Summer is around the corner and with the inevitable blooms of jellyfish, which are gathering biomass in the Mediterranean, I felt that perhaps it is appropriate to mention an important Gibraltar connection to one in particular.

The barrel jellyfish (Rhizostoma) are a group of large jellyfish whose umbrella can measure up to 60cm diameter and have a total length of up to 2 metres. These are one of the largest true jellyfish in ours waters. Their name is derived from the Greek rhizo, meaning root, and stoma, meaning stomach, and root stomach animals they are indeed. These animals have cauliflower tentacles which are so huge it gives it the barrel shape for which the group are referred to.

There are three known species in the Mediterranean and surrounding Atlantic area: Rhizostoma octopus, Rhizostoma pulmo and Rhizostoma luteum. The final one had not been sighted for six decades and researchers had started to doubt its existence at all. It was first discovered in the Straits of Gibraltar by Quoy and Gaimard in 1827. These gentlemen were French naturalists who were on a scientific expedition in the Mediterranean aboard the French corvette Astolabe. It was on this trip when they chanced upon this large jellyfish and were able to identify it as a new species.

These are one of the largest true jellyfish in ours waters.

Subsequent to the first sighting by Quoy and Gaimardin 1827, there was only one single scientifically confirmed sighting of the species in 1959, leading to the generally held belief that R.luteum had been a misidentified R.pulmo or R.octupus. This view has persisted until the first scientific confirmation in 2013. This sighting confirmed the morphometric differences as well as using genetic analysis to confirm it was indeed a separate species.

One of the obvious differences is that R.Pulmo and R.octopus lack the black appendages behind the oral arms. All three species have appendages but only R.luteums are black, making it very distinctive. Further, the bell of R.pulmo has a blue ring located at the marginal lappet and this is not present in R.luteum.

Rhizostoma jellyfish have a very typical scyphozoan life cycle, a sessile polyp stage which asexually produces miniature fully formed medusae and a sexual reproducing medusae stage. But according to a paper by Keinberger et al 2018, R.luteum is the only barrel jellyfish to brood its young. Indeed, it even boasts special canal structures in order to accommodate the behaviour; another distinguishing feature.

Rhizostoma are important micro-ecosystems, supporting various species directly on them or within their vicinity. Amphipods crustaceans can typically be found within the jellyfish body and these organisms make good use of any food scraps not consumed by the Rhizostoma.

These large jellyfish are also a good find for marine turtles as they are a full meal, if they can finish them! Some turtles can make impressive journeys in search of their food. Leatherback turtles normally found in waters around the UK, can periodically be found around Gibraltar during the summer months. They come in search of the increased number of jellyfish that are found during this time of year. As of late, they are treated to a feast.

As of late, they are treated to a feast.

Like most true jellyfish, R.luteum can give a nasty sting and should be avoided if seen for this reason. As a large and typically solitary animal, the chances of seeing one are low and due to its extremely small number of confirmed sightings since its original discovery, the animal remains as ‘not evaluated’ under the IUCN red list status. Subsequently, it is advised that if spotted then one should leave it well alone. But it is possible for these animals to swarm, just like any jellyfish species, in warmer waters where the plankton populations are blooming. 2002 saw the largest bloom of R.octopus off the south western coast of the UK. The large numbers gave scientists an important opportunity to study them, yielding valuable information on the species.

An area where the public can get involved is to capture the sighting which can significantly help to establish present populations. A free local mobile platform like NEMO makes adding these sightings very straight forward and the data are already being shared with researchers in the area. By simply reporting a sighting and legitimising the sighting with the addition of a photograph, it becomes very easy to assist scientists dedicated to untangling the present populations of the elusive R.luteum.

Gibraltar has been the centre of a number of significant scientific discoveries even though they might not have been recognised at the time; the Neanderthal skull was first found here. As the Rhizostoma luteum has no common name, I would humbly offer the ‘Calpean jellyfish’ as an appropriate adoption due to its discovery within the area of Gibraltar. This would help cement in the minds of the world the initial discovery in the Straits and further advocate our local marine environment.


Phylum: Cnidaria

Class: Scyphozoa

Habitat: Pelagic

Diet: Planktivore

Interesting Fact: Scyphozoa are an exclusively marine class of the phylum Cnidaria, referred to as the true jellyfish!