The deep-sea is the most extensive living space on Earth and is home to the largest biomass and number of individual animals in the world, making it of high importance in the biosphere. In the mesopelagic zone, animals struggle to find places to hide from predators and prey alike. In order to survive in this desolate habitat, animals have had to develop mechanisms to conceal themselves. Even in the blackness of the deep-sea, some animals are lighter than their dark background and others can be seen as silhouetted objects from down-welling light. As a result, mesopelagic animals have evolved means of camouflage to reduce or even eliminate their contrast to the surrounding darkness, allowing them to remain undetected. This generally takes one of four forms: cryptic colouration, silvering, transparency or bioluminescent counter-illumination.

Transparency

It is essential to distinguish between transparency and invisibility: any object that is invisible is transparent, but objects that are transparent are not invisible. An object that is transparent has a higher refractive index than its surroundings, this, along with a complex shape, makes it visible as a result of light scattering at its edges. To put this into context, a drop of water is transparent in air because there is a difference between its refractive index (n = 1.33) and the refractive index of air (n = 1.00029). However, that same drop of water placed into a puddle would be invisible, because its refractive index is the same of that of its surroundings. In the deep ocean, organisms make themselves less visible by coordinating their refractive index to that of the surrounding seawater.

Achieving 100% transparency is impossible, as an animal would need to be seawater themselves. Yet 91% transparency has been accomplished by gelatinous animals like comb jellies (e.g. Mnemiopsis leidyi), who have the ability to integrate seawater into their tissues. It is necessary for some tissues to remain opaque (e.g. retinae of eyes), however, this can be somewhat eluded by eliminating screening pigments, as seen in the amphipod Cystisoma sp. (Fig. 1) or by reducing the volume of the retina, restricting any distinctive areas to as smaller an area as possible. Another way of reducing distinction of opaque parts of the body is to take on a leaf-like flat form and thin out internal organs over a wider area.

Deepsea Amphipod (Cystisoma sp) from between 530-750m, Mid-Atlantic Ridge, North Atlantic Ocean
Figure 1: Deepsea Amphipod, Cystisoma sp (found between 530-750m, Mid-Atlantic Ridge, North Atlantic Ocean) demonstrating transparency and the elimination of screening pigments to maximise transparency.  Image available here.

Transparency is generally used for avoiding detection. However, there are some instances where it can be used to lure in prey. Some species of physonect siphonophores (e.g. Athorybia rosacea and Aglama okeni), use transparency over the majority of their body, excluding a few small regions. These small regions are pigmented in the shape of copepods and fish larvae, drawing in unsuspecting prey. Other species of siphonophore (e.g. Hippopodius hippopus and Vogtia) have evolved temporal shifts, in which they change from being transparent to being opaque when disturbed, startling organisms in close proximity.

 

Bioluminescent counter-illumination

Bioluminescent counter-illumination is used by the mesopelagic hatchetfish (subfamily: Sternoptychinae), a deep-sea species that is covered in a series of photogenic cells called photophores on the entirety of its ventral side. This mechanism is often used to effectively eliminate an animal’s silhouette so that it cannot be seen from below.

The photophores used by Sternoptychinae (Fig. 2) are lined with a reflective silvery covering and magenta-coloured filters, resulting in a blue bioluminescent light, mimicking the appearance of the surrounding daylight. The eyes of Sternoptychinae are specialised to ensure the bioluminescent radiation emitted from its ventral side is the same colour as the surrounding environment, while the animal travels to different depths.

Figure 2: Hatchetfish (insert scientific name) demonstrating the use of its photophores in bioluminescent counter illumination to eliminate its silhouette to animals viewing it from below. Image available from: http://www-tc.pbs.org/wgbh/nova/sciencenow/0305/images/04-glow-06.jpg
Figure 2: Hatchetfish (subfamily: Sternoptchinae); top photo a side view of the hatchet and the bottom photo exhibits a ventral view of a hatchetfish, demonstrating the use of its photophores in bioluminescent counter-illumination to eliminate its silhouette to animals viewing it from below. Image available here.

It is suggested that the cookie-cutter shark (Isistius brasiliensis) is an example of where bioluminescent counter-illumination can be a liability to an animal. I. brasiliensis has photophores that cover the entirety of its ventral side, except that of a small band (known as a dog collar). (Fig. 3). A study by Warrant and Locket (2004) states that this band is shaped in a way that is attractive to organisms that prey on I. brasiliensis. However, another study suggests that this, in fact, has a predatory use.

Figure ?: The cookie-cutter shark (Isistius brasiliensis) showing a small band of its ventral surface that is darkly pigmented, where the rest of its ventral side is covered with photophores. Taken from Widder 1998 (reproduced from Tinker 1978). Available at:
Figure 3: The cookie-cutter shark (Isistius brasiliensis); showing a small band on its ventral surface (bottom photo) that is darkly pigmented, where the rest of its ventral side is covered with photophores. Top photo is the side view of the organism. Image available here.

The dog collar of I. brasiliensis not only possesses no photophores but is also darkly pigmented and narrowed at each end, giving a fusiform-like silhouette. This shape is likely to attract prey of I. brasiliensis that are looking upwards for prey, suggesting that the band’s primary function is a lure. If this dog collar provided a significant disadvantage, it is likely that it would not be selected for naturally.

The continued evolution of camouflage in the deep sea is inevitable, as visual capabilities improve; predators that are well camouflaged will have to have a large field of vision to be able to spot prey before being detected, and visa versa. It is the hope of many scientists that it will some day be understood how organisms accomplish such exquisite mechanisms of camouflage, which could lead to the development of improved methods of camouflage for anthropogenic uses, such as polarisation camouflage in Naval technologies.

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