The Den Den Mushi are snails that are used for communication and/or visual aid throughout the world. They, for the most part, serve the same functions that telephones, fax machines, and cameras do in the real world.
They are classified as type "B" creatures, being "Small Friendly".
Den Den Mushi have the mysterious ability to telepathically communicate with each other, apparently through electronic signals; they can also perfectly mimic human speech. For generations, people have taken advantage of these abilities by attaching dials, receivers, and other accessories that convert the creatures into long-distance communicators.
When a proper number is dialed, the snail will launch a telepathic link with whichever snail('s mechanism) corresponds to that number. The signal is not infallible, but a well-kept specimen can reach across entire oceans. More elaborate mechanisms can even allow them to duplicate documents and photographs, akin to fax machines.
Overall, Den Den Mushi do not mind being used and tamed, as the arrangement gives them access to plenty of food. Their accessories are also not injurious - or even especially difficult - to remove, and may be done so at any time to return them to the wild.
The standard Den Den Mushi is roughly the size of a house cat, with the dialing mechanism and receiver built into its shell (or, in rarer cases, a completely separate device "plugged" into the back of its head). Specimens vary widely in color and shell patterns, and are often customized to signify and/or resemble their owners. The Marines, for instance, stamp their emblem on the shells of their Den Den Mushi.
Due to their size, Den Den Mushi are typically left stationary on desks and the like. However, many have improvised methods to carry them and their accompanying accessories in times of necessity.
On receiving a signal, a Den Den Mushi will make a purururu (プルルル?) sound, rendered identically to the real-life Japanese onomatopoeia for old-fashioned phone-rings. However, the anime (which renders the sound closer to purupuru) indicates that the snails are actually pronouncing these "rings" phonetically (and very dryly), rather than making any noise akin to a genuine ring. Similarly, it will make a gacha (ガチャ?) sound when the receiver is removed or replaced.
Den Den Mushi appear to have a separate signal for S.O.S. calls, which cause them to cry loudly.
During active use, a Den Den Mushi will relay not only the speaker's words and emotions (laughter, screams, etc.), but also distinctive facial traits and even injuries. It is also possible for two specimens to converse with each other using a third party.
Den Den Mushi are very lethargic by nature, usually sleeping whenever not in use (and in some cases falling asleep mere seconds after a call ends). They also do not appear to be sapient enough to understand any of the words they relay. In extreme situations, however, they are capable of emotions such as fear and gratitude.
When Oda was asked if the Den Den Mushi have a silent mode and if they can change their ring tone, he confirmed Den Den Mushi do have a "silent" mode. This is normally activated with a simple "shh" and a head nod. However, as they are living creatures they have to be trained to access this ability. The resulting volume depends on the effectiveness of their training and their diet.
There also exist many biological variants, with abilities not found in standard Den Den Mushi. These are usually much rarer and much more sought-after, and constitute a vital part of both legitimate news media and Underworld espionage.
Baby Den Den Mushi
A Baby Den Den Mushi (子電伝虫 Ko Den-den Mushi?) is small enough to fit in a palm or even be worn like a wristwatch, and is favored by many for portable communication. However, its signal is weaker than that of a full-grown specimen, and cannot be used for 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
The Black Den Den Mushi (黒電伝虫 Kuro Den-den Mushi?) is a species of Den Den Mushi that remains small for its entire lifespan, and enjoys "eavesdropping" on the signals of the standard variety rather than openly communicating. It can also memorize anything it intercepts, to be repeated at a later time.
White Den Den Mushi
The White Den Den Mushi (白電伝虫 Shiro Den-den Mushi?) can produce signals that neutralize the Black Den Den Mushi's eavesdropping abilities; as such, they can be attached to any standard Den Den Mushi to encrypt the user's calls.
Horned Den Den Mushi
The Horned Den Den Mushi (ツノ電伝虫 Tsuno Den-den Mushi?) can jam radio transmissions. The range and period of a transmission jamming caused by this type are unknown. This species was first used by a member of the Fire Tank Pirates before Sanji and Pudding's wedding as part of the Retrieval Team-Fire Tank Alliance's plan to take down Big Mom. After the Whole Cake Chateau collapsed, Mont-d'Or was able to communicate through a Den Den Mushi from Sweet City to the rest of Totto Land, alluding to the jamming disablement.
Visual Den Den Mushi
The Visual Den Den Mushi (映像電伝虫 Eizō Den-den Mushi?) is similar to the Surveillance Den Den Mushi.
The Proko (プロコ Puroko?) is able to receive, project and broadcast what the Cameko sees onto a bigger video screen. It is an older Visual Den Den Mushi. They are able to broadcast their frequencies between two relatively close locations.
The Cameko (カメコ Kameko?) is able to store images and videos. It is a younger Visual Den Den Mushi. While taking a photo, it create flashes of light with its eyes in order to perform flash photography. It has an antenna connected to the shell, that allows him to send signals and live video feeds to their connected Proko.
