Transparency Form of crypsis Involves modification of whole organism
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Transparency
Form of crypsis
Involves modification of whole organism
Found mostly in pelagic animals
Across many taxa
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Refractive index (n): Measure of how much the speed of light is reduced in a given medium relative to a reference medium – usually the speed of light in a vacuum (n=1).
Example: Refractive index of (fresh) water n= 1.333 or 1/1.33 = ¾ the speed of light in a vacuum.
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The region below the gray line has a higher index of refraction, and so light traveling through it has a proportionally lower phase speed than in the region above it
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• Transparent animals / objects– Do not absorb or reflect light
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Pelagic rare w/in group
Pelagic common
Transparency rare
Transparency common
PHYLOGENETIC DISTRIBUTION
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• Transparency
–Appears to have evolved multiple times
– Found in most major animal phyla
–Primarily pelagic
PHYLOGENETIC DISTRIBUTION
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• Terrestrial – extremely rare– Reflection
• Air low refractive index• Difference in ‘n’ between object and surrounding medium
surface reflection decreased transparency
– UV• Protective pigmentation
– Gravity• Skeletal structures
ECOLOGICAL DISTRIBUTION
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ECOLOGICAL DISTRIBUTION
• Benthic – rare– Match substrate• Pigmentation less costly than transparency?
– ShadowsTransparent animals may still cast shadows – seen in
benthic environments, not in pelagic ones
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ECOLOGICAL DISTRIBUTION
• Neustonic – rare– Match upwelling light• Blue or brown
– works from above but not from below
– UV• Protective pigmentation
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• Aphotic – rare– Red or Black pigmentation• Absorb bioluminescent light
ECOLOGICAL DISTRIBUTION
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MOST TRANSPARENT ANIMALS 10 MAJOR GROUPS (Pelagic):
CubozoansHydrozoansCtenophores (non-beroid)Hyperiid AmphipodsTomopterid PolychaetesHeteropodsPteropodsCranchiid squidThaliaceansChaetognaths
ECOLOGICAL DISTRIBUTION
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• Transparency of an animal depends on
– Fraction of light that passes through • Not absorbed or reflected (scattered)
– Contrast• Brightness of object relative to its background• Decreases with distance
– Visual capacity of viewer• Sighting distance• Adaptations to break transparency
INTERACTIONS
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Adaptations to break transparency:UV visionPolarization visionViewing angle (behavioral)
INTERACTIONS
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INTERACTIONSAdaptations to break transparency:
UV visionIncreased scattering of light in UV range
increased contrast
The advantage of UV vision shows in reef views in visible (left) and ultraviolet (right) light. In UV light, the fish are in much higher contrast to the background.
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INTERACTIONSAdaptations to break transparency:
Polarization visionCan detect changes in polarization of highly polarized oceanic light
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INTERACTIONSAdaptations to break transparency:
Viewing angle (behavioral)Snell’s Window condensed horizon effect increases contrast of transparent objects outside of “window”
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• Most organic molecules do not absorb light– Transparency is a matter of reducing light
reflections or scattering caused by light passing through media with different refractive indices.
ADAPTATIONS for TRANSPARENCY
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Transparent animals must compensate for their varied constituent refractive indices
ADAPTATIONS for TRANSPARENCY
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• Macro – Cloaking of non-transparent features• Eyes
– Compact retinas– Mirrors– Counterillumination– Separation of eyes
• Guts– Elongated– Vertically oriented (decreases view from above/below)– Mirrored/reflective– Counterilluminating bioluminescence – minimizes shadows
ADAPTATIONS for TRANSPARENCY
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• Macro – Cloaking of non-transparent features• Eyes• Guts
– Be flat• Light attenuation decreases exponentially as tissue
thickness decreases (Thinner = more light passes through)
ADAPTATIONS for TRANSPARENCY
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• Micro– Surface– Extracellular Matrix– Cellular
ADAPTATIONS for TRANSPARENCY
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• Micro– Surface• Moth eye surfaces
– Bumps have widths <1/2 the wavelength of incident lightCreate a refractive index gradient Decreases effective surface refractive index Decreases scatter
ADAPTATIONS for TRANSPARENCY
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WIDTH OF BUMPIS LESS THAN HALFA WAVELENGTHOF LIGHT
INDEX OF REFRACTIONDARK = BUMPSLIGHT = SURROUNDING MEDIUM
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• Micro– Extracellular Tissues• Average refractive index constant over distance ½ the
wavelength of incident light Low scattering
Due to destructive interference of scattered light
ADAPTATIONS for TRANSPARENCY
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• Micro– Extracellular Tissues• Average refractive index constant over distance ½ the
wavelength of incident light Low scattering
Due to destructive interference of scattered light Caused by densely packed similar objects
Example: Mammalian cornea and lens tissues densely packed so scatter is ordered and reduced
ADAPTATIONS for TRANSPARENCY
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Transparency and biomechanical properties of the cornea depend on the structure and organization of corneal stroma.
Collagen fibers and fibers interconnecting to the network formed collagen bundles, which were regular and parallel to the corneal surface
Barbaro, Mol Vis 2009; 15:2084-2093. http://www.molvis.org/molvis/v15/a224
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ADAPTATIONS for TRANSPARENCY
• Micro– Cellular Tissues• More complex• Necessary components with different refractive indices
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ADAPTATIONS for TRANSPARENCY
• Micro– Cellular Tissues• More complex• Necessary components with different refractive indices• Theoretical model:
– Size matters– Distribution– Refractive index– Shape does not matter that much
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ADAPTATIONS for TRANSPARENCY
• Micro– Cellular Tissues
• Theoretical model:– Size matters– Distribution– Refractive index– Shape does not matter that much
– Theoretical predictions: