Mantis Shrimp, female
(Odontodactylus scyllarus)
The male of this species is often bright green, but the female is more bland. These nocturnal predators feed on other crustaceans, worms, molluscs and fishes.

When detected this crustacean prefers to retreat into its shelter, but be aware: they are not defenseless and can do a great deal of damage to one's finger!