Mrs. Krabs was or is the mother of Pearl Krabs and the wife of Eugene Krabs. It is unknown if she is deceased or if she is divorced from Eugene. In a book, Mr. Krabs states that Pearl takes after her mother, suggesting that Mrs. Krabs is a whale. It is possible that Mrs. Krabs is Pearl's adoptive mother, but does not have custody over her. It is highly unlikely that she will ever make a physical appearance on the show. Pearl's lack of a mother is a reference to the phrase "mother of Pearl," which Mr. Krabs says when something surprises him. Stephen Hillenburg is against revealing the identity of Mrs. Krabs and has kept it a secret throughout the series.
The exclamation "mother of Pearl!" is used in the following episodes, usually by Mr. Krabs.