Brittlestar and seasquirts. Dark red-spined brittlestar (Ophiothrix purpurea, feather-like) and green barrel sea squirts (Didemnum molle) on coral. Brittlestars are echinoderms and are related to starfish. Ophiothrix sp. brittlestars are found worldwide, from tropics to Arctic and Antarctic regions. Green barrel sea squirts are native to the Red Sea and the tropical waters of the Indo-Pacific region. Photographed in Tulamben, Bali, Indonesia.