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Interesting Facts about Barrel Jellyfish
[Phylum: Cnidaria] [Class: Scyphozoa (true jellyfish)] [Order: Rhizostomeae] [Species: R. pulmo]
Despite its colossal size and menacing appearance, one particular species of sea jellies is the favourite food source for leatherback turtles.
This segment contains interesting facts about barrel jellyfish (Rhizostoma pulmo) including where they thrive best, what they eat, and how they reproduce.
Rhizostoma pulmo Habitats and Distribution
The superabundance of barrel jellyfish occurs most often in the coastal and shelf waters of temperate regions, and especially:
- North Eastern Atlantic Ocean (e.g. the North Sea, Irish Sea)
- The Mediterranean Sea (including the Adriatic)
- Black Sea (including the Sea of Azov in Eastern Europe)
- Western Atlantic (e.g. western and southern coastlines of Africa)
Higher water temperatures tend to increase the population blooms of these large sea jellies from late Spring to the end of summer.
Pro Tip: It is common to see swarms of different jellyfish species drifting around the southern and western shores of the British Isles. Still, the best months to see the UK's biggest jellyfish, the barrel jellyfish (sometimes called the dustbin-lid jellyfish or frilly-mouthed jellyfish), are the summer months (e.g. May to September).
Barrel Jellyfish Characteristics
The thick rubbery bell (head) is usually white or light blue with a purple or violet rim and domed (shaped like an umbrella).
Instead of tentacles, like most other jellyfish types, they have eight oral arms and sense organs that extend down from the bell. These finger-like structures contain the jellyfish's nematocysts (stinging cells) used to ensnare their prey.
They try to propel themselves in open oceans with pulses of the bell. But, movement is limited and they are weak at swimming. Hence, they drift into shallow marine coastlines with prevailing water currents and tidal swings.
Here's the thing:
The lion's mane jellyfish (Cyanea capillata) is the biggest of the species. However, the barrel jellyfish (Rhizostoma pulmo) is almost as big, measuring up to ninety (90) centimetres in diameter (3 feet) and weighing around thirty five (35) kilograms (77 pounds).
Despite their menacing appearance and enormous size, in fact barrel jellyfish are not highly poisonous for human skin. After direct contact, a mild irritation or burning sensation may occur. But it's generally considered to be non life-threatening.
Fun Fact: As part of a symbiotic relationship, it's commonplace to see juvenile fish taking refuge within the oral arms of the barrel jellyfish.
What Do Barrel Jellyfish Eat?
Even though the vast majority of jellyfishes are carnivorous, feeding on marine invertebrates and microscopic zooplankton, the barrel jellyfish is a planktivore.
After a victim has been immobilised with the poison, they use their oral arms to funnel the meal into their mouth.
The typical daily diet of a jellyfish usually includes:
- Amphipods (known as scuds or side-swimmers)
- Copepods
- Crabs
- Fish larvae (including eggs)
- Krill
- Planktonic organisms
- Shrimp larvae
Barrel Jellyfish Reproduction
The complex life cycle of jellyfish involves two forms of sexual reproduction. It often includes two stages, the fixed-position "sessile" form (also known as the polyp form) and the adult or "medusa" stage.
Asexual Reproduction (Polyp Stage)
Some polyps reproduce asexually - either through a process of budding or by dividing itself into smaller segments (known as strobilation).
- Budding polyps can produce new polyps that can either remain attached by the stem or they can completely detach and form new individuals.
- In strobilation, the polyp segments into ephyrae - which are disc-like sections that form immature medusae. They eventually detach themselves and develop into adult medusae to complete the life cycle.
Sexual Reproduction (Medusa Stage)
Depending on the species, there are some adult jellyfish that release sperm and eggs into the water column. This process means fertilisation can either occur internally inside the female jellyfish or externally in the water.
Once fertilised, the tiny eggs start to develop into a free-swimming planula larva - a phenomenon copied by some coral species and sea anemones. The larvae eventually find a suitable surface to settle on and develop into polyps.
Threats and Predators
The lifespan of barrel jellyfish is short and rarely continues beyond twelve (12) months. Nonetheless, they have an important ecological impact as part of the food web.
For instance, being ferocious planktivores means they consume large quantities of zooplankton. This action helps to regulate populations of planktonic materials and maintain a healthy balance within aquatic ecosystems.
But wait - there's more:
In the wild, their natural predators are sea turtles (especially leatherback turtles), ocean sunfish (Mola Mola), and some seabirds.
Other noteworthy threats to the long term survival of barrel jellyfish include underwater garbage pollution, habitat destruction, and unregulated commercial fisheries.