The Sazae-Oni produces a type of mucus that covers it's body to help protect itself from predators by attracting rocks, seaweed, shells, and sand over its body to make the yōkai's flesh taste horrible. During rainy days/nights, this lustful creature exits it's home to search for a mate. After finding a perfect human, the Sazae-Oni will first cover its victim with mucus to help soften the body and to hold him into place while mating. Afterwards, the Sazae-Oni will tear off the testicles for reasons unclear and will eat the body. Out of theory, Monstrologists believe that the Sazae-Oni uses the remaining sperm in the testicles for later fertilization or some believe they are used as trophies of each victim the single Sazae-Oni had taken. Being amphibious, the Sazae-Oni are mainly found in the deep water off the coast during the day and on the beach during the cool air of the night.
The Sazae-Oni in the illustration above is what happens when the yōkai move it's shell to a new location while picking up debris to avoid being eaten by predators. The mucus of the Sazae-Oni is so strong that even their shell can stick to their human sized bodies.