What do the Secret Seashells do?

In Link's Awakening, sometimes I come across Secret Seashells, and the text that pops up says that if I collect a lot of them, something good is bound to happen.

what will happen if I collect enough? (How many is enough, anyhow?)

They can be brought to the Seashell Mansion at various points to earn things.

• 5 shells gets you a bonus shell.
• 10 shells gets you another bonus shell.
• 20+ shells gets you the Level-2 Sword, capable of shooting beams at full health (as well as being more powerful in general).

There are several extra hidden around the island (about a total of 25). After you've cashed in 20 of them, any extra ones in chests you haven't opened will turn into 20 rupees.

(Bonus fun fact: you can put bombs in the wooden wall at the end of the Mansion's insides. For some reason, they fall into it!)

If you collect and bring 20 of them to the Seashell house, you'll get a more powerful sword. There should be a bit more than 20 in total, so you don't have to find all of them.