Jellyfish and Japweed at Studland

Yesterday on Knoll Beach at Studland Bay in Dorset, the two most common things washed ashore were great clumps of Japweed (Sargassum muticum) and large barrel-mouthed or dustbin-lid jellyfish (Rhizostoma octopus). There were at least a dozen jellyfish on the stretch of sand that I walked. They varied in size from about 20 – 60 cm diameter across the dome. The colours varied from crystal clear to pink and blue. They all seemed very fresh and I think maybe some of them were still alive or just expiring. As they washed to and fro in the waves, sometimes entangled in the Japweed, they turned this way and that, upright then upside down, inside and out. This species is becoming an increasing feature on south-west coasts over the last couple of summers. I first encountered these seashore creature on the Gower Peninsula in South Wales where they have long been a frequent find on the beaches. Click here to learn more about the Rhizostoma octopus jellyfish on Jessica’s Nature Blog.

Large Jellyfish at Rhossili


Yesterday (27th July 2014) I walked along Rhossili beach from one end to the other and back again – a distance of about 10 kilometres. I followed the high tide strand line most of the way and saw 16 large Barrel Jellyfish, also known as Dustbin-lid and Root-mouthed Jellyfish (Rhizostoma octopus Linnaeus) – but there could have been more. They were various sizes and states of maturity. I put a seashell beside each one I photographed to give an idea of scale. They were different shades of pink and blue colour. Their condition varied, too. Some were freshly dead and well preserved but others had been split or torn, and some were beginning to decompose by “melting” into the sand. They were lying at different angles. Some were dome upwards and others were upside down. All are harmless – no danger from stings to holiday makers. They are a relatively common sight on beaches of the Gower Peninsula in South Wales. However, they have been appearing in very large numbers along the Coast of Devon and Cornwall this summer, which is an unusual occurrence, and there has been a lot of coverage of the phenomenon in the media. There are more posts about Barrel or Dustbin-lid Jellyfish elsewhere in Jessica’s Nature Blog from sightings in previous years on Gower.


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Blue flotsam & blue jellyfish at Rhossili

Flotsam blue rubber glove: An inflated blue rubber glove bobbing ashore on the incoming tide at Rhossil Bay, Gower, South Wales, UK (1)

From a distance, from a very long way off, it might have been possible to mistake this piece of unusual flotsam for one of the many blue jellyfish that drift ashore at Rhossil Bay in Gower. Actually, just an inflated blue rubber glove – don’t ask me what it was doing there. Some of the things you find as flotsam on British beaches are intriguing.

Barrel-mouthed jellyfish: The blue dome of the Dustbin-lid Jellyfish, Rhizostoma octopus (Linnaeus),  stranded at Rhossili Bay, Gower, South Wales. UK (2) 

This picture shows what the real blue jellyfish looks like. You can see why someone might have thought the glove was one. Only this animal is very much bigger than the flotsam. The diameter of the dome can measure upto 90 cm across. This is the Dustbin-lid Jellyfish, Rhizostoma octopus (Linnaeus), also apparently called a Barrel or Root-mouthed Jellyfish.

The blue dome of the Dustbin-lid Jellyfish, Rhizostoma octopus (Linnaeus),  stranded at Rhossili Bay, Gower, South Wales. UK (3)

The colour of these jellyfish is variable. You can compare this specimen from Rhossili – with the top of the blue dome being uppermost and the oral arms just protruding from the dome or bell – with the specimen featured earlier in Monster jellyfish stranded on Whiteford Sands which was stretched out with the underside (sub-umbrella or oral surface) and oral arms entirely visible.  There’s also more information and pictures in the post Pink Dustbin-lid Jellyfish at Rhossili

The picture below shows one of a yet another colour variation drifting ashore in shallow water with the incoming tide at Rhossili. Unlike many jellyfish, this type does not have a ring of dangerous stinging tentacles around the outer edge of the large dome or umbrella. However, the upper surface of the umbrella (known as the ex-umbrella or aboral surface) is covered with groups of  tiny nematocysts or stinging cells that give a slightly matt appearance to the surface when it is out of water. 

