Jellyfish

SizeUnknown

LifespanA few weeks to a year or more

StatusNot Evaluated

About

Jellyfish

Jellyfish, also known as sea jellies, are gelatinous marine invertebrates belonging to the subphylum Medusozoa within the phylum Cnidaria. Possessing radial symmetry, their bodies consist of an umbrella-shaped main bell composed of mesoglea—a gelatinous tissue made up of 95 to 98 percent water—and trailing tentacles. Lacking a brain, heart, bones, blood, head, or specialized respiratory organs, jellyfish rely on a decentralized nerve net to interact with their environment. Specialized sensory lobes called rhopalia, located around the rim of the bell, contain statocysts, sensory pits, and occasionally ocelli to detect light, temperature, and chemical changes in the water. Locomotion is generated through rhythmic muscle contractions of the umbrella-shaped bell, which propels the medusa through open water. While most species drift passively with ocean currents, box jellyfish (Cubozoa) are more active hunters capable of swimming at speeds up to five centimetres per second. Many jellyfish exhibit striking bioluminescence; for example, the deep-sea species Atolla wyvillei uses light flashes as a burglar alarm to attract larger predators to attack its aggressors. Others produce green fluorescent proteins, which have become vital tools in modern scientific research. The life cycle of true jellyfish (class Scyphozoa) typically alternates between two stages. The free-swimming medusa is the sexually reproducing phase, releasing mature gametes from the stomach lining through the mouth. Fertilised eggs develop into planula larvae, which drift before attaching to hard surfaces on the ocean floor as sessile polyps called scyphistomae. These polyps can survive for much longer periods than adult medusae, reproducing asexually via budding or strobilation to produce new microscopic medusae. For scuba divers, encountering these primitive creatures offers a fascinating look into an ancient lineage that has existed for over 500 million years, predating the dinosaurs. Distributed throughout the world's oceans from coastal shallows to abyssal depths, sea jellies play a vital ecological role as both predators of small organisms and prey for leatherback sea turtles, ocean sunfish, whale sharks, and grey triggerfish. Under favourable conditions, populations can rapidly swell into massive swarms known as smacks.

Size

Unknown

Weight

A few grams to over 200 kg

Lifespan

A few weeks to a year or more

Status

Not Evaluated

Where to Find Them

Best Diving Locations

Diving Tips

Encountering jellyfish underwater requires divers to maintain strong spatial awareness and proper buoyancy control. Because most sea jellies are weak swimmers that move via pulsating bell contractions, their trajectory is largely dictated by ocean currents. Divers should maintain a safe distance and remain vigilant downcurrent from blooms, as trailing tentacles armed with stinging cells called nematocysts can extend well beyond the visible bell and drift invisibly in the water column. Stings from jellyfish vary from mild discomfort to severe injury or death. Cubozoans, or box jellyfish, are among the deadliest venomous animals on Earth and require extreme caution in regions where they occur. Wearing full exposure protection, such as a wetsuit or dive skin, provides a physical barrier against accidental contact with nematocysts. Divers should execute deliberate, controlled fin kicks to avoid creating water turbulence that could push fragile tentacles toward exposed skin. Additionally, divers exploring rocky substrates or macro environments should avoid disturbing sessile polyps or stalked jellyfish (Staurozoa) attached to seaweed and rocks.

Best Season

The presence of free-swimming jellyfish in coastal waters is strongly seasonal, primarily tied to water temperature and life-cycle dynamics. In many regions, the pelagic medusa stage of true jellyfish (Scyphozoa) is most evident during the warm summer months. As water temperatures rise, environmental conditions stimulate bottom-dwelling polyps to undergo strobilation, releasing large numbers of juvenile medusae into the upper water column. Because the medusa phase of most jellyfish species grows rapidly and matures within a few months, adult populations peak swiftly before dying off soon after breeding. As a result, divers are most likely to observe large numbers or swarms ('smacks') in coastal surface waters during late spring through late summer. In contrast, the sedentary polyp stage remains attached to the seabed year-round, while specialized deep-sea medusae and stalked jellyfish (Staurozoa) can be encountered across different seasons depending on local oceanic currents and depth zones.

Habitat

Jellyfish are found throughout the world's oceans, spanning diverse marine environments from warm equatorial reefs to polar waters. Scyphozoans, or true jellyfish, are exclusively marine organisms occupying every depth zone, from surface coastal waters to the deep ocean. While most species are planktonic and spend their adult lives drifting through open water, stalked jellyfish (Staurozoa) live anchored to rocks and seaweed in shallow intertidal zones, with some species recorded at depths greater than 3,000 metres. The dual life cycle of jellyfish involves both benthic and pelagic habitats. The polyp stage requires hard surfaces on the seabed, such as reef structures, rocks, or submerged vegetation, to attach and reproduce asexually. Once released, the medusa stage inhabits the open water column, where bell diameters typically range from 2 to 40 centimetres, though large species like the lion's mane jellyfish (Cyanea capillata) can reach 2 metres across. Favourable environmental conditions can cause massive aggregations in coastal bays and estuaries.

Diet

Jellyfish are carnivorous predators and opportunistic feeders that consume a wide range of marine organisms. Their diet primarily consists of small fish, fish larvae, shrimp, and tiny planktonic crustaceans such as krill, copepods, and amphipods, alongside microscopic plants like algae. Some species are cannibalistic, preying on other sea jellies, while others opportunistically feed on organic scraps discarded by larger ocean predators such as sharks and sea turtles. To capture prey, jellyfish use stinging cells called nematocysts, which coat their trailing tentacles, bell margins, and oral arms. Upon contact, these cells immobilise or kill prey before the oral arms waft the food toward the central mouth and into the stomach. The digestive system features interconnected radial canals lined with digestive enzyme-secreting cells and additional nematocysts. Certain species, such as filter-feeding rhizostome jellyfish, lack long marginal tentacles and instead strain microscopic plankton from the water column using specialized mouth openings along their oral arms.

Did You Know?

Fun Facts

A group of jellyfish is officially referred to as a 'smack' or a 'smuck'.

Jellyfish are composed of up to 98% water and lack a brain, heart, bones, or blood.

Having existed for at least 500 million years, jellyfish are older than the dinosaurs.

The lion's mane jellyfish (Cyanea capillata) is the largest species, reaching up to 2 metres across.

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