Br J Ophthalmol 2004;88:1113 1113 Br J Ophthalmol: first published as 10.1136/bjo.2004.049510 on 17 August 2004. Downloaded from Cover illustration...... You are what you eat

ebate over the smallest lenticular composition is not known, requires a definition of what the anatomy is. The consists of Dmakes an eye. The answer to this clear, concentrically arranged layers and question, generally given by those con- is poised above the pigment cup. The sidered authorities, is that an eye is an index of refraction of the lens is high organ that receives and recognises light at 1.52, but that would seem appro- and has the ability to define spatial priate since the organism is pelagic. detail. Specifically, an eye can compare Appropriate ray tracing documents that the amount of light coming from differ- rays striking the lens would come to ent directions. This definition means focus on the retinoid (the term used for that an unusual collection of photo- the presumed photosensitive portion of receptors that may perceive light, but the ocelloid), and the field of view is cannot tell direction or form of an thought to be about 30˚. The whole image, such as those found in the tail structure actually protrudes from the of some of the sea snakes, would not be cell, seemingly to point in different considered an eye. directions. A related species has been By most definitions then, the plank- Warnowia is an unusual ocelloid species documented to use its flagellum to because it also has a functional (green at mediate phototaxis or at least orienta- tonic , Erythropsidium, bottom of organism). The streaks you see on the must have among the smallest of , right are the chromosomes! tion relating to light stimulus. Other since the creature is only 50–70 mmin with an ocelloid have diameter. Next to nothing is known been documented to have a pigment single celled kingdom. Protists are about its genetics or visual mechanisms; within the eye cup that reflects and mainly aquatic, but terrestrial species nevertheless, we do know that this absorbs blue-green wavelengths with a remarkable eukaryote speaks volumes do exist in moist soils. Single celled they strong directional component. Such about evolution’s creativity and empha- may be, but simple they are not. These light was found to be focused on the sis on vision, although the term ‘‘vision’’ diverse and adaptable species often have basal portion of the longitudinal flagel- goes beyond an eye and must include specialised features including chloro- lum and presumably contributed to some degree of interpretation of the plasts, toxins (responsible for the red intracellular communication (Keimer image. It is doubtful that this organism tide), bioluminescence, and flagella, G, Reflective properties of different eyespot interprets any image. allowing them to occupy otherwise types in dinoflagellates 1999;150:311–23). Dinoflagellates are protists, or single challenging niches. Many are commen- These rather perplexing creatures celled nucleated organisms. Such single sals, saprophytes, or parasites––some- have been seen to chase prey, hence http://bjo.bmj.com/ celled eukaryotes (organisms that have times they parasitise each other. must be predators. And yet, they have cells with nuclei) have been traced at Certain flagellated protozoa are . Other related protists are least to the Cambrian period (see known to have phototaxis, possess an known to be so opportunistic as to be February 2004 BJO cover essay) or even eyespot, and can probably sense direc- autotrophic (photosynthetic) when light the Precambrian, but fossil records tion and perhaps even intensity of light is plentiful, and heterotrophic (eat other extend the reign of protists to at least with these grouped pigment granules. organisms) in the dark. These bizarre But one group of flagellates actually dinoflagellates, then, are neither plant

the Lower Middle Proterozoic era, about on September 23, 2021 by guest. Protected copyright. 1.2 billion years ago. Technically, a does better than that—the dinoflagel- nor , but have components of protist does not develop from a blastula, lates. each. so it is not an animal; it does not Our cover species, Erythropsidium,is Lynn Margulis, in her recent book, develop from an embryo, so it is not a one of several dinoflagellates that are Acquiring Genomes, asks a rather pro- plant; it does not develop from spores, known to have an ocelloid complex found question that would follow once so it is not a fungus; but it does contain enough to resemble an eye. One such one knows of these dinoflagellates and a nucleus so it is not a prokaryote or dinoflagellate is known to have a lens, their subcellular ocelloid. Could these bacterium. Protists are varied and most or hyalosome, that can change shape, an creatures have been the source of the unusual. Some protists, for example, ability that could be considered a form first eye? Perhaps, another protist or have 100 times the amount of DNA that of accommodation. There is also a clear metazoan did not evolve an eye, but human cells have. Most zoologists space overlying a pigment cup that is rather co-opted one from a dinoflagel- believe protists belong in their own filled with carotenoid globules and some late by ingestion. In that scenario, kingdom. unknown form of visual pigment. The evolutionary refinements and improve- Beetles are the most plentiful animal pigment cup, also known as the mela- ments eventually led to other forms of species (July 2004 BJO for review), but nosome, is probably of plastid (chloro- eyes including both those of compound the protists are probably more plentiful plast) origin and is divided into two and camera style. You are what you eat. still. Few would hazard a guess as to parts, including the retinoid or retinal I R Schwab how many nucleated algae, water body of precisely aligned membranes University of California, Davis, Sacramento, moulds, slime nets, diatoms, and other within a layer of carotenoid and mela- CA, USA; [email protected] such species exist, and we surely must nin pigments. A related dinoflagellate Photographs and essay advice (thanks for have an inadequate understanding and has been studied in an attempt to both) by FJR ‘‘Max’’ Taylor, University of count of this virtually uncountable analyse the lens, and although the British Columbia,Vancouver, Canada.

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