Jellyfish (Cnidaria and Ctenophora)

What is a jellyfish?

Most people associate the word “jellyfish” with the large, gelatinous creatures seen in harbours and stranded on beaches. These are known as Scyphozoans and they are cnidarians - belonging to the same group as sea anemones and corals. There are less than 10 Scyphozoan species in northern European waters (see below). However, there are over 100 species of small, less obvious, hydrozoan cnidarian species in the same area.

Which species do we see off Plymouth?

Of the 6 species of Scyphozoa we see off Plymouth, the moon jellyfish, Aurelia aurita is the most common. It is very tolerant of low salinity, so you often see it in estuaries and harbours. In some summers we receive a lot of records of the blue jellyfish Cyanea lamarcki. This species is found all-round the UK. Compass jellyfish Chrysaora hysoscella are also seen around the UK and are very photogenic underwater. The lion’s mane jellyfish Cyanea capillata is also reported but usually in lower numbers. In 2015, large numbers of the barrel jellyfish Rhizostoma pulmo were seen around the south west of England and Wales.

Are they becoming more common?

There has been more sighting and reporting of jellyfish in recent years, giving the impression that they have become more common. There is some evidence that in particular areas, possibly because of man’s influence on the ecosystem through over-fishing and eutrophication etc., that jellyfish numbers have increased, but better long-term data is required.

Every year there is a seasonal, variable increase in numbers of jellyfish (Cnidaria and Ctenophora) around UK coasts. Cnidarians are more obvious; off Plymouth we see 6 species of the larger Scyphozoa (this is what most people think of as jellyfish) and around 56 species of Hydrozoa (the smaller but often extremely numerous jellyfish). The species and numbers that occur change each year, because conditions – temperature, salinity, nutrients, food availability, weather, currents etc. vary from year to year. What turns up each summer can also change due to currents bringing species in from more oceanic areas; again these currents vary each year.

The type and number of cnidarians that we see will be affected by:

  • how good survival was in the previous year
  • conditions for the overwintering life stages, and
  • general physical and biological conditions in the spring and summer when reproduction is taking place.

Currents and tides can concentrate jellyfish and they themselves can gather at depths in the water column where food is concentrated.

All these factors mean that the species and numbers of jellyfish in UK coastal waters are variable and unpredictable from year to year.

Ocean warming should certainly have an effect on jellyfish distribution and numbers of some species. Ocean acidification due to higher dissolved CO2 could pose a threat to jellyfish and research is being carried out into the effects of lower pH on their orientation statoliths that are composed of calcium sulphate. In some areas of the world jellyfish numbers actually seem to be decreasing.

How long do jellyfish live?

The lifespan of a jellyfish depends on the species. The life cycle of most jellyfish involves an alternation between a polyp stage when they are fixed to the seabed, and a free-swimming (medusa) stage. During the polyp stage the jellyfish typically releases many free-swimming, medusoid stages, mainly in spring and summer. There are male and female medusoid stages and the fertilised egg develops into a tiny larva known as a planula that swims to the sea bed and transforms into the polyp stage. In autumn the medusoid stages usually die and only the polyps survive the winter to produce the following years’ generation.

Because most of the medusoid stages die in the autumn, at most they can only live for up to 6 months. Life span is very variable, depending on predation, food availability, temperature, species etc.  The large scyphozoan jellyfish can typically survive for around 2-6 months, while the smaller hydrozoan jellyfish may only live for days to a few weeks. Some deep-water jellyfish, because they live in a more stable environment, may survive for several years.

One species of jellyfish Turritopsis dohrnii is able to reverse a part of its life cycle, and has become known as the immortal jellyfish (see below).

What are the main predators of jellyfish?

Probably the most important predators of jellyfish are fish, seabirds, parasitic crustaceans etc. They are also fed upon by turtles and sunfish – the latter in particular are regularly seen in the inshore waters of the southwest peninsula.

Did you know?

  • Because they drift with ocean currents, jellyfish are classified as 'plankton'.
  • All jellyfish have stinging cells
  • The largest jellyfish is the lion's mane jellyfish, whose bell (main body) can be over 2 meters in diameter!
  • A group of jellyfish can be called a bloom, swarm, fluther or smack.
  • One species of jellyfish is known as the immortal jellyfish. See the blog at

Further information

You can download a jellyfish identification guide and find out how and why to report your jellyfish sightings from the Marine Conservation Society
Jul 3, 2018 By guba