visual range of animals: how some creatures exploit the spectrum
In this essay, I will
be exploring the different types of vision in ecology and explaining the
optical mechanisms which give some animals the ability to see a world that humans cannot. I will
begin this essay with a brief explanation of colour vision in humans which will
explain many of the foundations of the visual mechanisms which will be
discussed in this essay. I will then continue by exploring some of nature’s most
extraordinary eyes and the mechanisms behind them.
Colour vision in the Human eye
The Human eye consists two types of light sensitive cells, called rods and cones,
with the cone cells being responsible for the colour vision. Humans are said to
be trichromatic as the human eye has 3 types of cone cell; Short wavelength
(S), Medium wavelength (M), and Long wavelength (L). S, M and L cone cells have maximal absorption (?max) peaks at 445, 525 and 555-570 nm respectively due to
the differences in their photopigments 1. Thiis is the basis of Young- HelmHoltz
theory of colour vision .<---- credit to his theory?. A visual pigment consists of a protein, called an opsin, and a retinal-based chromophore which is the part of the photopigment that absorbs the light.2 S, M and L cone cells all share the same chromophore, 11-cis-retinal. However, each cell type contains a different opsin, each with a different amino acid side group which interacts with the chromophore to shift the ?max peak of the chromophore causing each cell type to absorb a different wavelength2.The reason for this is that, some opsins bind to 11-cis-retinal and cause there to a higher degree of planarity leading to a higher degree of conjugation between ? systems and so a smaller HOMO- LUMO energy gap, meaning the chromophore absorbs light of longer wavelengths (red)3. Other opsins cause rotations along single bonds which reduce the degree of conjugation and so increases the HOMO- LUMO energy gap and the light is blue shifted.3 STRUCTURE OF MANTIS COMPUND EYE UV Vision of the Mantis shrimp Stomatopod crustaceans, commonly known as mantis shrimp, are said to have the most complex vision of any animal researched so far. Their compound eyes allow them to see wavelengths of the visible spectrum and in the ultraviolet region. Their apposition compound eyes consist of thousands of individual visual receptors called ommatidia. There are 3 distinct regions to the eyes called the dorsal region, ventral region and the midband. Each ommatidia has a cornea which light passes through, the light then passes through the crystalline cone before reaching the photoreceptive rhabdoms, which consist of 8 retinular cells4,5. The midband rows 1-4 are responsible for colour vision and consist of 12 photoreceptors,8 of which are responsible for colour vision5. Each row consists of a set of retinular cells R1-R8, with the R8 cells being responsible for UV vision and the R1-R7 cells being responsible for colour vision between 400 and 700 nm5.However, experimentally the Mantis Shrimp could not distinguish between wavelengths the way humans can (1-5 nm apart) and so showed that it could not see more colours even though it has more photoreceptors5. This suggests that the Mantis shrimp does not discriminate between colours using the brain like humans, but instead recognises the general wavelengths associated with the 8 photoreceptors5. This allows for quick scanning of the surroundings which is advantageous for recognising possible predators and prey in the colourful reefs where they live. 5Figure 1: (A) shows the 12 photoreceptors of the Mantis Shrimp used to detect colours. (B) shows the eye of the Mantis shrimp, DH being the Dorsal region, MB being the 6 rows of the midband, the first 4 of which are for colour vision (CV) and the Ventral region (VH). Research of the stomatopod species Neogonodactylus oerstedii, showed that the retinular cells 8 (R8) of ommatidia in rows 1-4 of the midband region were sensitive to light in the UV range, with particularly sensitive peaks at 310, 320, 330, 340 and 380 nm 4. However, only 2 UV sensitive opsins were found in the R8 cells and so the mantis shrimp has only 2 visual pigments in the UV range, uv1 and uv2. Uv1 was found in rows 2-6 of the midband region and had a ?max of 334 nm. Uv2 was found in the R8 cells of row 1 of the midband, and had a ?max of 383 nm. This means that the 2 pigments are not the sole explanation behind the Mantis shrimp's ability to see 5 different UV wavelengths of light. Usually it is the opsin which tunes the chromophore and shifts its absorbnce peak to account for the for variety of wavelengths detected by the retinular cells. However, in the case of the mantis shrimp, there is another source of spectral tuning -UV absorbing filter pigments4. The filter pigments are used to shift the ?max of the visual pigments. A combination of filter pigments are used to shift the ?max values of the uv1 and uv2 pigments to highr qand lower wavelengths which allows the Mantis shrimp to have 5 disitinc UV absorption peaks4. Polarisation vision in the Mantis Shrimp Linearly plane Polarised light has all of its waves vibrating in the same plane. The retinular cells 1-71 contain rhodopsin , much like our own rod cells. However, in the mantis shrimp, the rhodopsin molecules of each cell are aligned parallel on their microvillli 6. This allows the excitable double bond of the chromophore to be aligned with the electric field of the incoming light, and so allows for the detection of linearly polarised light. Most vertabrates have a random arrangement