I'm with JayZS on this one - chub eat slugs! Simples. If they thought they were mussels wouldn't they spit them out having realised the terrible mistake?

I've observed chub taking slugs from the trailing marginal plants on a rising river, and at night you can hear the slurping noises as they feed on them. When using them on a low clear river as a stalking bait i'm sure it's just the fact that they make a nice loud plop as they hit the surface that attracts the chub. Many other baits would produce the same result it's just that the humble slug is readily available on the banks so you don't have to lug bait boxes about, has enough casting weight to be freelined and best of all - they're free!

Haven't caught a barbel on one yet though . . .