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If held by Clamperl, its Special Defense stat is doubled in battle. If Clamperl is traded while holding this item, it will evolve into Gorebyss.
A nice idea, but Clamperl's Special Defense (and, indeed, all of its stats) are so pathetically puny that even with this item in tow, you still aren't getting that much out of it. True, your Special Defense does exceed that of Blissey with maximum investment, but this is small consolation when your HP stat is so low you are finished by more or less everything anyway. Fortunately, Clamperl also happens to have DeepSeaTooth as well, which is a far more useful signature item.
Route 13 (requires Surf).
Route 13 (talk to Treasure Hunter - you receive one random treasure every day).
Wild Basculin - Blue Stripe Form (5%).