Jellyfish, ancient inhabitants of the world’s oceans for at least 500 million years, represent a wide array of species. The question of how many types exist is not straightforward, delving into the classifications of marine life. Many of these gelatinous animals remain undiscovered in the vastness of our planet’s waters.
Understanding What a Jellyfish Is
True jellyfish are marine invertebrates belonging to the subphylum Medusozoa within the phylum Cnidaria. Their bodies are predominantly composed of water, around 95%, giving them their translucent appearance. A defining feature is their umbrella-shaped bell, from which tentacles extend downwards. These tentacles are equipped with specialized stinging cells called cnidocytes, which contain nematocysts used for capturing prey and defense. Jellyfish exhibit radial symmetry, lacking a brain, heart, or specialized respiratory and circulatory systems, instead relying on a simple nerve net for sensory perception and coordination.
The Main Branches of Jellyfish Diversity
The diversity of true jellyfish is categorized into four main classes, each with distinct characteristics.
Scyphozoa
Scyphozoa, often referred to as “true jellyfish,” includes many of the larger, more familiar species, characterized by their prominent bell and long oral arms. There are approximately 200 known species in this exclusively marine class.
Cubozoa
Cubozoa, or box jellyfish, are distinguished by their cube-shaped bell and possess highly potent venom, along with more advanced sensory capabilities, including complex eyes with lenses and retinas. This group comprises around 36 to 49 species.
Staurozoa
Staurozoa, known as stalked jellyfish, are unique because they are typically sessile, attaching themselves to surfaces with a stalk. These small jellyfish, usually measuring 1 to 4 centimeters, have a trumpet or goblet shape and are primarily found in colder, shallow waters, with about 50 recognized species.
Hydrozoa
The medusa forms of Hydrozoa represent a highly diverse group, with many species being smaller than other jellyfish. While some hydrozoans live in freshwater, most are marine and often feature a velum, a muscular flap inside the bell margin that aids in propulsion. This class contains roughly 3,700 species.
The Ongoing Discovery of New Species
Determining an exact number of jellyfish species presents a continuous challenge due to the vastness and inaccessibility of ocean environments. Deep-sea exploration often reveals new forms, and many species are delicate, making their collection and study difficult. New discoveries are regularly made, such as a deep-sea jellyfish found in a volcanic caldera or a new box jellyfish with 24 eyes identified in Chinese waters. Researchers continue to identify previously unknown species, with thousands more likely waiting to be discovered across the global oceans, particularly in unexplored deep-sea habitats.
Not All Jellies Are Jellyfish
Many gelatinous marine animals are colloquially called “jellies,” but not all are true jellyfish.
Comb Jellies
Comb jellies, belonging to the phylum Ctenophora, are distinct from true jellyfish (Cnidaria). They move using rows of comb-like cilia and lack stinging cells, instead often employing sticky cells to capture prey.
Salps
Salps are another group of transparent, barrel-shaped marine animals that propel themselves by contracting muscular bands. These filter feeders are part of the phylum Tunicata, making them more closely related to vertebrates than to jellyfish.
Siphonophores
Siphonophores, such as the Portuguese Man O’ War, are frequently mistaken for jellyfish. However, a siphonophore is not a single animal but rather a colony of highly specialized individual organisms called zooids, which work together to float, capture prey, feed, and reproduce.