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The special effects budget was $50,000.
Universal Studios attempted to sue New World for spoofing Jaws (1975) , but Steven Spielberg saw the movie in advance and loved it. After that, Universal dropped the lawsuit.
While the two main characters are exploring the cluttered lab, a small two-legged humanoid lizard creature skitters across a countertop without their noticing. The stop-motion monster was an homage to Ray Harryhausen, and it was Joe Dante's hope to bring the creature back a few times throughout the film, growing bigger each time. Originally, he even hoped to end the film with a giant version of the creature attacking a pier. Unfortunately, they didn't have near the budget they would have needed to pull that off, so this remains its one and only cameo as Dante had to focus on the rubber fish.
Director Joe Dante once said that he and collaborator/B-movie producer Roger Corman would submit this and other movies to the MPAA, make the suggested cuts to avoid an X-rating, and then reinsert them before the films were distributed to theaters since the MPAA didn't follow up once it had assigned its final rating.
Steven Spielberg described this film as "the best of the Jaws (1975) rip-offs." He and Joe Dante later collaborated on Twilight Zone: The Movie (1983). Richard Dreyfuss, who played Hooper in Jaws, later had a cameo in the opening scene of the 2010 remake (Piranha 3D (2010)).
The piranha were done by attaching rubber puppet fish to sticks.