scallops can gallop

Fun fact: Stephen Hillenburg, the creator of Spongebob, majored in marine biology and worked as staff artist in the Ocean Institute

So were shell blades in Ocarina of Time!

Basically if you wiggle in a fluid you push that fluid around you. The scallop moved via muscles it has, and shoves water down and away from its destination, which pushes it towards it.

It moves by opening and closing rapidly, to suck in and force out water.

Another way to say that is "You don't have to be graceful to swim", and scallops certainly fit that bill.