To most people, scallops bring to mind butter, pan frying and deliciousness, but there’s much more to these unusual animals than meets the eye. For a start, they do actually have eyes, as many as 200 of them depending on the species, and they are hardly typical.
Evolution has only produced three different ways of forming an image on the light sensitive retina. Of these three possible solutions, the lens, is the most common and will, of course, be familiar to all of us. However, a very small set of animals have evolved vision without the use of a solid, crystalline lens. One approach, taken by the ancient deep-sea mollusc nautilus, is to use its narrow pupil to form a dim and blurry image, much like the aperture of a pinhole camera. The other solution found in nature is almost unbelievable; rather than using a crystalline lens to focus light, some animals use mirrors instead. It is by using mirrors, rather than a lens, that scallops can see the world around them and with a surprising amount of detail.
We use metals to make mirrors, but this is not an option available to animals. Instead, scallops use an organic compound called guanine which has a very high refractive index. It is guanine that gives fish scales their extraordinary shine. Interestingly, it is also used in many man-made products such as shampoo or cosmetics to give an attractive iridescent sheen. The scallop mirror consists of a layered mosaic of flattened square guanine crystals interspersed with cytoplasmic space, which has a much lower refractive index. It is this alternating stack of low and high refractive indices that makes the scallop mirrors so reflective. Due to the physics of how light interacts with structures of alternating high and low refractive indices, the total amount of light reflected will depend on its wavelength (ie the colour). The scallop mirror is tuned to reflect blue-green light, which given that they tend to live in shallow, coastal waters that are greenish in colour, means their eyes collect the available light very efficiently. It goes without saying that the brightness of an image is crucial in order for an animal to be able to see the world around them in any sort of detail. That image must also be in focus. So how exactly can mirrors be used by scallops to focus light?
It turns out it is all about the placement of the mirrors and the retina within the eye. Scallops have hemispherical eyes. Although they do not have a typical crystalline lens capable of strong refraction, they do have a gelatinous region at the front of their eye (roughly analogous to the vitreous humour in a human eye) that acts as a very weakly refracting lens. Light entering the scallop eye is refracted, but not to the extent that it forms a clear image on the back of the eye. It is at this point that the scallop eye diverges greatly from all other types. The scallop actually has two retinas, the proximal and the distal retinas, which sit back-to-back and rather than being at the back of the eye, this double retina is actually suspended towards the centre. Instead the back of the scallop eye is coated by a hemispherical, concave mirror. As a result, any light that reaches the back of the eye gets reflected. Due to the geometry of the concave mirror coupled with the weak refraction of the gelatinous lens, the reflected light is focused at a point midway between the back of the eye and the centre of curvature of the mirror; exactly on the back of the suspended retina. The result is a miniaturised, inverted but focused image of the outside world. Amazingly, this is the same way that some telescope designs work. So, in effect, scallops have miniature telescopes for eyes.
Figure 2: A close-up of scallop eyes (Image: Sönke Johnsen)
Recent re-examination of the scallop eye using the advanced imaging techniques of cryo-scanning electron microscopy and x-ray micro-computed tomography have revealed they are more sophisticated than initially thought. Rather than being completely spherical, the scallop mirror is slightly flattened in the centre, shifting its optical axis. The result is that the mirror has a variable focal length, depending on where in the field of view the light came from. Light entering the eye from directly in front (0° to 20°) is focused as described earlier; directly on the back of the retina. But light entering the eye more peripherally actually ends up being focused further up, towards the front of the retina. This suggests that the two parts of the retina, proximal and distal, have different roles. The distal part at the back of the retina is responsible for viewing the world directly in front of the scallop with a high degree of detail, while the distal part at the front is responsible for providing sharp peripheral vision.
There is a drawback to this design: unfocused light must first pass through the outer, proximal retina before it is reflected and focused on the inner, distal retina. There is no way for the scallop to tell whether the signal produced by the photoreceptors in the retina is from the unfocused light as it enters the eye, or from the focused image. As a consequence, the contrast of the scallop’s vision is reduced, as effectively the clear, focused image is overlaid with a very blurry image of the same scene. Despite this, scallops have remarkably sharp vision. They’re capable of detecting dark moving objects as small as 2° in size, which, although about 100 times worse than our visual acuity, is remarkable for a bivalve mollusc and is on a level with bees, crabs and some spiders. By using mirrors, the scallop has evolved both sharp frontal and peripheral vision. So what do scallops use this complex vision for?
