Short answers, pulled from the story.
Cnidaria is a phylum of aquatic invertebrates containing over 11,000 species, found in both freshwater and marine environments. The group includes jellyfish, sea anemones, corals, hydroids, and some of the smallest marine parasites. They range in size from microscopic parasitic myxozoans to the lion's mane jellyfish, which can exceed 2 m in diameter and 75 m in length.
A cnidocyte is a specialized stinging cell unique to cnidarians that fires a harpoon-like structure to capture prey or defend against threats. It can extend its thread in as little as 2 milliseconds at pressures as high as 140 atmospheres, similar to a scuba air tank. Cnidocytes can only fire once, and replacing them takes about 48 hours.
Chironex fleckeri, known as the sea wasp, has been described as the world's most venomous jellyfish and has been held responsible for 67 deaths. It is a box jellyfish and is nearly transparent, making it difficult to identify. Despite its reputation, most stingings by Chironex fleckeri cause only mild symptoms.
The earliest widely accepted animal fossils are cnidarians from the Ediacaran period, before the Cambrian Explosion. Molecular clock analysis of mitochondrial genes estimates the crown group of cnidarians to be almost 200 million years older than the Cambrian period. Auroralumina, a British Ediacaran fossil, is the earliest known animal predator.
Cnidarians can reproduce both sexually and asexually. Many species cycle through an asexual sessile polyp stage followed by a sexual free-swimming medusa stage, a process called metagenesis. Mass spawning events, such as those at the Great Barrier Reef where at least 110 coral species spawn simultaneously, can produce enough gametes to turn the water cloudy.
Large jellyfish of the order Rhizostomeae are commonly consumed in Japan, Korea, and Southeast Asia. The fishing season is restricted to daylight hours and calm conditions in two windows, from March to May and August to November. Jellyfish is very low in cholesterol and sugars, though poor preparation can introduce heavy metals.