These are two English puns. Who can help me explain?

To put it simply, the first one is that a professor told his students that some languages are double negative, and double affirmation is negative. The last student said, "Yeah, right." (double affirmation) to express their objections, so as to achieve humorous effect.

The second is that two fish see two mussels eating chocolate, and then they say they are two shellfish, which is the same as being too selfish.