Although they are shellfish, scallops live an unusually active life. Unlike oysters and mussels, scallops do not attach themselves to rocks, but roam freely across the sea bed. By opening and shutting their shell using a large adductor muscle (that is the bit we eat) to rapidly expel water through holes or gaps in their shells, they are capable of propelling themselves several feet through the water. There is something undeniably comical and inherently unexpected about watching a scallop ‘swim’. They use this ‘jet propulsion’ to avoid predators and to relocate to desirable habitats.
Figure 3: A cross-section of a scallop eye. Light (i) entering the eye first passes through the very weakly refracting gelatinous lens (l). The scallop retina (r) is not placed against the back of the eye, but ‘floats’. Instead, there is a mirror (m) at the back of the eye. Incident, unfocused light first passes through the retina before reflecting from the mirror. Upon reflection, this light is focused (f) on the back of the retina. The whole eye is surrounded by an opaque structure (o) that is commonly bright blue in colour
Surprisingly, one of the scallop’s main predators is the starfish. Although we tend to think of starfish as stationary, slow creatures that do not pose much of a threat, they are actually very mobile predators. They use hundreds of tubular feet to crawl around the ocean floor on the prowl (or more accurately, a crawl) for their next meal. Like us, starfish consider scallops to be a delicacy. Once they have detected the scent of a scallop, starfish attempt to ambush it quickly enough before the shell clamps shut. They then regurgitate their stomachs to swallow the soft body of the scallop whole, digesting it with their powerful enzymes. More familiar predators include fish, rays and lobsters. Fish tend to nibble on the scallop’s soft tentacles, so all it has to do is simply clamp its shell shut to prevent an attack. However, while their hard shell may deter fish, it does not offer much protection from the claws of a determined lobster. In this situation, the scallop’s only hope is to swim away to safety. Clearly, there is an imperative for the scallop to spot a predator before it gets too close, making sharp vision essential for survival. Vision also helps direct their evasive swimming manoeuvres. It would be ironic (and deadly) for a scallop escaping one predator only to end up swimming into the path of another. So the combination of sharp frontal and peripheral vision allow a scallop to spot potential predators and to escape safely.
Figure 4: Scallops can swim. They propel themselves through the water by rapidly opening and closing their shell, expelling water through small holes, effectively giving them jet propulsion
Despite their active nature, scallops are filter feeders rather than scavengers or predators. Those ‘teeth’ in the image at the beginning of this article are actually their gills, which scallops use to filter plankton, their food of choice from the water. There is strong evidence that they use visual cues to modify how they filter food. As discussed previously, to keep safe, it is advantageous for a scallop to minimise the amount that it opens its shell. However, if they do not open their shell wide enough, then they will miss out on larger pieces of food floating in the water column. Using a
laboratory set up with screens to simulate the appearance of plankton, researchers discovered that scallops use visual cues to determine how wide to open their shell when feeding. When shown a simulation of large food particles, the scallops gaped wider than they did when shown smaller food particles. Similarly, they gaped wider when the food appeared to be moving slowly than when it was moving more quickly. Presumably, the faster moving food is easier for them to filter than the slowly moving food, meaning they need more area across their gills to capture the same volume of food.
While we now understand the optics of the scallop eye in great detail and the role vision plays in the life of a scallop, questions still remain. For instance, it is not known why scallops have so many eyes. While it is likely to be a strategy for improving their field of view (insert the all-the-better-to-see-you-with joke here), this has yet to be proven. We do not even know if the different views from each eye are combined to form a ‘whole’. Nor is it clear whether there is a reason for the outside of the eyes to be such bright colours. It has been hypothesised that the optical structure of the blue material that forms the outer layer of the eye of the bay scallop acts as a sunscreen; preventing short wavelength UV light from entering the eye and causing damage. So in addition to being tiny telescopes with built-in sunglasses, it could be that scallop eyes have even more surprises in store for science.
Hopefully the knowledge of the scallop’s surprisingly sophisticated visual system has not put you off enjoying them as a starter, but has given a new appreciation of the amazing diversity of visual systems that have emerged through evolution. •
Dr Ilse Daly is a research associate at the School of Biological Sciences at the University of Bristol.
