Its description is: "After hours of study, you have managed to exploit the weaknesses of mirelurk and now have a damage bonus of 5 when you attack one."
The perk was intended to grant a bonus damage of 5 per hit against mirelurks, similar to the intended effect of Ghoul Ecology against ghouls.
Due to a misplaced condition, the Mirelurk Ecology perk has no effect. The perk is intended to check whether or not the creature being damaged is a mirelurk, but instead runs that check on the owner of the perk, i.e. the player, who is obviously not a mirelurk.
Behind the scenes
The Mirelurk Ecology perk was meant to be obtained after reading a book titled Marsh Hunting Tips Vol IV: Mirelurks. How the book was to be obtained is not indicated. The book still exists within the DLC's resources and, if somehow obtained, will grant the perk when used.
A similar book was created (and not used) for the Ghoul Ecology perk, titled "10 Indisputable Rules On Ghouls".