Transparency and camoflauge Flashcards
What does an objects transparency depend on?
- Object transparency depends in part on the difference between its refractive index & the refractive index of surrounding medium
- To become translucent you are trying to minimise the differences between your tissue properties and that of the seawater.
- The large difference causes surface reflections that substantially increase visibility.
What is a refractive index?
- Refractive index of biological material roughly proportional to density
- Ranges from 1.35 (cytoplasm) to 1.55 (protein)
- Refractive index of seawater ~1.34
What groups are dominated by transparent forms?
•Many taxonomic groups (e.g. cnidarians, ctenophores, salps, cranchid squid, hyperiid amphipods) are dominated by transparent forms.
•Transparency is the only form of camouflage in the pelagos that is “successful from all viewpoints & at all depths”
What are some other forms?
- Cryptic colouration (e.g. countershading) generally only successful from a given viewpoint at a given depths
- •Mirrored sides are successful from euphotic & upper dysphotic, but not from directly above or below
- •Counterillumination (camouflages animal by mimicking downwelling light using photophores) is metabolically expensive & only successful during moonlit nights or at dysphotic depths.
Primary barriers in tissues are light scattering, necessary opaque tissues (e.g. gut, retina) and transparent tissues lose UV protection - UV transparency
What is scattering?
Scattering
Caused by discontinuities in the refractive index of tissues Biological materials display a great deal of scattering
Transparent animals must be adapted to scatter as little light as possible
Primary barriers in tissues are light scattering, necessary opaque tissues (e.g. gut, retina) and transparent tissues lose UV protection - UV transparency
What is an adaptation to this?
Adaptations
Structural changes –cloaking of tissues that cannot be made transparent and body flattening
Bump phenomenon.
A phenomenon exploited by marine species where;
- Tiny bumps on body surface can enhance invisibility by reducing reflections
- Bumps with widths <1/2λ of light do not have their own refractive index
- Refractive index is average of bump’s index and that of surrounding medium
(Merges bump with surrounding medium
The bump doesn’t have its own reflective index)
What is driving transparency adaptations?
- For predators & prey the sighting distance is a critical factor.
- Sighting distance is the maximum distance at which an organism is detectable by an animal relying on visual cues
- Prey with short sighting distances reduce encounters with visually hunting predators
- “Ambush” predators (e.g. medusae, siphonophores, cydippid ctenophores) with short sighting distances → increase chances of catching visually orienting prey before detection and avoidance
Raptors (e.g. chaetognaths, heteropods) with short sighting distances → increase their chances of getting within striking distance before detection
- The visibility of a transparent individual generally depends more on its contrast than on its size
- Contrast is the difference between the darkest and lightest areas
- There are a number of mechanisms whereby contrast can be maximised to negate the camouflage benefits of transparency;
- UV-vision
- Polarisation vision
- Viewing at certain angles – “Snell’s Window”
UV dilemma
protection or concealment?
Consequence may be to live at greater depth – reduces UV exposure, however, potentially removes you from your food source, therefore many organisms use diel vertical migrations.
The dilemma may become more relevant due to reduction in ozone layer
A responsive increase in UV-protective pigments increases visibility at UV wavelengths (& possibly visible wavelengths), potentially resulting in increased predation or decreased feeding success
What is polarised light?
If we look at a beam of light from the front the electrical field vectors
are randomly orientated along the direction of propagation
This is unpolarised light
Polarisation is the preferential orientation of the field vectors and gives more contrast.
Underwater light is polarised, particularly in the horizontal
direction
A transparent object can affect this polarisation in 2 ways;
- It can depolarise it entirely
- If the object is birefringent (decomposition of a ray of light into 2 rays, sometimes called double refraction), it can rotate the plane of polarisation. It becomes much more visible.
Either change can affect an objects visibility
What is snells window?
Snell’s window is a phenomenon by which an underwater viewer sees everything above the surface through a cone of light of width of about 96 degrees.
This phenomenon is caused by the refraction of light entering the water, and is governed by Snell’s Law.
The area outside Snell’s window will either be completely dark or show a reflection of underwater objects by total internal reflection.
How does the snells window effect predator-prey interactions?
•This is useful in the euphotic zone where water can refract. Interaction of air and water – predators eyes facing upwards.
2 other uses of transparency apart from camouflage:
- Calycophoran siphonophores
Hippopodius hippopus & Vogtia are normally transparent but rapidly become opaque when disturbed - startle-type defence
- Physonect siphonophores Athorybia rosacea & Aglama okeni use pigmented
regions in the otherwise transparent body as lures Mimic copepods & larval fish
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