Den Den Mushi

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. 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 speaks through a Den Den Mushi, the Den Den Mushi will mouth the person's speech and display the person's emotions as well as take on distinctive physical traits of the person on the other end. 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 do not mind being used and tamed. 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.

Den Den Mushi

The Den Den Mushi appear in many varieties, depending on who owns them and who is talking. For example, 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. Den Den Mushi are sometimes designed to resemble their user.
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 Babys in field communications due to the range, carrying them around with the help of a shoulder strap.
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.
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.
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.
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.