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Transparency Form of crypsis Involves modification of whole organism

Transparency Form of crypsis Involves modification of whole organism Found mostly in pelagic animals Across many taxa. 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). .

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Transparency Form of crypsis Involves modification of whole organism

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  1. Transparency Form of crypsis Involves modification of whole organism Found mostly in pelagic animals Across many taxa

  2. 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.

  3. 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

  4. Transparent animals / objects • Do not absorb or reflect light

  5. PHYLOGENETIC DISTRIBUTION Pelagic rare w/in group Pelagic common Transparency rare Transparency common

  6. PHYLOGENETIC DISTRIBUTION • Transparency • Appears to have evolved multiple times • Found in most major animal phyla • Primarily pelagic

  7. ECOLOGICAL DISTRIBUTION • 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

  8. ECOLOGICAL DISTRIBUTION • Benthic – rare • Match substrate • Pigmentation less costly than transparency? • Shadows Transparent animals may still cast shadows – seen in benthic environments, not in pelagic ones

  9. ECOLOGICAL DISTRIBUTION • Neustonic – rare • Match upwelling light • Blue or brown • works from above but not from below • UV • Protective pigmentation

  10. ECOLOGICAL DISTRIBUTION • Aphotic – rare • Red or Black pigmentation • Absorb bioluminescent light

  11. ECOLOGICAL DISTRIBUTION MOST TRANSPARENT ANIMALS 10 MAJOR GROUPS (Pelagic): Cubozoans Hydrozoans Ctenophores (non-beroid) Hyperiid Amphipods TomopteridPolychaetes Heteropods Pteropods Cranchiid squid Thaliaceans Chaetognaths

  12. INTERACTIONS • 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

  13. INTERACTIONS Adaptations to break transparency: UV vision Polarization vision Viewing angle (behavioral)

  14. INTERACTIONS Adaptations to break transparency: UV vision Increased scattering of light in UV range  increased contrast

  15. INTERACTIONS Adaptations to break transparency: Polarization vision Can detect changes in polarization of highly polarized oceanic light

  16. INTERACTIONS Adaptations to break transparency: Viewing angle (behavioral) Snell’s Window  condensed horizon effect  increases contrast of transparent objects outside of “window”

  17. ADAPTATIONS for TRANSPARENCY • 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.

  18. ADAPTATIONS for TRANSPARENCY Transparent animals must compensate for their varied constituent refractive indices

  19. ADAPTATIONS for TRANSPARENCY • 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

  20. ADAPTATIONS for TRANSPARENCY • Macro • Cloaking of non-transparent features • Eyes • Guts • Be flat • Light attenuation decreases exponentially as tissue thickness decreases (Thinner = more light passes through)

  21. ADAPTATIONS for TRANSPARENCY • Micro • Surface • Extracellular Matrix • Cellular

  22. ADAPTATIONS for TRANSPARENCY • Micro • Surface • Moth eye surfaces • Bumps have widths <1/2 the wavelength of incident light Create a refractive index gradient  Decreases effective surface refractive index  Decreases scatter

  23. WIDTH OF BUMP IS LESS THAN HALF A WAVELENGTH OF LIGHT INDEX OF REFRACTION DARK = BUMPS LIGHT = SURROUNDING MEDIUM

  24. ADAPTATIONS for TRANSPARENCY • Micro • Extracellular Tissues • Average refractive index constant over distance ½ the wavelength of incident light • Low scattering • Due to destructive interference of scattered light

  25. ADAPTATIONS for TRANSPARENCY • 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

  26. 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

  27. ADAPTATIONS for TRANSPARENCY • Micro • Cellular Tissues • More complex • Necessary components with different refractive indices

  28. 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

  29. ADAPTATIONS for TRANSPARENCY • Micro • Cellular Tissues • Theoretical model: • Size matters • Distribution • Refractive index • Shape does not matter that much • Theoretical predictions:

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