Joe Dante's first solo feature, scripted by John Sayles, is a textbook example of how to get the most out of a Roger Corman budget and a shoddy premise. The effects money extends to a couple of impressive mini-monsters and some nastily chewed corpses but is otherwise so meagre that few mutant fish get screen time and have to be represented by bloodily churning waters.
However, Dante and Sayles fill in between monster attacks, traditionally the dull stuff, with cynical dialogue, visual invention and amusing tributes to old-time movies. One reason Dante's films are so engaging, often despite their nastiness (this is a rare horror film in which children are mutilated and killed wholesale), is that he fills them with performers he clearly loves to see work.
Among the friendly faces are Kevin McCarthy and Barbara Steele ('fish genetics is a very small field') as mad scientists, Keenan Wynn as a garrulous old-timer who tells jokes to his dog, Paul Bartel as a comically tyranncal summer camp commandant always clamping down on signs of fun but who nevertheless has a moment of real heroism during the piranha attack and Dick Miller as the crass tycoon ('Ralph the Swimming Swine is a national institution') whose party is spoiled by a minion who discreetly coughs 'the piranha ... they're eating the guests, sir'.
Sayles is clearly laughing at America in his premise (the fish were bred to be used in Vietnam) and the finale (when all else fails, the hero releases pollution into the river on the grounds that it killed everything else). The low-grade sequel, known as Piranha II: Flying Killers or Piranha, Part 2: The Spawning, was the directorial debut of James Cameron.