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So in all seriousness - what on earth in Shellder's biology causes this when it comes into contact with a Slowpoke?? Like it happens with Slowking too so?????????????????????????????????????????????????
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Runewuff
not much info is given, however, the Pokedex entries in some generations say the Shellder's venom on Slowking's head raises his intelligence somehow? (ewww, thanks Bulbapedia!)
I think they were just inspired by parasitic fish like ramoras and such.