Den Den Mushi

Den Den Mushi (電伝虫) 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 telepathically through radio waves. The people in 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. They also don't work when the animals themselves are sick.

Types of Den Den Mushi
The Den Den Mushi appear 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 appear to be general categories by which Den Den Mushi are classified.

Regular Den Den Mushi
The most common type of Den Den Mushi commonly seen. They are too big to be conveniently carried 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, enough to fit in one's palm. It's good for keeping in contact with fellow comrades in the same area, but incapable of inter-island 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. Their abilities to intercept can be blocked by their counterpart, the White Den Den Mushi.

Golden Den Den Mushi
The Golden Den Den Mushi (ゴールデン電伝虫), Golden Transponder Snail in the Viz Manga, is a special kind of Den Den Mushi which is only held by the Marine Admirals. Used to summon the Buster Call attack. The Den Den Mushi is immobile and its 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 with licence from Admiral Aokiji. 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 a silver snail that is bearded and old-looking. It has a shell that appears to be made 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 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 (映像電伝虫) is similar to the Surveillance Den Den Mushi, and it broadcasts what the Den Den Mushi sees to a bigger video screen. 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 between two relatively close islands.

Self-Propelled Visual Den Den Mushi
Shiki has a variant of the Visual Den Den Mushi called the Self-Propelled Visual Den Den Mushi (自走式映像転送電伝虫). As the name implies they are able to move and look around. They also appear to be close to the same size as the larger variant of the Visual Den Den Mushi and have attachments that allow them to project what they can see to a monitor.

White Den Den Mushi
The White Den Den Mushi (白電伝虫) can be attached to other Den Den Mushi and send out psychic waves that prevent interception; in a way, they are an opposite to the Black Den Den Mushi. They are rare and one was first seen used by Monkey D. Dragon to communicate with Emporio Ivankov.

Translation and Dub Issues
The name comes from the Japanese word "Dendenmushi (でんでんむし)" which means snail. Oda just uses different Kanji with the same pronunciation to create an entire new name. 電/でん/den means electric, 伝/でん/den refers to communication, and 虫/むし/mushi as snails are generally considered bugs in Japan. The Japanese word for phone "電話/でんわ/denwa" also begins with the kanji 電.

They are called Transponder Snails, Telesnails, and Snail-o-Phones in the English versions.

In the 4Kids dub, the voices of the Den Den Mushi did not match the person speaking into them.

Trivia

 * In one of Oda's SBS, 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.
 * Regular Den Den Mushi appear in both One Piece: Unlimited Adventure and Unlimited Cruise as catchable bugs, used for creating items.
 * The idea of using a snail for communication is rather ironic since snails are incredibly slow but can be used to instantly transmit signals over long distances.
 * Skypiea is the only location seen where they seem completely absent or unavailable for use. However, Dials, the remains of particular shellfish that live in the White-White Sea, are used constantly by the natives.
 * Another reference to the fact Den Den Mushi are living creatures can be seen in the Marineford arc, when Buggy and the rest of his crew were frozen by Aokiji and unfrozen by Akainu's attack, however, the camera Den Den Mushi had yet to be unfrozen, and when it sank in the seawater boiled by Akainu, it became dehydrated as a normal snail would due to the salt content of the ocean. After that, it got extremely scared by Buggy's crew and hid in his shell. It then started crying from joy after Buggy defended the snail, saying it had worked too hard and deserved a break.