Slang Meaning Of Puns

My dad, step mum and I were watching a nature program. The topic turned to a lake that contained the largest number of wild mussels in the country. Cue conversation:

Dad: "I went to a party there once."

-Skeptical silence-

Dad: "Yeah, I pulled a mussel".

I groaned, step mum rolled her eyes, refusing to acknowledge the joke while dad is cracking up at himself. It took him a good minute to compose himself.

(For those unfamiliar with the slang, in England "to pull" someone means scoring/picking someone up at a bar/club/party or whatever)

EDIT cant spell.