Kuroobi is a ray-type fish-man. Kuroobi is one of the strictest members in his crew and has always been wary of Nami’s supposed allegiance to Arlong. Kuroobi practices “Fish-Man Karate” (called “Merman Karate” in the now defunct 4kids dub) to fight his opponents. During the battle with the Straw Hats in the Arlong arc, he fights against Sanji whom he nearly defeats when the cook goes underwater to try and save Luffy. As a Fish-Man, Kuroobi has an advantage underwater. However, Sanji manages to force him up to dry land by breathing the air from his gills, suffocating him, and then promptly beats him into submission with his kicks.

(Source: Wikipedia)