Merman Gujutsu is a fighting style used by the merfolk. It utilizes tridents and swords (and possibly other related weaponry) to deliver fast and powerful attacks. So far only the princes of Ryugu Kingdom have been shown using it. In the FUNimation subs, this is also called Merman Combat.
Koin Ryusui So (光陰流水槍,Kōin Ryūsui Sō?, literally meaning "Flying Time Spear"): Used by Fukaboshi, he initiates this attack by spinning his trident rapidly. He then lashes forward and rapidly slashes his enemy with great strength. This attack is powerful enough to defeat some of the New Fish-Man Pirates' sea monsters. In the FUNimation subs, this is called Spear of the Ages.
Ryugu Romon (竜宮楼門,Ryūgū Rōmon?, literally meaning "Dragon Palace Tower Gate"): Used by Ryuboshi, he moves both of his swords forward rapidly, slashing his enemies numerous times. This attack was strong enough to defeat some of the New Fish-Man Pirates' sea monsters. In the FUNimation subs, this is called Dragon Tower Gate.
Shitchin Banpo (七珍万宝,Shitchin Banpō?, literally meaning "All the Treasure"): Used by Manboshi, this is a powerful slashing attack, performed with both hands on the sword's hilt. It was strong enough to defeat some of the gigantic sea monsters sent against the three princes by the New Fish-Man Pirates. In the FUNimation subs, this is called Seven Great Treasures.
↑ 1.01.11.21.3One Piece Manga and Anime — Vol. 64Chapter 631 (p. 14-17) and Episode 551,
The three princes of Ryugu uses their respective Merman Jujutsu techniques against the New Fish-Man Pirates' drugged up sea beasts.