Underwater camouflage

At the very deepest areas such as the benthic regions of the hadal zone, most animals use pale red and cream colors.

Some marine animals such as jellyfish have gelatinous bodies, composed mainly of water; their thick mesogloea is acellular and highly transparent.

[7] Some tissues such as muscles can be made transparent, provided either they are very thin or organised as regular layers or fibrils that are small compared to the wavelength of visible light.

At medium depths at sea, light comes from above, so a mirror oriented vertically makes animals such as fish invisible from the side.

A further complication for fish with bodies that are rounded in cross-section is that the mirrors would be ineffective if laid flat on the skin, as they would fail to reflect horizontally.

[10] Counter-illumination through bioluminescence on the underside (ventral region) of the body is found in many species that live in the open ocean down to about 1000 metres.

These have light-producing organs (photophores) scattered all over their undersides, creating a sparkling glow that prevents the animal from appearing as a dark shape when seen from below.

[11] Counter-illumination camouflage is the likely function of the bioluminescence of many marine organisms, though light is also produced to attract[12] or to detect prey[13] and for signalling.

Top/bottom countershading is common in fish including sharks, marlin, and mackerel, and animals in other groups such as dolphins, turtles and penguins.

[14][15] Mimesis is practised by animals such as the leafy sea dragon, Phycodurus eques, and the leaf scorpionfish, Taenianotus triacanthus, which resemble parts of plants, and gently rock their bodies as if swayed by a current.

[16][17] In the fish species Novaculichthys taeniourus, the rockmover or dragon wrasse, there is a striking difference in appearance between the adults and the juveniles.

It swims in a vertical position with its head pointing downwards, and behaves in a way that perfectly resembles the movement of a piece of seaweed: moving back and forth in the surge, as if it was inanimate.

[18] Self-decoration is employed by animals in different groups, including decorator crabs, which attach materials from their environment, as well as living organisms, to camouflage themselves.

For example, the Japanese hermit crab, Eupagurus constans, has the hydroid Hydractinia sodalis growing all over the shell that it lives in.

Another hermit crab, Eupagurus cuanensis, has the aposematic orange sponge Suberites domuncula which is bitter-tasting and not eaten by fish.

[19] Similarly, sea urchins use their tube feet to pick up debris from the bottom and attach it to their upper surfaces.

[22] Some fish which mimic seaweeds such as the frogfishes Antennarius marmoratus and Pterophryne tumida have elaborate projections and spines which are combined with complex disruptive coloration.

Many cephalopods including octopus, cuttlefish, and squid similarly use colour change, in their case both for camouflage and signalling.

The blackest species was in the predatory genus Oneirodes (dreamers) which reflected only 0.044% of ambient light, and was almost as black across the range 350 to 700 nm.

This distribution in turn implies that natural selection has driven the convergent evolution of ultra-blackness camouflage independently many times.