Survival in nature can be quite easy. Enough defenses invested, and one can make it quite safely through life. Examples are whales, roses and poison arrow frogs, investing in size, structures or chemistry in order to deter predators. The problem with these strategies is that they are costly, diverting energetic resources from growth and reproduction to survival. There is, however, a way of both eating the cake and having it! Such organisms fake their defense, mimicking both structures and colors of animals with real defenses. This specific kind of mimicry, where a harmless species has evolved to imitate the warning signals of a harmful species reaping the benefits but not paying the costs of defenses, is called Batesian mimicry. The prime example of Batesian mimicry would probably be the hoverflies. Their black and yellow barring probably mimics the pattern of yellowjackets and bees, making would-be predators think twice before trying to attack the harmless hoverflies.
There are a number of examples in the waters we as divers visit. I will give a couple of examples from mimics of flatworms. Flatworms are generally poisonous, warning potential predators that it would be a very bad idea to lunch on the flatworm.
The first example is juvenile pinnate batfish, which with its erratic movements and distinct color pattern does quite a good representation of a swimming flatworm
Second, we have the juvenile barramundis. They also have a distinct and very contrasty color pattern and swims with the same erratic movements as the batfish.
Third, another erratic swimmer is the juvenile harlequin sweetlips. With its big spots, contrasting against its white color, few predators take the risk of trying to eat it.
Then, there are of course other models for Batesian mimics. One is the banded snake eel, which probably mimics the yellowlipped kraits. It seems to be very efficient, as banded snake eels, in contrast to other snake eels, are often found swimming openly during day time.