Den Den Mushi

Den Den Mushi (電伝虫), called Transponder Snails, Telesnails, and Snail-o-Phones in the English versions, are snails that are usually seen with phones or fax machines attached to their shells.

About
They have the ability to communicate with each other through radio (thought) waves. The people of the One Piece world take advantage of this ability by attaching buttons and receivers to them. Den Den Mushi thus take the place of telephones and other similar machines in the world of One Piece. They are classified as type "B" creatures being "Small Friendly".

When a person speaks through a Den Den Mushi, the Den Den Mushi will mouth the person's speech and display the person's emotions. For example, when someone screams, the snail will scream as well. Den Den Mushi are usually active when someone answers the phone, but once they are hung up, they go to sleep. Despite acting like machines, they are actually animals. Since being raised by humans gives them access to plenty of food, Den Den Mushi don't mind being used and tamed. However, they don't work in extreme cold.

The name comes from the words "denwa" (電話), which means "phone" and "mushi" (虫), meaning "bug".

Types of Den Den Mushi
The Den Den Mushi can come out in many many styles based on where they are and who owns them. For example, marine Den Den Mushi have the marine symbol painted on their face or on their shell. Despite their varied appearances, there are however general specific groups for each and every Den Den Mushi.

Regular Den Den Mushi
The most common type of Den Den Mushi commonly seen. They are too big to carry around but their signal range is much further than that of a baby. They can also serve as fax machines if the right accessory is attached. They are capable of making long distance calls, and many are large enough to make a worldwide signal.

Baby Den Den Mushi
A Baby Den Den Mushi (子電伝虫) is a fairly small and portable Den Den Mushi. It's good for keeping in contact with fellow comrades in the same area, but incapable of international calls. They can also be used as speakers, in which to broadcast the user's voice throughout the area.

Black Den Den Mushi
A Black Den Den Mushi (黒電伝虫), Black Transponder Snail in the English versions, is a type of Den Den Mushi used commonly by the Marines, as listening devices or "wiretaps", to intercept calls between Pirates or other miscreants. They are actually a different type of species compared to other types of Den Den Mushi. They do not grow as they age and remain a small portable size for the rest of their lives. They enjoy eavesdropping, and dislike communicating with others.

Golden Den Den Mushi
The Golden Den Den Mushi (ゴールデン電伝虫) is a special kind of Den Den Mushi, used to summon the Buster Call attack. The Den Den Mushi is immobile and it's entire body appears to be made out of gold.

It is a very rare type of Den Den Mushi. After the button on its back is pressed, it sends a message straight to the Silver Den Den Mushi, telling the Marines to start the Buster Call.

So far in the series, it has only been shown to be used twice: once by Spandine on the Island of Ohara, and again, 20 years later, by Spandam. Spandine's use of it was intentional, although Spandam's was not; he used the Golden Den Den Mushi by accident because he thought it was a Baby Den Den Mushi.

Silver Den Den Mushi
The Silver Den Den Mushi (シルバー電伝虫) is a special type of Den Den Mushi, used as an alarm to alert the Marines through the Golden Den Den Mushi's Buster Call request. It is depicted as silver snail that is bearded and old looking. It has a shell that appears to be of solid silver. The snail is generally calm when nothing happens. However when it receives a Buster Call signal from a Golden Den Den Mushi, it gives off a loud vibrating like sound that alerts any and every Marine nearby.

Surveillance Den Den Mushi
The Surveillance Den Den Mushi (監視電伝虫) is a type of Den Den Mushi that acts as a video camera. They come in two sizes. The small Surveillance Den Den Mushi are mounted in specific locations around a building with an alarm connected to the shell. They are all connected to a larger Surveillance Den Den Mushi that is able to pick up the signals. These large ones are connected to a machine which produces video feeds to a set of monitors.

Visual Den Den Mushi
The Visual Den Den Mushi is similar to the Surveillance Den Den Mushi, and it broadcasts what the Den Den Mushi sees to a bigger video screen. Unlike the Surveillance version, the Visual one is used as a TV camera, and broadcasts worldwide. The smaller Den Den Mushi (which seems to be larger than the Surveillance variation) are used as the cameras, while the larger ones are linked to the screens to receive the signals. These visual versions seem to be able to broadcast their frequencies worldwide.

Trivia

 * In one of Oda's SBS questions, a fan asked Oda to apologize to a Den Den Mushi whose receiver was destroyed by Sanji. Oda responded to the fan's question by explaining that Den Den Mushi are actually born naturally without the receivers and buttons commonly seen on them. The people of the One Piece world attach these devices to the snails in order to use the snails' ability to communicate with one another. Breaking these devices attached in truth doesn't harm the animal at all.
 * Den Den Mushi tend to be costumed based on their owner's own looks. For instance, a Den Den Mushi that belongs to a masked individual, such a Spandam, would have a matching mask.