A Dustbin-lid Jellyfish, Rhizostoma octopus (Linnaeus), drifting ashore at Rhossili Bay, Gower, South Wales, UK (4)

An inflated blue rubber glove washing ashore at Rhossil Bay, Gower, South Wales, UK (5)

Revision of a post first published 11 July 2009


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Monster jellyfish stranded on Whiteford Sands

Stranded jellyfish: A large jellyfish, Rhizostoma octopus (Linnaeus), stranded on the beach at Whiteford Sands, Gower, South Wales,  27 June 2009.

Looking like some alien monster with big googly eyes, this huge jellyfish was one of many that get washed ashore and stranded on Gower beaches. There are often dozens on the shore of Rhossili Bay in various stages of decomposition. This particular fine and fresh specimen was seen on the rocks at Whiteford Sands. The Latin name is Rhizostoma octopus (Linnaeus).  Common names for it include Barrel-mouthed or Dustbin-lid Jellyfish. It has a large, fairly firm umbrella shaped dome about 90 cm across; the overall length – stretched out with its tentacles extended – was well over a metre.

The wet gelatinous texture is repellent to some people but the colours of the various parts are attractive with shades of pink through a milky hue to shades of blue. A narrow, scalloped, purple fringe trims the lower edge of the umbrella. Viewing this animal from the underside, all the detailed anatomy is revealed. There is a pattern based on a fourfold or eightfold repetition. I think it is a splendid-looking creature. 

Revision of a post first published 1 July 2009


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Pink Dustbin-lid Jellyfish at Rhossili

Jellyfish on Rhossili beach: Pink Dustbin-lid Jellyfish, Rhizostoma octopus (Linnaeus), on the strandline at Rhossili, Gower, South Wales, June 2009, upper surface (1) 

The Dustbin-lid, Barrel or Root-mouthed Jellyfish, Rhizostoma octopus (Linnaeus), comes in a variety of colours; and its shape will vary depending on the way it has been washed ashore; and according to how long it has lain on the strandline.

This jellyfish species can be blue or, like the pictures in today’s posting, pink.  The photographs show both upper and under surface views, together with a few explanations about its anatomy.

Jellyfish are Coelenterates; and this particular type belongs to the Class Scyphozoa. The large free-floating medusoid form is the dominant individual in the life cycle.

The bell, dome or umbrella can be up to 90 centimetres across, which seems quite big but is small in comparison with the 2 metre diameter of another Scyphozoan called Cyanea arctica – which (thankfully) lives in  the Arctic Ocean and not on British beaches.

Most Scyphozoans live in coastal waters where they can create problems for swimmers. The presence of stinging cells called nematocysts make encounters with these jellyfish unpleasant and sometimes dangerous. However, it is important to emphasise that the Dustbin-lid Jellyfish does not have have the circle of trailing stinging tentacles around the perimeter of the bell like the ones in some other more harmful types of jellyfish. There are stinging cells scattered on the outer surface of the bell (ex-umbrella or aboral surface) and on some of the mouth parts. The stinging cells immobilise small fish and planktonic animals that the jellyfish captures and eats. 

The bell or dome is filled with a firm, gelatinous, and fibrous substance called the mesoglea. This is thick in the middle but around the edge it thins out to form a more flexible, thinner area like a loose-hanging pelmet. You can see this in the picture above. This thinner outer perimeter of the dome  is scalloped into lobes called marginal velar lappets. The edges of the lappets are coloured purple. This jellyfish has a generalised nerve net type of nervous system with synapses; there are specialised sense organs grouped into structures called rhopalia located around the margin between the lappets. 

The jellyfish has a muscular system. It moves by means of a band of powerful circular fibres that form the corona muscle around the underside edge of  the bell (sub-umbrella or oral surface). When the corona muscle contracts the animal moves upwards through the water. When the muscle relaxes the animal sinks. The overall effect is one of pulsating movement. Sideways locomotion is through wave and current action and the animal has no control over this.

