Tuesday, May 27, 2008

The superior eyes of shrimp

Blogging on Peer-Reviewed Research

We mammals have been beaten again. Shrimp have more sophisticated eyes than we do, with the ability to see things we can't, and I'm feeling a bit envious.

There are a couple of general properties of light that can be captured and measured with a light detector. One is the amplitude of the light wave, which we see as differences in the intensity of light. This is the most basic measurement of a photoreceptor, sensing the raw amount of energy being transmitted. Another property is wavelength, which we perceive as the color of light. Many mammals are incapable of detecting the wavelength, having monochromatic vision, so we're actually among the fortunate lineages that have a sufficiently elaborate photoreceptor system that we can actually see the color of objects.

Don't get cocky, though: there are some features of light that we can't see, but that would be very useful. One is phase. You can have two light waves of equal amplitude and wavelength, which look identical to our eyes, but they can be a fraction of a wavelength out of phase relative to one another.


That may not sound particularly useful to you because you've never experienced it. However, many materials shift the phase of light as it passes through them; a transparent (to our eyes) piece of glass can actually change the phase of light, as can cell membranes. We can build instruments to detect phase shifts, and they are extremely useful in microscopy — by aligning light waves that pass through a specimen with an unperturbed reference wave, we can generate patterns of constructive and destructive interference that our eyes can see, and voila, the transparent cell is made visible.

One other ability that we lack, but that many other animals possess, is light polarization detectors. Again, this is a property you may not be able to appreciate until you experience it, but it's another powerful visualization tool. Light waves can be vibrating in any of multiple planes. For instance, the plane of vibration can be up and down, or it can be side to side, or at any angle in between. Light can also be circularly polarized, where the plane of vibration corkscrews through space. (How, you may wonder? It's actually not hard; circularly polarized light can be decomposed into two linearly polarized light waves of equal frequency, but with the planes orthogonal to one another and with each out of phase with respect to the other).

We can't see the polarity of light! Up and down, side to side, it's all the same to our eyes, which is unfortunate, because we are often in environments where the light is polarized. Sunlight, for instance, is partially polarized as it passes through the atmosphere, and has a plane of polarization perpendicular to the direction of the sun, a property that many insects can use for navigation. Light reflected off of shiny surfaces is also selectively polarized. Polarizing sun glasses, for instance, are useful because than can selectively filter out glare reflected from horizontal surfaces. As light passes through otherwise transparent materials, like, say, a transparent animal swimming through the ocean, it can also be selectively polarized, a useful property to take advantage of if you are hunting for small, nearly transparent animals to eat.

The cephalopod eye has polarization detectors, but if you really want some sophisticated light detection, look to the arthropods, especially mantis shrimp. Even a superficial view of the eye reveals that something special is going on: there are specialized ommatida (the facets of the compound eye) and a curious equatorial band of aligned and specially organized receptors.

A) Adult Gonodactylus smithii, or mantis shrimp, ˜7 cm long. The stalked apposition compound eyes are divided into a dorsal and a ventral hemisphere by an equatorial mid-band of enlarged and structurally specialised ommatidia. Inset. Mid-band position indicated by curved dark lines. The three pseudopupils (dark spots) visible within each eye indicate that the visual fields of the two hemispheres and the mid-band almost completely overlap at the equator of the eye, so that the three eye regions view the equatorial strip simultaneously. Photograph by R.L. Caldwell. B) Frontal view of the right eye to illustrate the division of the eye into a dorsal hemisphere (DH) and a ventral hemisphere (VH) by the equatorial mid-band formed by six rows of enlarged ommatidia, numbered row 1 to row 6 from dorsal to ventral. Mid-band rows 1-4 contain spectral photoreceptors; mid-band rows 5 and 6 are specialised for circular polarisation vision; the dorsal and ventral hemispheres for linear polarisation vision. Recording electrodes were lowered through corneal holes cut in the lateral half of the dorsal hemisphere, where the mid-band is ˜15° relative to the equator of the eye. The black scale bar is 1 mm, the axes refer to Dorsal, Medial, Ventral and Lateral. C) Electron micrograph of a longitudinal section through a mid-band row 6 rhabdom. The alternating layers of microvilli are highly ordered and in thinner layers than in hemispheric rhabdoms. The polarisation discrimination D of mid-band rows 5 and 6 retinular cells is twice as high as that of hemispheric cells due to a more crystalline microvillar structure: c.f. D̅mid = 0.340±0.061 with D̅hemi = 0.145±0.035

Kleinlogel and White dug deeper into that eye and found that the structures specifically support detection of polarized light, both linearly and circularly. They inserted microelectrodes into the photoreceptors and measure the responses as the eyes were flashed with polarized light, and got some beautiful electrophysiological data that showed wonderfully clean response curves. They have optimal polarization vision, capable of resolving all of the parameters necessary to detect the plane of circularly and linearly polarized light in their field of vision. This is powerful stuff; mantis shrimps are moving through a visual world rich with novel details beyond our imagination, able to detect qualities of light outside our experience. The authors see the importance, too.

Stomatopods are shallow-water crustaceans in a visual environment with a partially polarised background. Crustaceans are known to use polarisation for navigation; many stomatopod prey species are either reflective or transparent but change the polarisation of the light—an obvious possible driver of evolutionary change. Optimal polarisation vision provides all the information about polarisation of the visual field without confusion states or neutral points—giving the greatest ability to detect changes in both the degree and type of polarisation. This goes beyond simple contrast enhancement: optimal polarisation vision is analogous to the improvement afforded by stereo over mono vision in terms of increased information capacity.

The only conclusion possible is that it is completely false that God hates shrimp — he must like them very much indeed, and the biblical injunction against eating them must be because he likes them better than us.

Original here

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