Slithering slugs that look like Day-Glo jewelry, paper-thin flatworms floating and billowing like silk scarves, swarms of jellyfish, crocodile fish, scorpion fish — these are the tiny creatures of the sea that depend on coral reefs for survival. In the new nature documentary “The Last Reef 3D,” they squiggle and swim in front of your face, daring you to grasp at them. But the film wants to do more than impress with its underwater photography and 3-D effects. It’s a cautionary tale about the fragility of the reefs, but the film could use a little more urgency in making its case.

It begins with a bang — literally: footage of a nuclear blast on the Marshall Islands’ Bikini Atoll, where the United States tested nuclear weapons from 1946 to ’58. The fallout destroyed the undersea reefs. But, left alone for years, those reefs came back, we’re told by the narrator, in a testament to their resiliency.