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Leash Alien Icon
In Rising Tide, Leash Alien is an ability that Explorer units gain once you have researched Alien Lifeforms.
Leashing an alien can be attempted on an alien unit with 1 hex range, and will consume the remaining movement points of the explorer.
Leashing an alien unit will reduce the explorer's health by a moderate amount, and can kill the explorer if adequate health does not remain.
Leashing an alien will convert the alien to the team of the explorer, essentially gaining a new unit. This new alien unit is autonomous and not attached to the explorer, despite the ability term "leash".
Requires level 3 in 
Harmony.
Leashing cannot be used on Colossal Alien Units initially, but once you've researched Alien Domestication and have level 9 in Harmony you can leash Colossal units too.
Strategy[]
The (non-Colossal) aliens aren't the strongest units in the game, but the price is right. An Explorer can easily leash a few low level alien units back to back, quickly building up a small alien army for more exploring, traveling over miasma, and providing cover for your explorer.
Explorers can't heal while at sea, but you can use the leash alien ability at sea, so once you have, you'll have to travel land or a floating city to heal before you use the ability again, unless you're prepared for the explorer to die in the process.
The Explorer still takes damage even if the aliens are cyan (i.e. friendly).
Take note of where you are about to leash an alien unit, because aliens are not amphibious, so leashing land aliens on a small island is much less valuable than on a coastline of a large landmass.
Leashing ranged aliens, like the Manticore can also be much more valuable, as they can then gang up on enemy units and actually do some good damage.