Den Den Mushi are a species of telepathic snail that are used for vocal and/or visual communication throughout the 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, even imitating people’s voices. 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 transmit and 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?) (click in the English dub) 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. Furthermore, they don't just pick up signals from each other, as they can also receive warning signals from Territorial Sea Slugs.
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.
Habitat and DistributionEdit
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.
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 MushiEdit
A Baby Den Den Mushi (子電伝虫 Ko Den-den Mushi?) is simply a younger, and therefore smaller version of the standard snail.
It is small enough to fit in the palm of ones hand 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 MushiEdit
The Black Den Den Mushi (黒電伝虫 Kuro Den-den Mushi?) is a species of Den Den Mushi that 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.
A Black Den Den Mushi remain small, about the size of a small mouse, for its entire lifespan.
White Den Den MushiEdit
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 MushiEdit
The Horned Den Den Mushi (ツノ電伝虫 Tsuno Den-den Mushi?) can jam radio transmissions from other nearby Den Den Mushi. 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 MushiEdit
The Visual Den Den Mushi (映像電伝虫 Eizō Den-den Mushi?) are a type of Den Den Mushi that transmit images instead of sounds.
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 is an older Visual Den Den Mushi. It is very large, roughly the size of an elephant.
They are able to broadcast their frequencies between two relatively close locations.
Projector Den Den MushiEdit
A Projector Den Den Mushi is able to view an image with one eye and project said image at a larger scale using its other eye.
Surveillance Den Den MushiEdit
The Surveillance Den Den Mushi (監視電伝虫 Kanshi Den-den Mushi?) is a Visual Den Den Mushi assigned to the task of surveillance.
A number of Cameko are mounted in specific locations around the designated area. They are all connected to a singular Proko, which is connected to a machine which produces video feeds to a set of monitors. Each screen displays the vision of a different Cameko.
In the event that the Cameko spots something wrong, 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 people watching the monitors.
Self-Propelled Visual Den Den MushiEdit
The Self-Propelled Visual Den Den Mushi (自走式映像転送電伝虫 Jisō-shiki Eizō Tensō Den-den Mushi?) is a type of Visual Den Den Mushi that, as the name implies, move around on their own, as opposed to the standard Den Den Mushi that just sits in place.
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 MushiEdit
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, its sole function is to transmit a signal to its Silver counterpart; as a result, its only "accessory" is a single button at the apex of its shell, which immediately signals the proper receiving snail when pressed.
About the size of a Baby Den Den Mushi, apparently made of solid gold and completely immobile.
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 CP9 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 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, that serves as a receiving line for the Golden Den Den Mushi, generating a loud vibrating alarm sound.
It is about the size of a standard Den Den Mushi, apparently possessing a solid silver shell.
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).
Translation and Dub IssuesEdit
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 always assume the voices of their users. They also make a generic phone ringing sound effect.
- Den Den Mushi, for the most part, serve the same functions that telephones, fax machines, and cameras do in the real world.
- 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 their snails 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 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
- Communication - Wikipedia article on vocal communication
- Visual communication - Wikipedia article on visual communication