we know plankton and krabs have been playing poker together for 15 years

we also know this episode aired before the episode where pearl turned 16

while the non-continuity & non-chronological order of the series means that assuming that pearl was 15 in welcome to the chum bucket is a fool’s gamble - it’s reasonable enough that if there is an episode about her turning 16, her character was likely conceptualized as being 15 years old prior to that point. but either she was 15 or she was 16 in welcome to the chum bucket … and either way, that’s about as long as pearl’s been alive.

we also know that pearl is krabs’ biological daughter … through a combination of facts … and were given reason to believe that something happened to the mother of his child shortly after pearl’s birth that made him depressed. she’s not around anymore and no longer apart of their lives in any way shape or form.

we also know plankton and krabs were childhood best friends, going on to have an on-again-off-again friendship for years prior to spongebob getting a job at the krusty krab … and the two are shown to occasionally have moments where they truly, genuinely care about each other deep down, despite the rivalry …

conclusion: plankton might have started playing poker with krabs to cheer him up after the death of his wife