Hide in plain sight
Despite not appearing quite as extreme as the freezing cold waters of the polar region or the crushing depths of the deep-sea, the featureless environment of the pelagic zone provides unique and challenging conditions for the species living here.
The most upper limit of the pelagic is the epipelagic. Ranging from around 200m deep to the ocean surface, this region is very well-lit which aids in high visibility. One would expect both predator and prey to be hindered by the high visibility in the epipelagic. Both are constantly exposed with nowhere to hide, no rocks to take cover behind, no benthic fauna to scurry into, to survive here, one must be able to hide in plain sight.
The commonality of complex adaptations for crypsis in pelagic species compared to other habitats suggests that exposure in this region is a strong selection pressure. The three most common for this zone are countershading, transparency, and silvering. All three require light from the surface to function, hence why organisms most often display them in the epipelagic.
Countershading is the name given to a species that expresses dark colouration on its dorsal side and a lighter coloured ventral region (Figure 1). Despite being first recognised as a crypsis mechanism over 100 years ago, and common throughout a wide range of marine species, exactly how countershading protects an organism still lacks understanding. In 1896, A. H. Thayer hypothesised that an organism achieved camouflage via countershading through a mechanism called ‘self-shadow concealment‘. Thayer suggested that the variation in colouration is to conceal the animals shadow. Light from the surface means that the ventral side of a fish is usually in shadow and that a predator could identify its prey by the darkened underside.
Another proposed mechanism is ‘background matching‘. Background matching may be particularly useful in the pelagic zone as predators can attack their prey from any direction, but most likely, they’ll attack from above or below. The lighter ventral side is harder to make out against the bright surface above, and the darker dorsal region blends in against the black abyss beneath. Because of this, one may expect an organism that exhibits countershading to be vulnerable from the side, yet Thayer went on to argue that the dorsoventral gradation in colour diminishes cues such as shading and contour; making the three-dimensional animal appear less rounded and thus harder to identify as prey.
Transparency is common in plankton and jellyfish found in the epipelagic. It requires the whole of the organism to be successful as opposed to just the surface layer to achieve the illusion, as is the case with countershading and silvering. Transparent species are mostly water; meaning that when light from surrounding water crosses the surface tissue, the direction of the light is virtually unchanged. However, the organism cannot achieve full transparency due to the slight variance in refraction between the gelatinous body of the organism and seawater. In addition, there are always certain parts of an organism that must remain opaque to function. An example would be the retina, having to absorb light, it cannot be transparent. Therefore, many species have their eyes on the end of long protruding stalks to distance them from the main body.
Another part of the body that is hard to remain transparent is the stomach. The stomach itself is not opaque, but much of what the organism has consumed will be. Therefore, the partly digested food matter will make it vulnerable to the large, visual predators that roam these waters. To combat this, many transparent species have their stomach coated in reflective ‘mirror’ like tissue. This strategy is effective as the light being reflected by the stomach is no different to the light behind the organism, thus giving the illusion of invisibility. The same principle applies to silvering in many fish species.
Most commonly recognised in sardines (Sardina pilchardus), Silvering is the process by which the skin maintains its high reflectivity so as not to stand out from its surroundings. Usually, when light reflects off a surface, it becomes polarised, causing a decrease in reflectivity. Drops in reflectivity can be dangerous for pelagic prey, as some predators are sensitive to polarised light, which aids in the locating of countershaded prey.
To combat this, silvering fish have evolved a complex layer of tissue beneath the scales called the “stratum argenteum” (Figure 2). This layer of tissue contains cytoplasm and guanine crystals. The crystals themselves are known as birefringent, meaning the refractive index is dependent on the direction and polarisation of the light. However, the crystals are arranged in layers with two populations, either with the optical axis of the crystal parallel to the long axis or perpendicular to the crystal plane. The orientation of the crystals and the layering of the stratum argenteum results in polarisation to occur over various angles as opposed to just one. This arrangement nullifies the polarisation of light upon reflection, causing the sardine to reflect light from almost every angle.
What does it mean?
It is clear that specialised camouflage techniques have developed in epipelagic species to combat the stress of exposure. As predicted earlier, the high visibility of these waters hinders both predator and prey. Therefore, countershading, transparency and silvering have evolved so that the organism remains as unseen as possible in this featureless environment. Also, the non-polarising effect exhibited by Sardina pilchardus has been adapted, and is now used in sunglasses with polarised lenses by fishermen. Polarised lenses allow for the fisherman to avoid glare from the water, which helps to see the fish beneath (Figure 3).