Say it ain't so ... there's no Muskrat Love left for Captain & Tennille ... they're getting divorced ... and the Captain tells TMZ he didn't see it coming.

Toni (Tennille) filed legal docs in Arizona, claiming the marriage was "irretrievably broken." They've been married for 39 years.

If you don't know ... you should. They had a string of hits in the 70s, including "Love Will Keep Us Together," "Muskrat Love" and "Do that to Me One More Time."

They were discovered by Neil Sedaka at The SmokeHouse restaurant in Burbank, near NBC.

Daryl Dragon (The Captain) is 71. Yeah, it's Dragon.

Daryl tells TMZ ... "I don't know why she filed. I gotta figure it out for myself first." He also said they're still living together in the same house.

According to the divorce docs, obtained by TMZ, there's special mention about health insurance coverage, and that seems relevant, because in 2009 Toni blogged that Daryl had been diagnosed with Parkinson's. In 2011 Toni updated the message, saying Daryl's tremors were so bad he was embarrassed to go out in public ... to restaurants, the movies, etc.

Here's what's unclear -- whether it's possible Toni really filed for health insurance reasons ... whether his coverage might be better if they were divorced.

The blog has an update on Jan 16, 2014, saying "The Captain & Tennille appeared to the public as them being the ideal model for a 'rock-solid' married pair. But almost all people naturally evolve over time, & sometimes hidden feelings start to be uncovered ..."

It's a sad day for muskrats.