Clamperl is known as the Bivalve Pokemon and has two different evolutions in Brilliant Diamond and Shining Pearl.
The Water-type Pokemon can evolve into the gracious Gorebyss or the ferocious Huntail. They remain pure Water-types without a secondary typing added after the evolution takes place.
If you want to evolve Clamperl in Pokemon Brilliant Diamond and Shining Pearl, you will need to catch one, give it either the Deep Sea Scale or the Deep Sea Tooth, then trade it away.
How to catch and evovle Clamperl in Pokemon Brilliant Diamond and Shining Pearl
Clamperl can be found in the Grand Underground of Pokemon Brilliant Diamond and Shining Pearl. The Fountainspring Cave, Riverbank Cave, and Still-Water Cavern biomes may have one after you acquire the National Pokedex.
Otherwise, you can catch one with the Super Rod on Route 219 or Route 221. The encounter rate there is extremely rare, however. Once you've caught one, you can focus on evolving it.
How to evolve Clamperl into Gorebyss
Evolving Clamperl is the simple part. Getting everything in order to do it is where the challenge comes from. You will need to find the Deap Sea Scale item to start the process.
This can be found in the Central Zone of the Grand Underground. There is also a 5% chance that a wild Gorebyss or Relicanth will be holding one after you catch it and put it in your party or PC box.
After you acquire the Deep Sea Scale, give it to Clamperl to hold. Now, find a friend to trade with. Trade Clamperl and it will evolve, then have your friend send it right back to you.
How to evolve Clamperl into Huntail
Evolving Clamperl into Huntail works almost exactly the same as it does for Gorebyss in Pokemon Brilliant Diamond and Shining Pearl. The main difference is the held item being used.
Clamperl evolves into Huntail while holding the Deap Sea Tooth rather than the Deep Sea Scale. It can be found in the Southwestern Zone of the Grand Underground.
There is a 5% chance for a wild Pokemon to be holding it, too. You'll find it on wild Carvanha, Sharpedo, and Huntail. Trade the Clamperl holding the Deep Sea Tooth for it to become Huntail and initiate a trade once more for it to return.