There is a sapient jellyfish species resembled the box jellyfish found to be lurking in the ocean today, a team of researchers took a sub and dived into the abyss to look for it. What kind of non-invasive tests (in-situ) can be performed on this jellyfish to see if it is sapient? Don't expect it to talk back or open a jar! However incomplete fossil record shows that they could evolve some sort of neuron cells similar to those found in human and current theory suggest they could be extremely intelligence.