Change your preferences here.
If the holders current species is Clamperl, its Special Attack is doubled.
When a Clamperl is traded while holding this item, it evolves into Huntail.
The DeepSeaTooth is the only item that makes the otherwise frail and useless Clamperl a powerhouse, especially in Trick Room. When holding the DeepSeaTooth, Clamperl's mediocre 271 maximum Special Attack is boosted all the way up to 542. This will make Clamperl's STAB Surf (and other special attacks) a very powerful attack. It should go without saying that there is absolutely no reason to use DeepSeaTooth on any Pokemon other than Clamperl.