You may not eat scallops with quite the same gusto again. The humble shellfish of the phylum Mollusca, a staple seafood delicacy, could have seen the fishermen coming — with hundreds of elegantly designed eyes. Now that scientists have had a detailed look at the tiny eyes of scallops, superlative adjectives are rising to the surface. Phys.org says:
Scallops may look like simple creatures, but the seafood delicacy has 200 eyes that function remarkably like a telescope, using living mirrors to focus light, researchers said Thursday [Emphasis added.]
Like a telescope? Yes! The eyes of Pecten maximus use biological concave mirrors, like the Newtonian telescope design. And there’s more design to talk about.
Scientists have known since the 1960s that these shellfish that inspired the Shell Oil logo had “eyes” of some sort, but it was difficult to dissect them. Now, new imaging techniques that freeze them before they can dry have allowed researchers from Lund University and Weizmann Institute to see them in detail and model how they work.
One thing that is interesting about the paper in Science by this team of ten researchers is that there is no mention of evolution. No, not even phylogeny, ancestor, mutation, or natural selection. The focus is on its unique functional design.
Although multilayered retinas have infrequently been observed in other animals, in these cases, they are used to enhance light sensitivity or act as spectral filters. In contrast, in the scallop, the upper and lower parts of the retina seem to be specialized for discriminating different fields of view. Thus, at the highest hierarchical level of organization, the complex 3D shape of the scallop eye mirror appears to be controlled to focus light from a broad field of view onto two retinas placed at different heights above its surface.
How Is the Scallop Eye Constructed?
Unlike other eyes in the animal kingdom, the scallop’s visual system uses mirrors in addition to a weakly refracting lens. The light path passes through a cornea, then an iris, then a lens, then through crystals of guanine stacked like tiles. The crystals form a biological concave mirror that reflects the light back through the system and onto a unique two-layered retina.
Some other animals, including spiders and beetles and even silver-haired monkeys, use guanine crystals for spectacular visual displays. This instance, though, deserves a new kind of design prize for the ultimate in functional art:
Perhaps the most complex optical function of guanine crystals in nature is in image formation. This function demands an extremely high degree of ultrastructural organization because light must not only be reflected but also focused. The hierarchal organization of the scallop mirror is finely tuned for image formation, from the component guanine crystals at the nanoscale to the overall shape of the mirror at the millimeter level. The scallop controls the crystal morphology and spacing to produce a tiled multilayer mirror with minimal optical diffraction aberrations, which reflects wavelengths of light that penetrate its habitat and are absorbed by its retinas. The mirror forms functional images on both retinas, which appear to be specialized for different functions.
What can you say but “Wow! That’s amazing”? We tend to think of vertebrate eyes as the best, but for its needs, the “simple scallop” has achieved optical nirvana.
Even Jerry Coyne was impressed. Over at Why Evolution Is True, he posted some of the best pictures and videos of scallops and their amazing eyes. But when it came to explaining them, all he could say was:
The mirror reflecting light onto an image-detector is precisely the way reflecting telescopes work, though human-constructed mirrors are very different from those of the scallop. In fact, I don’t think humans are capable of making mirrors like this bivalve does. As Leslie Orgel once said, evolution is cleverer than you are.
Ring the gong for that show!
The electron micrographs show tile-like crystal squares arranged like roof tiles in stacks. Everything in this arrangement is for a purpose, they explain:
The key to the functionality of the mirror lies in the regular square plates of β-guanine, which constitute the mirror’s basic building blocks. This unusual square morphology differs markedly from the theoretically predicted prismatic growth form of guanine. In this morphology, the crystal face with the highest refractive index (n = 1.83) is preferentially expressed, as is also the case in many other highly reflective natural photonic systems. The crystals are arranged so that the high-refractive-index faces are oriented toward the direction of the incident light across the mirror (fig. S1), creating a highly reflective surface. The square-plate morphology is also optimized for tiling. Each layer of the mirror is formed from an almost perfectly tessellated mosaic of two-dimensional (2D) squares — closely resembling the segmented mirrors used in reflecting telescopes. In Euclidean geometry, there are only three possible ways to completely tile a surface using regular congruent polygons: with equilateral triangles, with hexagons, or with squares. Crystal tiling minimizes surface defects at the crystal interfaces that would cause optical diffraction effects (which would result in a reduction in the image contrast) and optical loss owing to transmission of light through the mirror. Thus, at the lowest hierarchical level of organization, the scallop controls crystal growth to produce a crystal morphology that minimizes surface defects in the mirror and enables the formation of a highly reflective surface.
There’s more. The surface of the concave mirror is not perfectly spherical, but has a variable curvature with a flattened base. This sends the reflected light off-axis in two directions, to the proximal retina for the lower visual field, and to the distal retina for the upper visual field. “The nonspherical symmetry and tilt of the mirror produce more complex vision than was previously imagined,” they explain. “A simple on-axis, spherical mirror would not result in opposite sides of the visual field being focused as distinctly separate images at different heights above the mirror.”
How Does It Work?
Although it’s impossible to know for sure what the creature actually perceives, ray-trace models indicate that the scallop obtains more finely focused vision for nearby objects that move (triggering defense or escape behaviors), along with a wide field of peripheral vision that “could provide useful information to control and guide its movement while swimming with jet propulsion or to assess static features of its habitat.” The two types of focus also expand the dynamic range of vision, similar to how rods and cones overlap in brightness sensitivity in vertebrates.
One more question: Why does the scallop need 200 of these light detectors? For that, we have to consider the brain of this creature:
What benefit does the scallop receive by having up to 200 eyes located on the periphery of its semi-circular mantle, spanning ~250°? Ray tracing reveals that the images formed on both retinas of one eye vary substantially in focal quality across their visual fields. Interestingly, the optic nerves from nearly all of the eyes project on to the lateral lobes of the parieto-visceral ganglion (PVG), the site of visual processing in scallops. We speculate that neural processing in the PVG can combine the visual information from the substantially overlapping and differently focused views from multiple eyes, allowing the scallop to improve visual acuity relative to the isolated eye and potentially to determine the depth of features in the environment. This would offset the drawback of limited areas of well-focused vision in individual eyes.
This is a complete system, in other words, with all the contributing parts working together to optimize visual acuity. A short video by the AAAS (see the top of this post) puts the whole picture together.
It’s so good, the authors conclude, human engineers would do well to imitate it:
The crystal morphology, multilayer structure, and 3D shape of the scallop’s eye mirror are finely controlled to produce functional images on its two retinas. Understanding the strategies that organisms use to control crystal morphology and arrangement for complex optical functions paves the way for the construction of novel bio-inspired optical devices. In particular, the resemblance of the scallop’s tiled, off-axis mirror to the segmented mirrors of reflecting telescopes provides inspiration for the development of compact, wide-field imaging devices derived from this unusual form of biological optics.
So is design science a science stopper? Does scallop vision not make sense except in the light of evolution? We rest our case.