Stranded jellyfish: Pink Dustbin-lid Jellyfish, Rhizostoma octopus (Linnaeus), on the strandline at Rhossili, Gower, South Wales, June 2009, under surface (2) 

Looking at the underside of the Dustbin-lid Jellyfish you can see that the animal design is based on multiples of  four, that is tetramerous symmetry. Beneath the bell – but unfortunately not properly visible in this particular specimen – is a large four-sided tubular manubrium that has a short, pillar-like, basal stem and sixteen epaulets – each with frilled mouth openings. 

Beneath the epaulets, the manubrium divides into four pairs of oral arms. These are clearly visible in the photographs radiating out from a central point. Each arm consists of three winged portions; and three winged and elongated terminal appendages. The arms have many mouth openings. All the mouths carry food through the manubrium into a stomach cavity with four gastric pouches housed in the dome.

I think that’s more than enough anatomical information for one post. I will continue with the details regarding digestion and reproduction together with more pictures at another time. 

Rhizostoma octopus (Linnaeus): Pink Dustbin-lid Jellyfish, Rhizostoma octopus (Linnaeus), on the strandline at Rhossili, Gower, South Wales, June 2009, closer view of the under surface (3) 

Revision of a post first published 4 June 2010 


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Dead & decomposing jellyfish

Dead & decomposing jellyfish 1 - Decomposing jellyfish on the strandline surrounded by hundreds of small holes made by burrowing sand hoppers in the locally damp sand

Dead jellyfish that wash ashore and end up on the strandline  seem to slowly melt into the sand and disappear. Even during life there are not many animals that feed on them except turtles. In death they don’t seem to be very appetising either; though sometimes birds may peck and crabs may claw at the remains.

I thought for a long time that the thousands of sand hoppers – Talitrus saltator (Montagu) – frequenting the strandlines were tucking in to the jellyfish feast as well as all the other detritus. However, as far as I can find out from books and the web, these small amphipod crustaceans are vegetarian and only eat seaweed and other vegetable remains.

This leaves a puzzle because, if you look closely at the photographs of dead jellyfish (mostly Rhizostoma octopus L.) in this post, you will see that that almost all of them are surrounded by hundreds of small holes in the sand. These holes lead to the burrows where nocturnal seashore creatures like the sand hoppers hide during the daytime. Maybe their presence can be explained by the fact  that these amphipods need a humidity of 90% minimum at all times to survive. The deliquescing jellyfish leaks moisture into the sand beneath and around it – providing an excellent microclimate for hopper habitation.

For more information from Jessica’s Nature Blog about this and other types of jellyfish found on Gower beaches click Jellyfish.

Dead & decomposing jellyfish 2 - Decomposing barrel-mouthed jellyfish on the sandy strandline.

Dead & decomposing jellyfish 3 - Decomposing dustbin-lid jellyfish on the sandy strandline.

Dead & decomposing jellyfish 4 - Deliquescing Rhizostoma octopus jellyfish on the sandy strandline.

Dead & decomposing jellyfish 5 - Sand-covered Rhizostoma octopus jellyfish on the strandline of a Gower beach in South Wales.

Dead & decomposing jellyfish 6 - Sand-covered dead Rhizostoma octopus jellyfish on a sandy Gower beach.

Dead & decomposing jellyfish 7 - Decomposing 'dustbin-lid' or 'barrel-mouthed' jellyfish (Rhizostoma octopus Linnaeus) on a sandy beach with mussel shells.

Dead & decomposing jellyfish 8 - Decomposing 'dustbin-lid' or 'barrel-mouthed' jellyfish (Rhizostoma octopus Linnaeus) on a sandy beach with mussel shells.

Dead & decomposing jellyfish 9 - Decomposing 'dustbin-lid' or 'barrel-mouthed' jellyfish (Rhizostoma octopus Linnaeus) on the shelly sand of a Gower beach.

Dead & decomposing jellyfish 10 - Decomposing 'dustbin-lid' or 'barrel-mouthed' jellyfish (Rhizostoma octopus Linnaeus) almost completely 'melted' into the sand of a Gower beach.

Dead & decomposing jellyfish 11 - A vague blue gelatinous shadow on the beach where a Rhizostoma octopus jellyfish has completely decomposed, deliquesced and soaked into the sand.  


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