That awkward moment when you pop outside to show your date your pet goldfish only to discover that "Cleo" is being devoured by a fish-eating spider ...

Field guide Jérémy Schalkwijk catalogued Cleo's final moments in February last year in Barberton, South Africa. The arachno-assassin responsible for dragging the fish from its pond and hauling it up a near-vertical rock is known as a nursery web spider. And fish murdering is kind of their thing.

The semiaquatic arachnids belong to a family of spiders called the Pisauridae and they are known for their ability to stroll on still water and even dip beneath the surface to escape predators. They are most impressive, however, when on the hunt.

The spiders will rest their front legs on the surface of a still pond or stream while their hind legs remain anchored to a rock or plant near the water's edge. Any disruption to the water's surface caused, for example, by a fish or an unfortunate bug that has fallen in, triggers the nursery web spider into action.

The arachnid nabs its prey using strong front legs and quickly injects it with a deadly neurotoxic venom. The toxins not only immobilise the prey, but also turn its insides into a nutritious soup, which the spider can consume once on land.

While the sight of a spider tucking into some fresh sashimi may seem unusual, fish-munching spiders are more common than you might think. A study in 2014 revealed that spiders in eight of the world's 109 arachnid families are able to catch and eat fish. And some of them make off with some very hefty meals.

While Schalkwijk, who is familiar with fishing spiders, was startled by the size of this arachnid's meal, it's actually below par. According to the results of a survey sample analysed in the 2014 study, some fishing spiders can capture prey that is five times their own size. However, the average captured fish is around 2.2 times as long as the spider responsible for ensnaring it, which makes this fishing spider's quarry fairly small.

Shortly after Schalkwijk snapped some photos, the spider hauled its prey out of sight to dine in peace. "It was a crazy, crazy sight," he told The Sun. "You don't see that every day."

Header image: Charles J Sharp/Sharp Photography (cropped and edited lightly)