Visual Den Den Mushi are a type of transponder snail specialized in the One Piece universe, primarily used for visual communication and surveillance. The creatures are really an extended version of the common Den Den Mushi concept by adding visual capabilities to what is already a versatile communication system existing in One Piece.
The main role of the Visual Den Den Mushi is to transmit and receive visual information, meaning it works more like video calling or surveillance devices. In simple terms, they work on the same basis as regular Den Den Mushi but have increased image capture and display capability. This produces long-distance visual communication in a world where technology is often merged with living things.
One of the more notable variants of the Visual Den Den Mushi has to be the Monitoring Den Den Mushi, also simply referred to as a Cameko. These are specifically used for surveillance purposes and often installed at various locations to monitor certain areas. The Cameko system is capable of providing constant visual observation; it thus serves as a tool for security and intelligence in the world of One Piece.
The most advanced version is the Self-Propelled Visual Den Den Mushi, which is a very rare variant and belongs to the character Shiki. This kind may move by itself and, hence, afford mobile surveillance or communication. Now, with this kind of ability to move autonomously, Den Den Mushi is capable of doing things that add another dimension to their potential, really showing that there is a possibility for evolution and adaptability to different needs for these creatures.
Like their audio-only counterparts, Visual Den Den Mushi could also reproduce the speaking person's face expression. This thus adds a personal touch to long-distance communication, in that the person speaking through it could express emotions and facial expressions as his or her words are conveyed. Contrarily, while they can express and imitate expression, they can't be completely changed into another appearance. For example, when a Visual Den Den Mushi is sporting a mustache, it means the snail itself has one—not merely mimicking the whiskers on his face.
When it comes to appearance, the Visual Den Den Mushi is typically larger than the regular Den Den Mushi, often sporting extra eyes or lenses that would lend to amplifying their visual capabilities. Most characters in One Piece have personalized Visual Den Den Mushi resembling their appearance or character. Brook, being a skeletal musician character, has a Visual Den Den Mushi mimicking his unique skull features.
Visual Den Den Mushi in One Piece typically goes in conjunction with a major plot point or dramatic reveal. They are used to broadcast events of vital nature, much like a public execution or a government announcement, to a wide audience. They allow Oda to provide information to multiple characters and the audience simultaneously with often dramatic effect.