Makes the Submarine's darts pop up to 3 bloons each.
~ Description from BTD 5 iOS
Barbed Darts is an upgrade for the Monkey Sub that appears in BTD5 and BTD6.
In BTD5, it 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 Monkey 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, as first upgrade of Path 2. It increases the pierce of the Monkey Sub. Unlike BTD5, Barbed Darts does still grant additional pierce when you purchase Airburst Darts. It in addition costs $450 on Medium.