Get on a winning streak in Las Vegas and the mokes crowd around and act like they're the lucky ones.

So it goes one morning in the backstage depths of Shark Reef, the splashy aquarium exhibit that draws a million-plus visitors a year to Mandalay Bay, the tropical-themed casino resort on the south end of the Vegas Strip. Here, inside a clear 20-gallon tank, two little fish get lucky in the lewd sense. The male, an inch long, with iridescent blue patterning and flouncy, dark-edged fins, dogs a less flashy female that now and again lets him squeeze close and vibrate.

Each look at fish hey-hey increases the joy and fulfillment of four human

onlookers—Cynthia Martinez, assistant field supervisor for the U.S. Fish and Wildlife Service in southern Nevada; Jack Jewell, general curator of Shark Reef; Gordon Absher, vice president of public affairs of Mandalay Bay's parent company, MGM Mirage; and me.

Jewell provides the play-by-play. "They're getting ready now," he says. "They do a little shimmer, that's it . . . They'll repeat this many times." The curator is wiry and very charged up about fish husbandry, a field in which he's famous for his work with captive sharks. He has a grand-slam moment when our little fish get it on next to the side of the tank and a whitish speck appears and drifts downward—a fertilized egg.

"I can't believe it," he says. "You just got to see a Devils Hole pupfish egg! Let me tell you right now, that is a momentous occasion. Almost nobody gets to see that!"

Big grins all around. Cynthia seems especially pleased—which makes sense, because she's one of the principals on a multi-agency team trying to ensure the continued existence of Cyprinodon diabolis. The pupfish, so named because they chase one another around like playful puppies, lives naturally in only one place, a bottomless pool in a bizarre fissure in the earth in a detached chip of Death Valley National Park in southwestern Nevada. The surface area of the pool is 318 square feet—less than a standard room at Mandalay Bay—giving plausibility to the claim that C. diabolis has the smallest habitat of any vertebrate in the world. It also occupies one of the worst fish habitats anywhere, with oxygen-poor water geothermally heated to 93 degrees, and a starvation diet based on algae and microorganisms. A trout would go belly up in minutes.

Rare and at risk to begin with, the species is inches from extinction, due to a mysterious population drop in the home habitat, made worse by an accidental massacre perpetrated two years ago by researchers. (More on that later. It's so awful to recall that Cynthia tells the story in fragments, as if every word hurts.) Meanwhile, disasters have wiped out three artificial refuges designed to maintain reserve populations off-site. In September, when numbers usually peak, adult fish in Devils Hole numbered just 85—well below the 400 biologists would be comfortable with. Figure in naturally low fertility, a life span of about 11 months, and a dearth of young surviving to adulthood and the picture looks troubling indeed for the fish and for those who care deeply about them, which seems to include everybody with any connection.