erythropsidinium ocelloid dinoflagellates

http://www.newscientist.com/article/d...

The planktonic dinoflagellate Erythropsidinium possesses an ocelloid and piston. It is not necessary an ocelloid or an eye just only to detect light. More simple organelles with pigments, i.e., eyespots, are able to differentiate between light and darkness. The question is whether a unicellular organism, is able to see or at least to differentiate shadows.
A unicellular organism cannot see, I mean to interpret an image as humans and other vertebrates. In order to see, you need two organs: a brain and at least one eye, the second eye is useful to calculate distances. The eye is only translating the light signal, and the brain to interpret the image. The eye and the brain are multicellular structures, and in consequence unicellular organisms cannot see because they have not eyes or brain. However, Erythropsidinium possesses the most elaborate photoreceptor organelle among the unicellular organisms. A cut of the organelle reveals a structure analogous to the eye. It has the retina-like, melanosome, and hyalosome, like the liquid inside our eyes, and the lens-like. The lens of Erythropsidinium concentrates the light on the retina as in our eyes. However, the ocelloid of Erythropsidinium probably cannot project on the retina a real image. The reason is that the ocelloid of Erythropsidinium lacks a convergent lens. Even, if it is able to project a real image, there is not brain or nervous system to interpret the image. Erythropsidinium concentrates the light in its photoreceptor, retina, and when an object, a potential prey or predator, creates interference, the ocelloid of Erythropsidinium could register the position, the transparency and size. That is all the information that Erythropsidinium needs to know. I mean to find a suitable prey and to escape from potential predators.
While other unicellular organisms of the marine plankton are moving continuously, Erythropsidinium stays quiet, apparently observing around, even moving the ocelloid in different directions, and when disturbed it escapes moving vigorously the piston. No other organism in the nature has a piston. Both organelles, the ocelloid and the piston, give a competitive advantage.
Erythropsidinium and other relatives with an ocelloid are found in the euphotic zone, illuminate layer, of the ocean, Gómez 2008, Eur. J. Protistology 44, 291-298. Obviously, they have a competitive advantage when light is available. It is widespread in the open ocean, but with low abundance usually less than 10 cells per liter. The transparency of the open ocean waters favors the visual predators with an ocelloid. However, even if Erythropsidinium is an efficient predator, the density of potential preys in the open ocean is low and it cannot reach high abundances. Coastal waters are more productive, higher density of potential preys. However, the turbidity reduces the transparency and visual predators are less competitive.
The formation of eyes, the organs of extreme perfection for Darwin, must be evolved independently in different times. Studies based on transmission electron microscopy by Greuet in the late 1960´s revealed that the ocelloid of Erythropsidinium consist of a cornea-like surface layer, a lens-like structure, a retina-like structure with stacked membranes, and a pigment cup, all assembled in a single cell. Based on these observations, Gehring 2005, J. Hered. 96: 171-184, proposed a hypothesis for the origin of the eyes. He stated that because dinoflagellates are symbionts in corals, and other cnidarians, dinoflagellates, i.e. Symbiodinium, might have transferred the genes required for photoreception to the cnidarians, and further to other animal groups. However, this feature is not very common in the nature. Certainly, the probability of gene transfer would increase in organisms living in symbiosis. For example, if Erythropsidinium or their relatives could live in symbiosis with animals, we will have more reasons to consider the hypothesis of the gene transfer. Gómez et al. 2009, J. Eukaryotic Microbiol. 56, 440-445, provided the first gene sequence of Erythropsidinium. None of the close phylogenetic relatives of Erythropsidinium lives in symbiosis with animals. However, the results revealed that Erythropsidinium belongs to a group with photosynthetic dinoflagellates with chloroplasts of different origins. Greuet observed that the ocelloid looked like a chloroplast during its formation. We can interpret that Erythropsidinium is able to transform a chloroplast into an elaborate photoreceptor organelle. Although we cannot consider that Erythropsidinium has an eye. This evidences that eye-like structures converge into the same morphology and the complex photoreceptors have different origins in the evolution.