Well, this is the way I think it.
It said Shellder helps Slowpoke go on two legs, and Slowpoke, in turn, helps Shellder go on dry land.
And if you know, if the Shellder is lost, Slowbro devolves into Slowpoke.
But look at Shellder. It's small, and only has a tongue. If it evolved like that, Slowpoke could stay in that form for FIVE SECONDS.
But Slowbro's Shellder has fangs, chomping firmly onto Slowbro. I believe...
Shellder transforms so he doesn't lose Slowbro.