Primus is in the market for a new drummer ...

because Tim Alexander abruptly quit the hard rock group ...

informing his bandmates via electronic communications. The band announced Tuesday that Tim quit Primus "effective immediately" ..

. after firing off an email to founding members Les Claypool and Larry LaLonde . Primus says they were blindsided when Tim decided to quit .

.. and, the band says he went radio silent after dropping out, aside from one response where he told them he had "lost his passion for playing.

" Tim joined Primus way back in 1989 ...

though he took a 7-year hiatus in 1996 before returning. The band's biggest hit was the 1993 classic "My Name is Mud" ..

. and Primus recorded the OG theme song for "South Park." Les and Larry say they're disappointed to lose Tim but they respect his choice .

.. and, the band says it's still planning to take the stage Dec.

31 for a New Year's Eve gig at the Fox Theater in Oakland, without Tim of course. Tim had open heart surgery back in 2014 ..

. but it doesn't sound like health is a factor in his departure..