All jellyfish belong to the animal phylum Cnidaria, which includes sea anemones, corals, and other organisms. Their defining feature is the possession of cnidocytes, specialized cells capable of delivering stinging attacks.
Within the vast Cnidaria family, there are over 10,000 species, with approximately 4,000 belonging to Medusozoa, a group encompassing all creatures commonly recognized as jellyfish. Jellyfish, specifically, refer to the adult medusa stage in their life cycle. Among these 4,000 medusozoans, they can be categorized into four distinct groups.

Scyphozoa represents the most recognizable jellyfish, including the larger and more vibrant varieties that often interact with humans. They are commonly referred to as "true jellyfish" due to their prominence. Scyphozoans typically exist in their medusa form and consist of at least 200 species.

Cubozoa, known as box jellyfish, derive their name from their box-shaped bells. Some cubozoans, like the sea wasps (found in the genus Chironex), possess some of the most potent venoms known to man. Additionally, cubozoan jellyfish display a more advanced nervous system compared to other jellyfish, featuring complex eyes with lenses, corneas, and retinas. Some even engage in intricate courtship behaviors, unusual for jellyfish. Approximately 48 known species inhabit primarily warmer waters. In 2011, a new species named Tamoya ohboya was discovered through a public naming contest by jellyfish expert Allen Collins and colleagues at the Smithsonian.

Staurozoa, or stalked jellyfish, differ from their counterparts as they do not swim freely but instead attach themselves to rocks or seaweed. With their trumpet-like shape, they primarily inhabit cold waters. Around 50 species of staurozoans exist, renowned for their distinctive blend of beauty and camouflage.

Hydrozoa comprises the largest group of jellyfish, with an estimated 3,800 species. The medusa stages of this group are often inconspicuous and small, while the bottom-dwelling polyps, known as hydroids, form extensive colonies. Notably, many hydrozoan species skip the jellyfish stage entirely and reproduce through gametes in their polyp or hydroid stage. In 2016, researchers discovered what they believe to be a new hydrozoan species of Crossota in the Mariana Trench, located 12,140 feet (3,700 meters) below the ocean surface. This unique Crossota jellyfish, resembling a glowing spaceship, defies the typical hydrozoan life cycle by spending most of its life as a large medusa. Hydrozoa encompasses approximately 3,700 species.

Siphonophores form colonial organisms composed of specialized individuals called zooids. These zooids originate from a single fertilized egg, rendering them genetically identical. Each zooid fulfills a specific role, such as swimming, feeding, prey capture, or gamete production.

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