in The Spongebob Squarepants Movie (2004), Mr Krabbs is shown adding "with cheese" to the menu at the Krusty Krab. it is a well known fact that cheese comes from milk. and milk only comes from mammals (specifically female mammals.) however, the only recurring female mammalian characters in Spongebob Squarepants are Pearl Krabs and Sandy Cheeks. and since milk is only produced post-childbirth- and there arent any more underwater squirrels to fuck Sandy, we must assume that Pearl is being used and abused by Mr Krabs to create cheese for his krabby pattys. this is most likely the reason Mr Krabs adopted her in the first place.