The Manta droid subfighter was a droid submarine fighter craft used by the Trade Federation. The Confederacy of Independent Systems and Quarren Isolation League used these craft during the Clone Wars.
After military defeats on Naboo, Maramere, and other water-covered planets, the Trade Federation began investing in the research and development of submersible fighter-craft. The Galactic Republic legislation that prohibited the Trade Federation from developing military droid hardware was only a temporary setback for the project because Trade Federation officials just moved the prototypes away from Republic investigators and bribed or disposed of anyone who discovered their work in progress.
The devout Xi Charrian engineers developed the subfighter while residing on remote private estates, isolated from the rest of their Haor Chall religious order. As a result of their isolation, their design talents were affected and the end product did suffer a number of design limitations. Nonetheless, the resulting manta droid subfighters were still impressive fighting machines when released in large shoals.
The sleek torpedo-like droid body was capped in front of a large wedge-shaped diving plane. The forward edge of the plane had an electromotive field, that when merged with repulsorlift-driven supercavitation vectrals incorporated into the subfighter's design, resulted in a very fast and surprisingly maneuverable underwater vessel. The twin barrels recessed in the plane sport laser cannons or torpedo launchers, and a few models even featured articulated barrel assemblies that allow both types of weapons on the same fighter.
The Xi Charrians were unable to reproduce the requested variable geometry feature found in their Vulture droid starfighter designs on the manta droid subfighters. The actuation servomotors required to change the fighter's shape in deep ocean pressures proved too difficult to engineer. As a compensation for this failure to the Trade Federation, the Xi Charrians provided more aquatic craft. This included a larger carrier craft and hydrodynamic adaptor kits for B1 battle droid-operated MVR-3 speeders. Despite the perfectionist Xi Char considering the manta droid a failure, the Trade Federation and Quarren Isolation League were quite satisfied with it.
During the Clone Wars, the Confederacy used these droids as one of their primary underwater weapons alongside aqua droids against the Republic's own water-based weapons and units. In 22 BBY, they employed them in the Battle of Mon Calamari against a Republic army of clone SCUBA troopers led by Jedi Master Kit Fisto, but Fisto and his army, aided by the Mon Calamari Knights, devastated the manta droid navy and destroyed the crab cannon superweapon, effectively winning the battle for the Republic.
Following the Clone Wars, a number of surviving manta droids remained on Mon Calamari, and formed their own community. Over time, they created a loose alliance with the Quarren, hunting ocean predators in exchange for modifications, repairs and equipment.