Category: Eyes & other Visual Systems

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The eye, so central to human existence, expression and emotion, is one of the main Darwinian paradigms. How, it was once pondered, could such a superb piece of biological engineering have evolved? Now the mystery may have evaporated in as much as we have perfectly adequate explanations for how the eye evolved, but the many fascinating examples of convergence show that both the limited number of options as well as the sheer sophistication throw new light on the way we understand the evolution and its potentialities. Perhaps the classic case is the convergence seen in the camera eye between cephalopods and vertebrates. The convergence, however, is much more striking in that the camera eye has evolved independently in at least six other groups: the alciopid annelids, three groups of snails and most astonishing of all the cubozoans or box-jellies, which lack any kind of brain. There are many different sorts of eye, but other than the camera eye by far the most successful is the compound eye. This too is strikingly convergent, having evolved at least five times: in the arthropods (at least twice), the sabellid annelids, the bivalve molluscs and, remarkably, in the brittle stars, the eye of which has a composition of calcite and so is specifically convergent on those of the extinct trilobites.

Other examples of convergence in the eye relate to its many functional requirements, such as correction for spherical aberration, ultraviolet absorption, the development of an accessory retina, the ability to detect polarized light and and perhaps most remarkably so-called double-eyes such as these employed by some fish to see both in water and in air. In addition, we find the independent evolution of the fovea. This is particularly interesting because the fovea is well known in the camera eye, but not only can occur in the compound eye but even more remarkably a direct equivalent is found in other sensory systems, notably the tactile sense and echolocation.

The eye is an important area for the study of molecular convergences, including for example crystallins (proteins accounting for transparency of the lens) and opsins (key proteins involved in the transduction of photons into electrical signals). Whilst the opsins of animal eyes probably have a single origin, this protein is convergent with the bacteriorhodopsin of bacteria. Another classic case is the evolution of spectral sensitivities that can be traced as far back as the bacteria. These sensitivities include striking examples such as ultraviolet and colour vision, and even trichromatic vision has evolved several times.

