The influence of light in underwater camouflage mechanisms Hollie-Ann Hatherell and Rachel Lane, CoMPLEX UCL Reflectance Animals such as herrings and sardines have vertical stacks of guanine which act like mirrors. Due to the symmetry of the distribution of light underwater about the vertical axis, onlooking Predators see a reflection of the region behind the animals. Transparency Common in invertebrates in the epipelagic zone, transparency requires minimum absorption and scattering of light. Absorption can be minimised through low levels of highly pigmented molecules such as haemoglobin [3]. Minimisation of scattering is achieved by a constant refractive index across the body, achieved via strategies such as adjusting the arrangement of internal cellular and extracellular components. Also, flattening to ~1mm gives 95% transparency [4]. Model of Robustness In [2] a model of the greatest distance at which an animal can be seen (sighting distance) is constructed for coloured and mirrored camouflage strategies. Predictions were made for environments other than that in which the animal is optimally cryptic to test the robustness of the mechanism. [1] Animal Camouflage: Mechanisms and Function. Martin Stevens, Sami Merilaita, Cambridge University Press, 2011. [2] Cryptic coloration and mirrored sides as camouflage strategies in near-surface pelagic habitats: Implications for foraging and predator avoidance. Sönke Johnsen & Heidi M. Sosik. Limnol. Oceanogr., 48(3), 2003, 1277–1288 2003, by the American Society of Limnology and Oceanography, Inc. [3] Hide and Seek in the Open Sea: Pelagic Camouflage and Visual Countermeasures. Sönke Johnsen. Annu. Rev. Mar. Sci. 2014. 6:369–92. [4] Hidden in Plain Sight: The Ecology and Physiology of Organismal Transparency. Sönke Johnsen. Biol. Bull. 2001. 201: 301–318. Colouration Wavelengths greater than ~580nm are quickly absorbed by the water and the majority do not reach depths greater than 10m. After roughly 50m, the predominant wavelengths remaining are around 480nm (blue-green). Vividly-coloured fish, such as the Magenta ‘dottyback’ or Pseudochromis paccagnellae, may appear conspicuous to humans, but actually appear blue at lower depths due to the lack of yellow/red wavelengths [1]. Introduction Effective camouflage is essential for the survival of many animals. This is complex in pelagic environments due to changing light conditions. A wide range of camouflage mechanisms have evolved across species to account for this. These mechanisms, although effective in specific pelagic zones, may not be robust in other situations. Figure: Three camouflage strategies at different depths of the sea, taken from [3]. Figure: Depth at which each wavelength is diminished to 1% of its value as it enters the water, taken from [3]. Conclusion Predictions from [2] suggest that mirroring is a more robust form of camouflage as the animal remains cryptic in a greater variety of surroundings and lighting. Both mechanisms were not effective in the epipelagic zone, where transparency may be more favourable. The advantage of transparency is that it is effective from all angles and depths compared to mirroring and colouration [4]. Figure: Sighting distance for 1m long coloured organism viewed at a shallow depth rather than its optimally cryptic depth of 50m, taken from [2].
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The influence of light in underwater camouflage mechanisms Hollie-Ann Hatherell and Rachel Lane, CoMPLEX UCL
Reflectance Animals such as herrings and sardines have vertical stacks of guanine which act like mirrors. Due to the symmetry of the distribution of light underwater about the vertical axis, onlooking Predators see a reflection of the region behind the animals.
Transparency
Common in invertebrates in the epipelagic zone, transparency requires minimum absorption and scattering of light. Absorption can be minimised through low levels of highly pigmented molecules such as haemoglobin [3]. Minimisation of scattering is achieved by a constant refractive index across the body, achieved via strategies such as adjusting the arrangement of internal cellular and extracellular components. Also, flattening to ~1mm gives 95% transparency [4].
Model of Robustness In [2] a model of the greatest distance at which an animal can be seen (sighting distance) is constructed for coloured and mirrored camouflage strategies. Predictions were made for environments other than that in which the animal is optimally cryptic to test the robustness of the mechanism.
[1] Animal Camouflage: Mechanisms and Function. Martin Stevens, Sami Merilaita, Cambridge University Press, 2011. [2] Cryptic coloration and mirrored sides as camouflage strategies in near-surface pelagic habitats: Implications for foraging and predator avoidance. Sönke Johnsen & Heidi M. Sosik. Limnol. Oceanogr., 48(3), 2003, 1277–1288 2003, by the American Society of Limnology and Oceanography, Inc. [3] Hide and Seek in the Open Sea: Pelagic Camouflage and Visual Countermeasures. Sönke Johnsen. Annu. Rev. Mar. Sci. 2014. 6:369–92. [4] Hidden in Plain Sight: The Ecology and Physiology of Organismal Transparency. Sönke Johnsen. Biol. Bull. 2001. 201: 301–318.
Colouration
Wavelengths greater than ~580nm are quickly absorbed by the water and the majority do not reach depths greater than 10m. After roughly 50m, the predominant wavelengths remaining are around 480nm (blue-green). Vividly-coloured fish, such as the Magenta ‘dottyback’ or Pseudochromis paccagnellae, may appear conspicuous to humans, but actually appear blue at lower depths due to the lack of yellow/red wavelengths [1].
Introduction
Effective camouflage is essential for the survival of many animals. This is complex in pelagic environments due to changing light conditions. A wide range of camouflage mechanisms have evolved across species to account for this. These mechanisms, although effective in specific pelagic zones, may not be robust in other situations.
Figure: Three camouflage strategies at different depths of the sea, taken from [3]. Figure: Depth at which each
wavelength is diminished to 1% of its value as it enters the water, taken from [3].
Conclusion Predictions from [2] suggest that mirroring is a more robust form of camouflage as the animal remains cryptic in a greater variety of surroundings and lighting. Both mechanisms were not effective in the epipelagic zone, where transparency may be more favourable. The advantage of transparency is that it is effective from all angles and depths compared to mirroring and colouration [4].
Figure: Sighting distance for 1m long coloured organism viewed at a shallow depth rather than its optimally cryptic depth of 50m, taken from [2].
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