Not many animals are known to eat sea cucumbers. There is a good reason. Most of them have chemicals in their body, which probably doesn't taste too good but more importantly may intoxicate the potential predators.
Humans have found a way to deal with these toxins. In the preparation of the bêche-de-mer, sea cucumbers are boiled for a long time which breaks down these chemicals.
If toxicity is a good way to avoid generalist predators, other animals have evolved to become specialists and can deal with the cocktail of toxins found in the skin and organs of sea cucumbers. In coral reefs, among the specialist predators, Tonna perdix is known to feed regularly on sea cucumbers, and in particular, on the species of the genus Stichopus.
After finding a Tonna perdix during a reef walk on Heron Island, I decided to keep it in a tank hoping to observe its feeding behavior. A few days later, Rob brought me back some Stichopus (that I can't identify to the species level) and decided to put it in the tank with the Tonna perdix. Before the Stichopus even touched the bottom of the tank, the Tonna perdix became really active. After a few minutes crawling around the tank, Tonna perdix used its proboscis to detach its prey from the wall of the tank, and in just a few seconds, the gastropod extended its proboscis around the sea cucumber swallowing it whole. Holly Heiniger had her camera with her to record this.
Sea cucumbers have also evolved ways to escape predation. In particular, Stichopus can shed its body wall to only leave pieces of it to the predator. However, in this case, the attack was so fast that the sea cucumber didn't seem to have any time to escape.