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This table lists all the Topics which are part of the Category "Eyes & other Visual Systems"
Topic title Teaser text Availability
Bioluminescence Flying through the air on a summer's evening or sparkling in the ocean you may see magical flashes of light that signal some of nature's most enchanting creatures, those that are bioluminescent. Available
Monochromacy in mammals Underwater environments are dominated by blue light. Ironically, whales and seals cannot see blue, because they have independently lost their short-wavelength opsins. Available
Four-eyed fish n/a Unavailable
Ultraviolet (UV) vision in insects and vertebrates n/a Unavailable
Independent eye movement in fish, chameleons and frogmouths One of the most surprising convergences amongst animals is that seen between a small fish that lives in coral sands, known as the sandlance, and the lizards known as chameleons. Available
Bats: Insights into convergence Bats show a fascinating array of convergences, from echolocation to flight to nectar feeding. Vampire bats can even detect infrared radiation, while others might be able to see into the ultraviolet end of the spectrum. Available
Camera eyes in cubozoan jellyfish On each of the four club-like extensions (rhopalia) near the base of the cubozoan jellyfish bell there are two camera-eyes, one pointing upwards and the other downwards. Available
Trichromatic vision in mammals Who has not enjoyed the splash of colour in a market: gorgeous red peppers, the green of basil and what on earth are these purple vegetables over there? All thanks to trichromatic vision, another story of convergence. Available
Protistan eye-spots and warnowiid dinoflagellate eyes Warnowiids propel themselves through the water, but unlike other dinoflagellates which are photosynthetic, they are hunters and the chloroplast is employed to make the lower part of the eye. Unavailable
Mammalian adaptations to underwater vision One of the more obvious eye adaptations in whales is the extraordinary capacity of the pupil to change the size of its opening, from very small when in surface sunlit water to very large when exploring the abyssal gloom. Unavailable
Light sensitivity and optics in sponges Some of the silica spicules of glass sponges are very long, and extraordinarily have a striking similarity to the optical fibres employed in the telecommunications industry. Unavailable
Nocturnal colour vision in moths, geckos and aye-ayes Nocturnal colour vision is clearly convergent, and found in groups as disparate as the hawkmoths (insects), geckos (reptiles) and aye-aye (mammals). Unavailable
Infrared detection in snakes Warm-blooded rodents watch out! There are heat-sensing predators on the prowl... Available
Ancient opsins and vision in extinct animals Spectral tuning of the eye generally depends on key substitutions of amino acid sites in opsin proteins. Available
Mitochondrial lens formation in flatworms In some of the flatworms (platyhelminthes) the lens is formed from mitochondria, and it is intriguing to speculate whether a mitochondrial enzyme has been co-opted to provide a crystallin. Available
Corneal nipple arrays in insect eyes Anti-reflection coating? Not only on mobile phone displays, but also on insect eyes... Available
Vision in echinoderms Among brittlestars and sea urchins we find visual systems that in some ways rival the arthropods in the form of compound eye-like structures. Available
Light sensitivity and eye-spots in bacteria Light sensitivity based on opsins is well documented, notably in the cyanobacterium Anabaena where it is involved with photosynthesis and in particular the production of key pigments. Unavailable
Compound eyes in arthropods It is clear that amongst the arthropods as a whole the compound eye has evolved at least twice, and possibly even more times. Available
Compound eyes in sabellid annelids Compound eyes have evolved convergently in the annelids, notably amongst the sabellids, where they evidently serve as an optical alarm system. Available
Camera eyes in alciopid annelids There is a striking example in the group known as the alciopids, which are pelagic polychaetes. The similarity of their camera eye to the vertebrate eye has attracted considerable comment. Available
Pinhole eyes in Nautilus and giant clam The pinhole eye has evolved not only in the Pearly Nautilus, but also in another group of molluscs, the bivalves and specifically the giant clams (Tridacna). Available
Compound eyes in ark clams Read on if you want to know more about bivalves with burglar alarms… Available
Camera eyes in gastropod molluscs The fast-moving cephalopod molluscs are famous for their camera eyes, but why on earth have gastropod snails, which are not exactly known for their speed, evolved this superb visual organ at least four times? Available
Scanning eyes in molluscs and arthropods Some sea snails have a linear retina. What a hopeless arrangement, to see the world through just a narrow slit! Not quite, because they have come up with a rather intriguing trick to extend their visual field - and it's a trick too good to use only once. Available
Telephoto eyes in animals Pursued by the paparazzi? Watch out for those animals equipped with telephoto lenses... Available
Camera-like eyes in arthropods Arthropods are famous for their compound eyes, but some groups have had a fair crack at evolving the optically superior camera eye… Available
Strepsipterans: convergent halteres and eyes Strepsipteran females spend their whole life inside a wasp. The males are rather more exciting, particularly in terms of convergence… Available
Beetles: insights into convergence The beetles are probably the most diverse animal group on earth, so it is not at all surprising that they provide many fascinating insights into convergence. Available
Crustaceans: insights into convergence Whilst predominantly marine, quite a number of crustaceans have invaded freshwater habitats and even more interestingly a few demonstrate terrestrialization, effectively freeing themselves from their aquatic ancestry. Available
Gastropod molluscs: snail shell anatomy Snail shells typically form a helical spiral, but within this geometry there is a considerable degree of convergence. Unavailable
Crystallins: eye lens proteins Whereas typically technology demands furnaces, so that the glass for a lens is produced at hundreds of degrees Celsius and then requires most careful grinding, so nature calls upon proteins known as crystallins. Available
Transparent tissues: eyes, bodies and reflective surfaces Read on if you want to know about the numerous animal equivalents to the invisible man... Available
"Colour vision" in Firefly squid The Japanese firefly squid (Watasenia scintillans), which inhabits the deep ocean, has three visual pigments located in different parts of the retina that are likely to allow colour discrimination as they each have distinct spectral sensitivities. Available
Retinal sensitivity changes in vertebrates and cuttlefish In vertebrates the sensitivity of the retina changes during the growth of the animal. In invertebrates this only occurs in the cephalopods, or at least cuttlefish, where this sensitivity has been acquired convergently. Unavailable
Polarized light detection in arthropods, fish and cephalopods In bees detection of polarized light from different quadrants of the sky is an important component in their navigation. Unavailable
Annelids: insights into convergence Notable instances of convergence involving annelids include luminescence, moulting and the independent evolution of both compound eyes (e.g. in sabellids) and camera eyes (in alciopids). Unavailable
Asymmetric eye use in octopus, dolphins and birds In a number of cases one eye is used in preference to another. This convergent phenomenon is found in octopus (cephalopods), dolphins, birds, and other animals. Unavailable
Bioluminescent reverse eyes in squid Normally one thinks of an eye as a structure that allows light to pour into the body, but in at least some squid (cephalopods) the opposite has been achieved. Unavailable
Eyes lenses in animals The majority of complex eyes not surprisingly do possess a lens and they are an excellent example of convergent evolution, especially in terms of camera-eyes and compound eyes, as well as employment of crystallins. Unavailable
Camera eyes of cephalopods The remarkable similarity between the camera eyes of cephalopods and vertebrates is one of the best-known examples of evolutionary convergence. Available
Ultraviolet (UV) absorption in vertebrates and cephalopods In some vertebrates (fish, mammals) and cephalopods we find an interesting convergence whereby some of the incoming ultraviolet is screened out. Unavailable
Octopus and other cephalopods: convergence with vertebrates What could be more different from us than the alien-like octopus? Hold on. Look it in the eye and think again. Available
Camera eyes in vertebrates, cephalopods and other animals Camera eyes are superb optical devices, so it is not surprising that they have evolved several times. But why, of all animals, in the brainless jellyfish? Or for that matter in a slow-moving snail? Available