The blue jellyfish (Cyanea lamarckii) is a strikingly beautiful medusa found across the northeastern Atlantic and North Sea, recognizable by its vivid blue or violet coloration and long, trailing tentacles. Smaller and less dangerous than its close relative the lion’s mane jellyfish (Cyanea capillata), the blue jellyfish is nonetheless a capable predator and a common sight on British, Irish, and Scandinavian coastlines — particularly during summer blooms.
What Does the Blue Jellyfish Look Like?
The blue jellyfish is one of the most visually distinctive jellyfish in European waters. Its identifying features:
- Bell diameter: Typically 15–30 cm, though large individuals can reach 45 cm
- Color: The bell ranges from pale blue to deep violet-blue, sometimes with yellowish-brown fringes at the bell margin. Color intensity varies with age and the individual — juveniles often show brighter, more saturated blue tones.
- Tentacles: Eight clusters of long, fine tentacles trail from the bell margin. These can extend 1 meter or more in large adults and contain nematocysts (stinging cells).
- Oral arms: Four frilly, ribbon-like oral arms hang beneath the bell and assist with feeding
- Bell structure: The top of the bell is smooth and slightly domed; the underside contains the complex oral arms and tentacle clusters
Blue Jellyfish vs Lion’s Mane — How to Tell Them Apart
These two Cyanea species overlap in range and are frequently confused. Key differences:
- Color: Blue jellyfish are blue or violet. Lion’s mane jellyfish are typically red, orange, brown, or yellow — rarely blue.
- Size: Lion’s mane are significantly larger — bells commonly 50+ cm, occasionally over 1 meter in Arctic waters. Blue jellyfish rarely exceed 45 cm.
- Sting: Both sting, but lion’s mane stings are more potent and more likely to cause significant irritation.
- Range overlap: Both are common in UK, Irish, and North Sea waters. Lion’s mane prefer colder, northern waters; blue jellyfish are more common in warmer southern parts of the range.
Where Do Blue Jellyfish Live?
Blue jellyfish are found across the northeastern Atlantic from the western Mediterranean and Bay of Biscay north through the English Channel, Irish Sea, North Sea, and into Scandinavian coastal waters. They are most abundant in the open ocean and coastal waters of the British Isles — Ireland’s west coast and Scottish sea lochs are particularly productive areas.
They are pelagic animals, drifting with currents rather than actively navigating. Onshore winds concentrate them near coastlines, producing the blooms that wash them onto beaches. Blooms are most common from spring through autumn, peaking in summer when water temperatures are highest and prey (plankton, small fish) most abundant.
What Do Blue Jellyfish Eat?
Blue jellyfish are active predators of zooplankton and small fish. They feed by trailing their tentacles through the water — any contact with prey triggers the firing of nematocysts, which paralyze or kill small animals. The oral arms then move food toward the central mouth.
Prey includes:
- Copepods and other small crustaceans
- Fish larvae and juvenile fish
- Small medusae (other jellyfish species)
- Arrow worms and other gelatinous zooplankton
Despite lacking a brain, eyes, or centralized nervous system, blue jellyfish are effective predators — their simple nerve net detects light, gravity, and chemical signals that help them maintain position in the water column.
Does the Blue Jellyfish Sting?
Yes — blue jellyfish sting, though their sting is generally mild compared to lion’s mane or Portuguese man o’ war. Contact with the tentacles causes a burning or itching sensation in most people, typically lasting 20–60 minutes. In sensitive individuals, more significant irritation, redness, and localized swelling can occur.
The sting is rarely dangerous for healthy adults. Children, elderly individuals, and those with known jellyfish sensitivities should take more care. As with all jellyfish stings:
- Remove any visible tentacle fragments carefully — do not rub
- Rinse with seawater (not freshwater, which can trigger unfired nematocysts)
- Apply a cold pack for pain relief
- Seek medical attention if symptoms are severe or widespread
Interestingly, dead or beached blue jellyfish can still sting — nematocysts remain active for hours after the animal dies. Avoid handling stranded jellyfish with bare hands.
Blue Jellyfish Life Cycle
Like all scyphozoan jellyfish, blue jellyfish have a complex two-phase life cycle:
- Medusa phase (the jellyfish we see): The adult, sexual stage. Males release sperm into the water; females capture it and fertilize eggs internally. Fertilized eggs develop into free-swimming larvae.
- Polyp phase (scyphistoma): Larvae settle on a hard substrate and develop into a tiny polyp. The polyp reproduces asexually by budding off juvenile medusae (ephyrae) through a process called strobilation. Each polyp can produce dozens of ephyrae.
The medusa phase lasts approximately one year — most blue jellyfish do not survive winter. The polyp phase can persist for several years, generating new medusae each spring.
Key Facts
- Scientific name: Cyanea lamarckii
- Common names: Blue jellyfish, blue fire jellyfish
- Bell diameter: 15–45 cm
- Color: Blue to violet
- Range: Northeastern Atlantic, North Sea, Irish Sea
- Sting: Mild to moderate — rarely dangerous
- Diet: Zooplankton, small fish, other jellyfish
- Lifespan: ~1 year (medusa phase)
Frequently Asked Questions
Are blue jellyfish dangerous?
Blue jellyfish are not considered dangerous to healthy adults. Their sting causes temporary irritation but is not medically significant in most cases. They are far less potent than the Portuguese man o’ war or box jellyfish.
Why do blue jellyfish wash up on beaches?
Blue jellyfish are carried by currents and wind. Onshore winds push them toward coastlines, where shallow water and wave action can strand them on beaches. They have no way to actively swim against a current — they drift passively and are at the mercy of ocean conditions.
What eats blue jellyfish?
Ocean sunfish (mola mola), leatherback sea turtles, some seabirds, and larger jellyfish species all eat blue jellyfish. Some nudibranchs (sea slugs) also prey on jellyfish polyps. Despite appearing fragile, blue jellyfish are a significant food source for several ocean predators.
Are blue jellyfish becoming more common?
Jellyfish blooms generally — including blue jellyfish — appear to be increasing in frequency and intensity in many parts of the world. Contributing factors include ocean warming (which expands suitable habitat and extends the blooming season), overfishing of jellyfish predators, and coastal eutrophication (nutrient pollution that fuels the plankton jellyfish feed on). Whether blue jellyfish populations are genuinely increasing or just more frequently reported is debated among researchers.