Surveillance Den Den Mushi
The Surveillance Den Den Mushi (監視電伝虫 Kanshi Den-den Mushi?) is a Visual Den Den Mushi used for surveillance. The Cameko are mounted in specific locations around a building. They are all connected to a singular Proko, connected to a machine which produces video feeds to a set of monitors. In the event that the Cameko spots an intruder, it will send a signal to the Proko, 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.
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 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 Proko and have attachments that allow them to project what they can see to a monitor.
Golden Den Den Mushi
The Golden Den Den Mushi (ゴールデン電伝虫 Gōruden Den-den Mushi?) is an extremely rare breed of Den Den Mushi, apparently made of solid gold and completely immobile. About the size of a Baby Den Den Mushi, its sole function is to initiate a Buster Call attack; as a result, its only "accessory" is a single button at the apex of its shell, which immediately signals the proper receiving line when pressed.
According to Spandam, permission to access a Golden Den Den Mushi can only be granted by Marines of Admiral rank or higher. Only two specimens have been witnessed thus far, both used by Cipher Pol agents:
- Spandine of CP5 used one, supposedly granted by then-Admiral Sengoku, during the Ohara Incident.
- Spandam himself, as director of CP9, accidentally used one (having mistaken it for his personal Baby Den Den Mushi) during the Straw Hat Pirates' invasion of Enies Lobby. His was supposedly granted by then-Admiral Aokiji.
When activated, the Golden Den Den Mushi apparently automatically transmits its location to the receiving line, to guarantee maximum speed of response.
Silver Den Den Mushi
The Silver Den Den Mushi (シルバー電伝虫 Shirubā Den-den Mushi?) is a special type of Den Den Mushi, apparently possessing a solid silver shell, that serves as Marineford's receiving line for the Golden Den Den Mushi. It is about the size of a standard Den Den Mushi, but does not appear to transmit any voices, instead generating a loud vibrating sound to alert all nearby Marines that a Buster Call has been ordered.
The specimens receiving both Spandine's and Spandam's Buster Call orders were visibly aged, with white mustaches and beards; it is unknown if this is inherent to the breed, or simply an indication that they have been used much longer than regular Den Den Mushi (or, possibly, that only one specimen exists and is linked to every Golden Den Den Mushi in active service).
Habitat and Distribution
Not much has been detailed about the natural range of Den Den Mushi, but they appear capable of surviving anywhere ordinary snails might (indeed, it remains unknown if there are "ordinary" snails lacking their abilities). Specimens have inhabited grasslands and forests as far into the New World as Dressrosa, and even the highly isolated "phantom island" of Zou.
At least some breeds of Den Den Mushi seem to be amphibious, as Fukaboshi was seen using one underwater. However, they (or, at least, the mechanisms linked to their telepathy) are universally unsuited to extreme cold.
In terms of human distribution, Den Den Mushi do not appear to be nearly as universal as real-life telephones. While fairly ubiquitous on Marine bases and ships, few have been seen in the possession of civilians outside the news media. Where pirates and other rogues are concerned, they tend to grow more common as one approaches the New World; the Straw Hat Pirates were not known to own one until they acquired the Thousand Sunny at Water 7, but around the Sabaody Archipelago, even small-timer Duval owned one with a specified number.
Translation and Dub Issues
電(Den?), meaning electric (a common component in the words for most electric appliances, telephones ( 電話Denwa?) included)
伝(Den?), meaning communication
虫(Mushi?), meaning insect (the only kanji reflecting the original meaning, snails traditionally being lumped in with insects and most other garden pests)
Most English translations, professional or otherwise, do not attempt any kind of similar pun, and variously call the creatures Transponder Snails, Telesnails, or Snail-o-Phones (or Shock Bugs, in the case of the English localization for One Piece: Unlimited Adventure).
In the 4Kids dubbed anime, these creatures do not assume the voices of their users.
- Some isolated areas do not have access, or choose not to use Den Den Mushi:
- 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. 
- The Tontatta Kingdom's dwarves release the snail back to the wild.
- Mokomo Dukedom do not have the knowledge on how to turn the wild snails on Zou into communication devices.
- Wano Country uses Smart Tanishi (スマートタニシ Sumāto Tanishi?, literally meaning "Smart River Snail").
- 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, they 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.
- It is yet unknown whether all snails naturally possess telepathic abilities, or if Den Den Mushi are a distinct species. At the very least, they do not require special breeding to be converted into communicators; Franky indicated that he could easily do so with the wild specimens of Zou.
- In the 1850s, the Frenchman Jacques Toussaint Benoit built a contraption known as the Pasilalinic-sympathetic compass also known as "snail telegraph" based on the hypothesis that mating snails form a permanent telepathic link and can be used to send and receive long distance telegrams.
- 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