Numa (沼,Numa?) means "swamp" or "bog" in Japanese.
In the Viz Manga and FUNimation subs, it is called the Swamp-Swamp Fruit.
Strengths and Weaknesses
With the Numa Numa no Mi, Caribou can create bottomless swamps which are impossible to escape from. If someone is caught by the swamp created by Caribou, they will sink more if they struggle. The swamp also serves as a storage space, allowing Caribou to store an infinite amount of whatever he has captured inside his body. The storage space can also be used to help carry a large surplus of food which remains edible over long periods of time, with Caribou noting that he has a month's worth of rations stored within himself. He is able to keep several weapons within himself and bring them out when he needs them. While he can keep creatures alive when they are in his swamp, he can also use the swamp to drown his opponents if he wishes to.
Like most of the other Logia users, Caribou cannot be hit or sliced as his element is intangible, and any attacks will be absorbed into his body harmlessly. Though more like Marshall D. Teach, Caribou cannot allow the attacks to phase through his body; instead, it must be absorbed. The thick liquid produced by the fruit's power is mud which is a half-water and half-earth element, found near bogs, with Franky even describing it as "mud-like".
The major weakness of this fruit is that the user cannot transform back to their normal form if they are confined in a space that their normal body cannot fit in. Due to this weakness, Franky was able to capture and seal Caribou in a barrel easily. Also this fruit has yet to demonstrate offensive capabilities other than 'drowning' opponents into the swamp. Also, since his mud is just basically earth that is mixed with water, this leaves him vulnerable to individuals who can manipulate water, such as Jinbe. Other than that the user is affected by the standard Devil Fruit weaknesses.
With his Devil Fruit powers, Caribou can create bottomless swamps by turning his lower body into a swamp. Via this transformation, he can capture living beings like he did when he used his powers to capture some mermaids. Should his targets prove to be too large to be captured by normal size, Caribou can expand his body to increase the surface area, and wrap around the target in question to drag them in faster as he did so with Shirahoshi. His "bottomless" swamp can also be used as a storage space. In fact, his fighting style consists of storing several weapons like Gatling guns and scythes inside his body. He can also hide people inside his body for stealth purposes.
Caribou can also use it to choke his opponents by encasing their head in a sphere of swamp. Like other Logia users, he can evade (or in his case, absorb) attacks by turning his body into the related element, which is in this case, swamp. However, his over-reliance on this defense, which he incorrectly believed to be invincible, led him to have little natural defensive power when his ability was dampened by attacks imbued with Busoshoku Haki.
By staying as his mobile swamp form, Caribou gains impressive stealth attributes allowing him to sneak away or approach unnoticed and eavesdrop on conversations by sticking to obscure or hard to see places so that he can listen onto conversations. He can also extend parts of his body, such as his ear, to aid him in his eavesdropping. This ability also aided in his kidnapping spree either by laying a trap on the ground to swallow the mermaids or simply absorbing them directly into his body upon meeting them should he not have the element of surprise on his side.
Numa Numa no Gatling Gun (ヌマヌマの機関銃,Numa Numa no Gatoringu Gan?, literally meaning "Swamp Swamp Machine Gun"): A large Gatling gun with a crank mechanism comes out of Caribou's "bottomless" swamp stomach and he shoots his opponents with it. It was first seen used against Pekoms. In the Viz Manga and FUNimation subs, this is called Swamp-Swamp Gatling Gun.
Anime and Manga Differences
In the manga, Caribou first revealed his powers by choking Drip and absorbing a cannonball fired by Mounblutain. In the anime, he revealed his Devil Fruit during the battle against the Marines when a Marine soldier struck him with a sword, and countering by drowning the soldier in the process. Regardless of showing off his powers earlier than the manga, the remnants of the Fake Straw Hat Crew remained unaware of his powers until he attacked Drip with it.
This fruit's power that revolves around absorption, as well as granting the user an infinite space within the body, makes it very similar to the Yami Yami no Mi.
The Numa Numa no Mi is the first Logia-type and the first new Devil Fruit in general revealed after the timeskip.