I always wanted to answer this question.

She's smaller than a normal whale and doesn't seem to need air. Draw your own conclusions.

She cant be adopted because in 'Whale of a birthday' Mr.Krabs hung up a banner he had since she was born. IT read; It's a boy" but he crossed out boy and wrote girl to save money.

but how can a krab give birth to a whale

Easy therory: Mr. Krabs was married to a whale. Anyway, it's a CARTOON.

oh that makes pertect secse

With help from gogetenkusu's idea, I have made a theory(even though it's not important)!

Mr. Krabs married a whale and something happened to her. But they had a baby. Like the mother, it was a whale. But since the father is a krab, it was smaller and was able to breath through the water.

Thank you gogetenkusu!

i can imagin it now

mrs. krabs - honey im pregnant

mr.krabs - is that cost anything?

Spongebob is a cartoon so anything can happen. Even a Krab giving birth to a whale.

yeah.

yeah.

Yes. Mr. Krabs does not love Pearl very much, so he just adopted Pearl.

Yeah I agree that Pearl was born into Mr. Krabs' Family. After Mr. Krabs fought in a war(maybe it was The Battle Of Bikini Bottom) he was in hard times but then managed to get the Rusty Krab which soon advanced into the Krusty Krab which changed the retirement home to a restaurant thus moving the retirement home into what possibly became Shady Shoals. After becoming the krabby patty successor he came across a beautiful whale and they soon became interested in each other and formed the love chain which resulted them getting married and having a baby. Though the connection to a crab and a whale was likely to be the cause of Pearl looking a little on the small side and also caused an effect for her to stay in the water completely without having to come up for air like what regular whales do by periodically diving out of the water for air and shooting water out of their hump-hole then diving back into the water. Though something must have happened to the mother since they never showed her in the whole SpongeBob story so far. That seems to be one of the connections that is still left out of this story and events. For information about Krabs fighting in the war look into The Krusty Krab Training Video and The Krusty Sponge. So if Krabs married a whale they both must have thought that even though they were two totally different sea creatures and species they agreed that they were made for each other. In conclusion that shows that a story being told about the mother and Krabs fighting in a war with a possible connection to The Battle Of Bikini Bottom can both be episodes that can be aired and thought out in the future.