Short answers, pulled from the story.
Brittle stars are echinoderms in the class Ophiuroidea, closely related to starfish but distinct in that their central disk is sharply marked off from the arms and all internal organs remain confined to the disk. In starfish, digestive and reproductive organs extend into the arms; in brittle stars they do not. Brittle stars also move by wriggling their flexible arms rather than using tube-foot suction.
Over 2,000 species of brittle stars are known today, spread across around 270 genera and 16 families. More than 1,200 of those species live in deep waters below 200 metres. Brittle stars are the most abundant group of living echinoderms, outnumbering sea stars.
Most brittle star species reproduce sexually, releasing eggs and sperm into the water for external fertilisation. Many brood larvae inside internal sacs called bursae, and a few species including Amphipholus squamata are viviparous, nourishing embryos through the bursa wall. Some species, notably the West Indian brittle star Ophiocomella ophiactoides, reproduce primarily by splitting the disk in two, a process called fissiparity, which can yield up to 15 new individuals per year.
Brittle stars can readily regenerate lost arms or arm segments as long as at least one arm remains attached. They deliberately discard arms to escape predators, in a strategy similar to lizard tail autotomy. Members of the family Amphiuridae can also regrow gut and gonad fragments lost along with the arms.
Over 60 species of brittle stars are known to produce light. Most emit green wavelengths, though a few blue-emitting species have also been identified. Both shallow-water and deep-sea species are bioluminescent, and the light is thought to deter predators.
Brittle stars are found in all major marine provinces, from polar regions to the tropics. Shallow species hide among sponges, coral, rocks, and sand from the low-tide line downward, while deep-water species extend to abyssal depths beyond 6,000 metres. More than 1,200 of the known species live below 200 metres.