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 itindex of refractionphase speed

5. Transparent animals / objects – Do not absorb or reflect light

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

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

8. 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

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

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

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

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

13. 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

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

15. Adaptations to break transparency: UV vision Increased 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.

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

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

18. 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

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

20. 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

21. 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

22. Micro – Surface – Extracellular Matrix – Cellular ADAPTATIONS for TRANSPARENCY

23. 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 ADAPTATIONS for TRANSPARENCY

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

26. 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

28. 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

29. 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 http://www.molvis.org/molvis/v15/a224

30. ADAPTATIONS for TRANSPARENCY Micro – Cellular Tissues More complex Necessary components with different refractive indices

32. 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

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