the most recent, and most interesting
. Researchers using a non-symbiotic species of hydra have discovered that its stinging cells exhibited a sensitivity to light - they are more likely to fire at low levels of light or in darkness, while bright light actually inhibits their firing. That's interesting in its own right - hydra prey on water fleas and other small swimming animals, whose activity may correlate with the activity of the hydras' stinging cells. But there's more. That activity is regulated by a species of light-sensitive chemical, opsin
, which is also found in the visual systems of higher animals, including mammals. So although hydras don't have physical structures analogous to eyes, they are photosensitive, and that photosensitivity is regulated by a chemical that has a very similar function in the human eye. The 'eye' of the common ancestor of hydras and the eyes of higher animals (fish, fowl, mammals, us) share a common pathway.
Parenthetically, it would be fun to examine to role of opsin in the behaviour of green hydra, which will migrate towards a bright light shone in one corner of their culture dish, presumably to maximise the photosynthetic output of their symbiotic algae. It might also be interesting to discover if green hydra's feeding behaviour is diurnal, or if it is just as active feeding by day as at dusk, or night (the polyps of reef-forming corals seems to photosynthesise by day and feed by night, getting the best of both being a plant and a predator).
There's also an important evolutionary angle, as one of the researchers, Professor Todd Oakley, points out: "What good is half an eye? Even without eyes there are other functions for light sensitivity that we may not be thinking of."
This is precisely the problem that Charles Darwin raised in On The Origin of the Species
, in a sentence that's often quoted by opponents of evolutionary theory:
To suppose that the eye with all its inimitable contrivances for adjusting the focus to different distances, for admitting different amounts of light, and for the correction of spherical and chromatic aberration, could have been formed by natural selection, seems, I freely confess, absurd in the highest degree.
Darwin went on to say (and this is the bit that his opponents often miss out):
When it was first said that the sun stood still and the world turned round, the common sense of mankind declared the doctrine false; but the old saying of Vox populi, vox Dei, as every philosopher knows, cannot be trusted in science. Reason tells me, that if numerous gradations from a simple and imperfect eye to one complex and perfect can be shown to exist, each grade being useful to its possessor, as is certainly the case; if further, the eye ever varies and the variations be inherited, as is likewise certainly the case; and if such variations should be useful to any animal under changing conditions of life, then the difficulty of believing that a perfect and complex eye could be formed by natural selection, though insuperable by our imagination, should not be considered as subversive of the theory. How a nerve comes to be sensitive to light, hardly concerns us more than how life itself originated; but I may remark that, as some of the lowest organisms in which nerves cannot be detected, are capable of perceiving light, it does not seem impossible that certain sensitive elements in their sarcode should become aggregated and developed into nerves, endowed with this special sensibility.
On The Origin of the Species, 6th Edition, Chapter 6
Darwin goes on to describe examples of possible transitional forms. The photosensitive 'eye' of the hydra is one such, and may help us understand 'how a nerve comes to be sensitive to light', one of the first steps in the evolution of the complex mechanism that is helping you read this.