Makes the Submarine's darts pop up to 3 bloons each.
~ Description from BTD 5 iOS

Barbed Darts is the first upgrade on path 1 for the Monkey Sub in BTD5. It costs $255 on Easy, $300 on Medium, $325 on Hard and $360 on Impoppable. It enables the sub's darts to go through three bloons at once, as opposed to the intial one. This upgrade can be useful, as the sub's darts are homing. However, the Airburst Darts upgrade renders this upgrade pointless, as the dart will split into three upon impact. Unless the player didn't want to purchase Airburst Darts or wanted to get Advanced Intel, this renders Barbed Darts functionally useless.

In BTD6, this upgrade returns, but the upgrade is now the first upgrade of Path 2, as well as no longer being on the same path alignment as upgrades the Monkey Sub had in BTD5. However, it also makes the Sub hit up to 4 Bloons per shot. It in addition costs $450 on Medium.