of rhodopsin molecules in their rod and cone cells and so the alignment of the double bond of the chromophore and the direction of the electric field cannot be achieved.6 Polarised light can exist both linearly and circularly. Circularly polarised light is a phenomena in which the electric field of the light wave rotates around the direction of travel of the wave6. Extraordinarily, the Mantis shrimp is the only known animal to detect and utilise this light, however, because the electric field rotates in circularly polarised light, it can no longer align with the excitable double bond of the chromophore, and so the Mantis shrimp cannot rely on this mechanism alone. They have evolved a way in which to convert the circularly polarised light into linearly polarised light, which can then be detected by the linear polarisation sensitive R1-R7 cells. It has been hypothesised by Marshall et al that the R8 cells introduce a quarter wavelength phase difference, which converts a circularly polarised light beam into 2 orthogonal linear ones6,2,8,9. The microvilli in the R8 cells in midband row 5 are aligned parallel to each other, but perpendicular to the microvilli of the R8 cells of midband row 63,4, which means the R8 cells of row 5 will cause a orthogonal path difference to the R8 cells of row 66,7,8. This allows the eyes to differentiate between right hand and left hand polarised light6. It is thought that the detection of circularly polarised light in Mantis Shrimp allows for a unique commuication signal only between Mantis Shrimp7. Behavioural experiments of the Mantis Shrimp have shown that circularly polarised light could be used as a way to communicate aggression between Mantis Shrimp as they have circularly polarised light patterns which are highlighted in their defensive position.7 5Figure 2: A diagram of circularly polarised light. 7Figure 3: A Mantis Shrimp in its defensive position, showing the circularly polarised light reflection patterns on its body. Polarised light vision has the potential to cause 'false colours' in animal vision, which is a large disadvantage when identifying prey. The R1-R7 cells have absorption peaks at 500 nm and the R8 cells have an absorption peak at 350 nm which could lead to false colours. However, to account for this, the colour sensitive cells of rows 1-4 of the midband are polarisation insensitive due to the random arrangement of the rhodopsin containing microvilli9. This means the brain can distinguish between absorptions of light in the visible spectrum from polarisation sensitive cells in rows 5 and 6 and the colour sensitive cells of rows 1-4 of the miband. Infrared vision of the Zebra fish The longer wavelengths of the spectrum are also used by animals as an evolutionary advantage. One example of this is in the fresh water Zebra fish (Danio rerio), fresh waters cause a greater degree of scattering and absorption of shorter wavelengths of light which means and so longer wavengths of the near infrared region are favourable for detection by the eye . The Zebra fish do this using the chromophores 11-cis retinal and 3,4-didehydroretinal, the balance of these chromophores allows the Zebra fish to switch its maximum absorbance peaks depending on the specific conditions of the water. The red-shift is caused by the additional conjuagted double bond in 3,4-didehyroretinal, this increase in conjugation causes a smaller HOMO-LUMO energy gap than that in 11-cis-retinal. This means that 3,4-didehyroretinal absorbs even longer wavelengths, past the visible spectrum and into the Near infrared. It is a specific gene cyp27cl which codes for an enzyme that causes a conversion of 11-cis retinal to 3,4-didehydroretinal and allows the fish to adjust its wavelength absorption to fit the environment.6 7Figure 3: The structures of 11-cis-retinal and 3,4-didehydroretinal. Showing the additional double bond in 3,4 didehyroretinal. Infrared vision of Snakes Infrared vision is also an essential evolutionary trait in snakes of the taxonomic families; Boidae, Pyhtonidae and Viperidae8,9. This exploitation of the spectrum is an essential part of predation in these snakes, as they must rely on their pit organs to detect the infrared waved emitted from prey when they are unable to see at night. However these snakes use a completely different organ to create the infrared image called pit organs. These pits are found between the eyes and the nostrils, and are 1-3 mm in diameter10. The pit organs detect infrared radiation by acting as pinhole infrared cameras each with a temperature sensitive membrane at the back of the organ12. The infrared sensitive membrane receptors are fired and the infrared image is then merged with visual signals from the visible region of the spectrum detected by the eye. This takes place at the optic tectum12.The membrane of the pit organ is known to have thousands of receptors that were thought to detect changes in temperature as small as 0.001?C11,11. 12Figure 4: a) Shows the position of the pit organ b) shows the structure of the pit organ. http://1.bp.blogspot.com/-jQbQSAr7xbM/UvpwoKuBRkI/AAAAAAAAAoM/6FHBscvkC_E/s1600/snake+pits.jpg 13Figure 5: The comparison between normal vision (left), and Infrared vision (right). http://strikerattleroll.blogspot.co.uk/2014/02/rattlesnakes-superpower-seeing-in-dark.html 5 .J. Anderson, ReliaWire, 2018. THIS IS NOW 10 7 A. Morshedian, M. Toomey, G. Pollock, R. Frederiksen, J. Enright, S. McCormick, M. Cornwall, G. Fain and J. Corbo, The Royal Society, 2018. 12 1.bp.blogspot.com, 2018. 13 V. profile, Strikerattleroll.blogspot.co.uk, 2018.