The Golden Den Den Mushi is used for ordering the Buster Call and seems to be made of rigid gold. It's strange and unusual for the rest of the species.
Is it an actual living being, or (unlike regular Den Den Mushis) some kind of mechanically improved, or purely mechanical device?
In Chapter 386 Spandam is holding the Golden Den Den Mushi and mentions that it is a precious species, indicating that it is in fact, a living snail.
The Den Den Mushi are actually snails which use radiowave to communicate which each other. Here is a picture of a wild Den Den Mushi without any devices attached to it:
Here is what its wikia page has to say about it:
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" [...]
The gold Den Den Mushi is a rare kind of Den Den Mushi.Here's its wikia's description:
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.
Now, all Den Den Mushi are mechanically improved. However, it is not confirmed whether or not the golden Den Den Mushi is clothe in gold even in its wild form (which I doubt).
The golden snail only communicates with the silver Den Den Mushi, shown above. So far, it has been shown to be limited to alerting the Marine about a Bluster Call request from the Silver Den Den Mushi.
Golden Den Den Mushi is the variation of normal Den Den Mushi, as stated here. However, this Den Den Mushi is immobile and entire body are made out of gold.
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's also said that Golden Den Den Mushi is a very rare type of normal Den Den Mushi
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, Golden Den Den Mushi is technically is a very rare snail and not a mechanical device.
P.S.
If we're talking about "mechanically improved" the Den Den Mushi itself just a snail with can communicate with each other telepathically through radio waves. Peoples in One Piece world attaching button and receiver to them. So you can say that Den Den Mushi are mechanically improved snails.
