One of the most fascinating behaviors to observe underwater are the diverse approaches to not being seen (crypsis).
At the same time it is one of the biggest challenges for scuba divers to find these creatures. In the marine environment the survival of many animals depends on their ability to blend in with the environment.
Two main reasons make this behavior essential and many clever tactics evolved to achieve it.
As divers we might miss dozens of critters every single dive because they look like a coral, a rock or are see-through. So most of the other marine animals won’t see them either.
Camouflage and mimicry are not as one might think only used by weak or slow creatures but even sharks at the top of the food chain show certain features to blend in with the surrounding waters.
Why the effort?
Not to be seen of course but by whom? Most of us first think of hiding from a predator. However, many animals use it as tactic to hunt. They remain unseen by their prey until it is too close to escape.
Several creatures just sit somewhere pretending not to be there and wait for prey to pass by in front of their mouth at which point they can suck them in.
Another reason to use these tactics is for reproduction. Cuttlefish can display two different colors one on the side of the female to impress her and a threatening white color to the side of a rival.
Not being there vs being something different
Camouflage is pretending not to be there and mimicry is pretending to be something different. The difference is that the camouflaging animal is not seen at all and the mimicking animal is seen by its predator or prey but it does look like something different.
These two behaviors are quite similar and some animals can do both. The mimic octopus is a good example. It can resemble something dangerous like a sea snake or pretend not to be there by looking like a rock on sand.
Camouflage (disguise) and mimicry (deceit) overlap when an animal pretends to be something uninteresting like sea weed. It is camouflaged looking like its surroundings by using mimicry.
The different methods
Several methods are used by marine animals to blend in with the environment pretending to be something different or not to be there at all.
1. Don’t be there
Lower animals pretend to just not being there by being transparent. Most of the small free swimming creatures are doing this like jellyfish and plankton. In addition shrimp and other small critters living on the ground are at least partly see-through.
2. Don’t be there II
If an animal can’t be transparent it can have the same color and form like its habitat. This is one very common tactic that can be observed a lot by animals living on the sea floor or in coral reefs.
Animals living on the sandy bottom are often sand colored. A cryptic shrimp living on a sponge can’t be found very easily as it copies the color and texture of a sponge.
It is believed that even brightly colored reef fish in shallow water are well camouflaged in. The reefs are often very colorful where patches of light would almost look like the bright coloring of the fish.
3. Have confusing colors
Similar to having the same color like the background it also works to have disruptive colors. These colors break up the outlines of the animal and often hide the eyes as well. To achieve this the coloration does include strong contrasts with colors similar to the environment to blend in perfectly.
4. Be red
Red is the first color to fade out at low light situations for example at dusk, under a rock or at depth. Most red fish live below the depth of red light penetration or hide during the day to only come out at night. During these times they are difficult to see.
4. Be a mirror
Animals which are too big to be transparent and move around in the open sea use reflection to remain unseen. The effect is called silvering. Pelagic fish are like a silver mirror which shows the ocean around instead of the fish. Most fish in the upper ocean are camouflaged by this effect.
5. Look like the deep sea and the ocean surface at the same time
Another type of camouflage used by pelagics is countershading. This is achieved with a dark top to imitate the dephts of the ocean and a lightly colored belly to resemble the surface. So no matter where the predator or prey is it remains difficult to make out a silhouette. Many bigger fish including sharks use this coloration. Some even combine it with silvering on the sides.
6. Switch the lights on
A more fancy version of the countershading is counter-illumination where the fish use bioiluminescence on the belly to resemble the ocean surface. This is an active process some animals are able to change the amount of light to perfectly camouflage themselves against a background. The light can either be produced by the animal itself or by a luminescent bacterium. This type of camouflage is mainly seen in animals that live deeper in the ocean. One example for this is the cookiecutter shark. The bobtail squid is a critter living in the shallower water using bioluminescence from bacteria to blend in with its surrounding.
7. Actively imitate the surroundings
Octopi and cuttlefish are able to actively change body texture and coloration to blend in with different environments. They can quickly change their appearance when moving around. This makes them much more flexible than animals which have a fixed color and will only remain unseen on one specific type of background.
8. Look like something uninteresting
The animal imitates a plant or an animal from the habitat which does not mean a threat for the prey or is unedable for predators. Examples for this behaviour is a leafy sea dragon looking like sea weed.
Alternatively many animals hide in holes in the sand, under coral or rocks. Some have really clever tactics to hide away like the coconut octopus who hides in coconuts, between shells or in bottles.
10. Decorate yourself
Animals like the decorator crab adds materials from the environment to itself which let them camouflage in with the surroundings or if poisonous animals are used as decoration it makes them inedible for predators.
The following are similar tactics where the animal is seen by the predator or rival
Look like something dangerous or disgusting
This behavior is very similar to the one described above. Instead of pretending to be something boring the animal mimics something poisonous or dangerous to the predator. This behavior is called batesian mimicry. A good example is the coleman shrimp which lives on the poisonous sea urchin and blends nicely in with the stingy spikes.
Add some more eyes
A type of mimicry is distraction with eyespots. One way this works is that the predator gets confused by the eye spots and attacks from the back. Like this the prey has a better chance to get away as the more sensitive head is not hit in the first attack.
Fake it till you make it
Different ways have evolved to momentarily distract a predator by a threatening or startling behavior or display. One nice examples are mantis shrimp who spread there front limbs to rivals to show their strength. This is even done by newly molted shrimp which could not get into a fight without hurting itself.
Other examples are a sudden display of eyespots or pretending to be much bigger like octopi which turn white and spread out there arms to look as large as possible.