The Den Den Mushi are snails that are used for communication or visual aid. They are usually seen with phones or fax machines attached to their shells.
They have the ability to communicate with each other telepathically through radio waves. The people of the One Piece world take advantage of this ability by attaching buttons and receivers to them. According to Franky, this process is quite simple, as when he offered to make communicators for the Mink Tribe from the wild snails of Zou. 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".A Den Den Mushi makes a ring-ring-ring (purupurupuru in Japanese) sound, to inform people on the receiving end that someone is calling, in a similar way to old fashioned telephones. When a person calls or speaks through a Den Den Mushi, the Den Den Mushi will mimic the person's speech and display their emotions as well as take on the distinctive physical traits of the person on the other end. For example, when someone screams, the snail will scream as well. They also seem to be able to change their eyecolor and even if they lost some teeth(ex. Caesar talking over Law's Den Den Mushi) whenever someone is talking through. 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 do not mind being used and tamed. The snails appear to be amphibious, as Fukaboshi was seen using one underwater. However, they do not work in extreme cold. They also do not work when the animals themselves are sick. When an S.O.S. signal is sent out, rather than ringing normally as in a regular phone call, the Den Den Mushi cries loudly. When a Den Den Mushi returns to the wild and their receivers are removed, they become a regular snail.
Types of Den Den MushiEdit
The Den Den Mushi appear in many varieties, depending on who owns them and who is talking. They are sometimes designed to resemble their user. Marine Den Den Mushi typically 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 MushiEdit
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. The Marines are shown to often favour these over Babies in field communications due to the range, carrying them around with the help of a shoulder strap.
Baby Den Den MushiEdit
A Baby Den Den Mushi (子電伝虫 Ko Den-den Mushi?) 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 MushiEdit
The Black Den Den Mushi (黒電伝虫 Kuro Den-den Mushi?), 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 MushiEdit
The Golden Den Den Mushi (ゴールデン電伝虫 Gōruden Den-den Mushi?), Golden Transponder Snail in the Viz Manga, is a special kind of Den Den Mushi which is only held by the Marine Admirals. It is 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, which is the signal for 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, twenty years later, by Spandam with false permission from Admiral Aokiji. While Spandine's use of it was intentional, Spandam's was not; he used the Golden Den Den Mushi by accident because he thought it was a Baby Den Den Mushi, which he was also carrying, and wrongfully pulled out the Golden Den Den Mushi and activated it without looking to see which Den Den Mushi of the two he had actually grabbed.
Silver Den Den MushiEdit
The Silver Den Den Mushi (シルバー電伝虫 Shirubā Den-den Mushi?) 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 an elderly snail that is bearded and old-looking. It has a shell that appears to be made of solid silver and similar in design to the Golden Den Den Mushi. Unlike regular Den Den Mushi, when this one 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 MushiEdit
The Surveillance Den Den Mushi (監視電伝虫 Kanshi Den-den Mushi?) 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 antenna 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. In the event that the smaller one spots an intruder, it will send a signal to the larger one, which will cause its red eyes to begin flashing in an alternating manner, and make a buzzing noise to alert the security guards watching the monitors.
Visual Den Den MushiEdit
The Visual Den Den Mushi (映像電伝虫 Eizō 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. 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 MushiEdit
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 Jisō-shiki Eizō Tensō Den-den Mushi?). 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 MushiEdit
The White Den Den Mushi (白電伝虫 Shiro Den-den Mushi?) can be attached to other Den Den Mushi. They send out psychic waves that prevent calls from being intercepted; in a way, they are the opposite of the black Den Den Mushi. They are rare and one was first seen used by Monkey D. Dragon to communicate with Emporio Ivankov.
Camera Den Den MushiEdit
This Den Den Mushi is able to take photos, creating flashes of light with its eyes in order to perform flash photography. This species was first seen being used by Koala in the underground trading port in Dressrosa.
Translation and Dub IssuesEdit
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 entirely 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, but also mainly translates to "insect". The Japanese word for phone, 電話 (でんわ/denwa; also read with the same characters in Chinese) also begins with the kanji 電.
In the English versions of the manga and anime, they are called Transponder Snails, Telesnails, and Snail-o-Phones. In the English version of One Piece: Unlimited Adventure, they are called "Shock Bugs".
In the 4Kids dub, the voices of the Den Den Mushi did not match the person speaking into them.
- 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 does not harm the animal at all.
- 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. 
- Regular Den Den Mushi living in the wild appear in both One Piece: Unlimited Adventure and Unlimited Cruise as catchable bugs, used for creating items.
- In One Piece: Unlimited Adventure, the are said to communicate with one another using radio waves. During Spandam's Special attack he uses a Den Den Mushi to call authorize an artillery strike.
- 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. It then became very happy, loyal, cooperative and grateful to Buggy, following a running gag where Buggy had won favor with people by looking impressive through pure dumb luck.
- Snail - Wikipedia article on snails in general
- Telephone - Wikipedia article on the telephone
- Pasilalinic-sympathetic compass - Wikipedia article on a concept similar to Den Den Mushi