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).

Example: Refractive index of (fresh) water n= 1.333 or 1/1.33 = ¾ the speed of light in a vacuum.

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

• Transparent animals / objects– Do not absorb or reflect light

Pelagic rare w/in group

Pelagic common

Transparency rare

Transparency common

PHYLOGENETIC DISTRIBUTION

• Transparency

–Appears to have evolved multiple times

– Found in most major animal phyla

–Primarily pelagic

PHYLOGENETIC 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

ECOLOGICAL DISTRIBUTION

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

ECOLOGICAL DISTRIBUTION

• Neustonic – rare– Match upwelling light• Blue or brown

– works from above but not from below

– UV• Protective pigmentation

• Aphotic – rare– Red or Black pigmentation• Absorb bioluminescent light

ECOLOGICAL DISTRIBUTION

MOST TRANSPARENT ANIMALS 10 MAJOR GROUPS (Pelagic):

CubozoansHydrozoansCtenophores (non-beroid)Hyperiid AmphipodsTomopterid PolychaetesHeteropodsPteropodsCranchiid squidThaliaceansChaetognaths

ECOLOGICAL DISTRIBUTION

• 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

Adaptations to break transparency:UV visionPolarization visionViewing angle (behavioral)

INTERACTIONS

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.

INTERACTIONSAdaptations to break transparency:

Polarization visionCan detect changes in polarization of highly polarized oceanic light

INTERACTIONSAdaptations to break transparency:

Viewing angle (behavioral)Snell’s Window condensed horizon effect increases contrast of transparent objects outside of “window”

• 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

Transparent animals must compensate for their varied constituent refractive indices

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

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)

ADAPTATIONS for TRANSPARENCY

• Micro– Surface– Extracellular Matrix– Cellular

ADAPTATIONS for TRANSPARENCY

• 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

WIDTH OF BUMPIS LESS THAN HALFA WAVELENGTHOF LIGHT

INDEX OF REFRACTIONDARK = BUMPSLIGHT = SURROUNDING MEDIUM

• 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

• 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

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

ADAPTATIONS for TRANSPARENCY

• Micro– Cellular Tissues• More complex• Necessary components with different refractive indices

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

ADAPTATIONS for TRANSPARENCY

• Micro– Cellular Tissues

• Theoretical model:– Size matters– Distribution– Refractive index– Shape does not matter that much

– Theoretical predictions: