MICHELLEHAYWOOD
Vision Thing
Michelle Haywood takes a deep dive into the extraordinary vision of the humble scallop
The Isle of Man has established itself an excellent reputation for wellmanaged scallop fisheries . The Crown Dependency has its own political system and control over its territorial seas . Buoyed by the research carried out in the Liverpool University Marine Lab based in Port Erin , there was a pioneering project to close an area of seabed over 30 years ago . Port Erin Closed Area was the first study to show that closed areas could enhance fisheries by providing a breeding ground where overspill would populate adjoining fishing grounds . But the scallops saw it coming ...
Molluscs have evolved some sort of eye many times over . Some have compound eyes ( a bit like an insect ). There are a wide range of different structures and special light sensitive proteins in play across various species . Scallops have up to 200 eyes , each about 1mm in size along the edge of their mantle . And scallop eyes are very different to our own in structure .
In our eyes , the retina sits at the back of the eye and special light-responsive proteins in our cone and rod cells absorb the light . Behind that , a dark pigmented sclera prevents any light being reflected . In the scallop , eye light passes through a cornea , through a pupil and lens and through two retinas before it hits a mirror at the back . Yep , a mirror .
The mirrored layer is made from crystals of guanine . That ’ s the same molecule that makes up part of DNA ( the G of the ATCG pieces ). There are 20-30 layers of guanine crystals arranged in closely tiled plates , like a mosaic . These crystals reflect light around 500nm wavelengths exceptionally well . The mirror later reflects the light onto the inside surface of the proximal ( closest ) and distal ( furthest away ) retinas .
This in turn generates a signal to the optic nerve , which connects to the scallop ’ s adductor muscle . That ’ s the bit that you eat – its primary function is to pull the two shells together for either closing the shell or propulsion . So as a diver ( or other predator )
“ So , the scallop has an eye that is more akin to the structure of a telescope ”
swims over a scallop , the scallop senses that and closes its shell .
So , the scallop has an eye that is more akin to the structure of a telescope than that of the human eye . It is effectively a reflective mirror sending all the light onto two retinas . The images on the distal retina are better focussed than the ones on the proximal retina , so this is how scallops discern shapes . The proximal retina is more sensitive to light levels and becomes more important at night .
Light is detected by opsins , light-sensitive proteins that change shape when a photon of light hits them . Humans have four opsins . Scallops have 12 . These proteins are so important to scallop survival that the genes have been duplicated and over time those copies have mutated slightly . The end result of these changes is to generate novel opsin proteins that can respond to different wavelengths of light . It ’ s not quite technicolour vision , but it ’ s close . Having 12 opsins increases the scallop ’ s ability to detect changes in the environment , such as incoming predators ( aka divers ) and respond accordingly .
Scallop eyes aren ’ t just responding to light and dark . We ’ ve all seen scallops close without any shadow falling on them . Scallops are capable of spatial vision . Each retina has different photo receptor cells with different opsins . The lens of the eye isn ’ t totally in line with the mirrored layer . This allows the scallop eyes to focus at different distances , something we achieve by changing the shape of our lens . Scallops can also adjust the amount of light entering by changing the size of their pupil , so that the light-sensitive proteins don ’ t become overwhelmed .
Scallops sense their environment in a way that directs where they will move to . They can look at different substrates and swim towards a more suitable patch of seabed . The humble scallop possesses only a rudimentary nervous system with a few bundles of ganglions . There ’ s no brain to think through the responses that the scallop makes . But next time you swim past a scallop , just be aware that 200 tiny eyes saw you coming , although they probably won ’ t remember you when you swim back past them again . �
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