StingingCnidaria

Cnidarians are diverse marine invertebrates, common residents of reef environments from shallow intertidal zones to depths of several hundred metres. All cnidarians possess specialized stinging cells, cnidocytes - named for the Greek 'knidē' (nettle) - primarily on their tentacles. These cells eject microscopic harpoon-like structures (nematocysts) that inject venom for prey capture and defence, capable of causing reactions from mild skin irritation to severe envenomation in humans. Their body plan is typically radial, with a central mouth surrounded by tentacles. Body forms vary between free-swimming medusae (jellyfish), with bell-shaped bodies less than 1 cm (0.4 in) to over 2 metres (6.6 ft) in diameter, and sessile polyps, such as corals and anemones, which attach to hard substrates and range from a few millimetres to over 1 metre (3.3 ft) across.

This group includes jellyfish, sea anemones, corals, and hydroids, commonly found on hard substrates, soft bottoms, and within the water column. Most cnidarians are suspension feeders or predators, capturing plankton, small fish, or other invertebrates using their stinging tentacles. Many corals and anemones host symbiotic algae (zooxanthellae) within their tissues, which contribute significantly to their varied colouration and nutrient acquisition, especially in sunlit shallow waters.