The director sat alongside stars Zoe Saldaña, Sam Worthington, Sigourney Weaver and others in Japan to promote the much-anticipated sequel -- he kicked off the press conference by saying, "Welcome to Pandora" ... and then the controversial show began.

The dolphins in a tank performed tricks with the help of trainers ... and as the conference continued, Cameron mentioned how much he loved a dolphin's intelligence and sociability ... before joking, "I'm sure everybody asked their permission to be in the show."

That joke did not land with animal activist groups like PETA -- because its spokesperson told Plant Based News, "Why on Earth didn't [Cameron] pause for just five minutes to consider whether he should have allowed himself to be seen as endorsing the cruelty of marine parks?"

Shockingly, Cameron's been an animal advocate for years ... he says he's been a vegan for a decade, and even produced a documentary alongside the director of the 2009 flick, "The Cove," the Oscar-winning animal rights film that exposed the cruelty of dolphin hunting in Japan, of all places.

Even "The Cove" director, Richard O'Barry, is calling out Cameron for his hypocrisy in using captive dolphins to promote a film whose central plot is about conserving ocean resources.