Root mouthed sea jelly at Long Beach

Sea life: Root-mouthed sea jelly

We had a huge number of sea jellies in False Bay at the start of the summer – the usual compass and box jellies, and then some other very large visitors whom I hadn’t met before. Root mouthed sea jellies (Rhizostoma pulmo, formerly known as R. octopus, and called barrel sea jellies, sea mushroom jellies, or dustbin lid jellies elsewhere in the world) are the largest known sea jellies, can grow to up to 1.5 metres in diameter and can weigh tens of kilograms. They are usually white, yellowish, or blue, and we’ve seen specimens of various hues this summer.

A root-mouthed sea jelly
A root-mouthed sea jelly

Root mouthed jellies have eight robust tentacles with frilly edges that emerge from the centre of the bell; they do not have long trailing tentacles, and are harmless to most humans. They may cause a mild rash if you have sensitive skin, but you shouldn’t touch them anyway.

Surface of the root mouthed jelly's bell
Surface of the root mouthed jelly's bell

The frilled arms each equipped with a mouth are used to filter plankton out of the water, which is then channeled directly into the gut. Their appearance in False Bay corresponded with a massive plankton bloom that was visible both above and below the water – I haven’t figured out if they came in (involuntarily) with the water bearing the plankton, or followed it into the bay!

Frilly tentacles (these ones have some sand on them)
Frilly tentacles (these ones have some sand on them)

Predators of jellyfish include sunfish, turtles (particularly leatherback sea turtles), some birds, whale sharks, some crabs, and certain kinds of whale (such as the humpbacks). We also witnessed some extremely happy sea anemones eating dead compass sea jellies that had gotten caught on the reef or died and fallen to the sand during the invasion described above. Suffice it to say, looking at the conservation status of most of the jellyfish predators listed here, we’re all going to be having a lot more sea jellies in our future unless something changes, fast.

Tentacles of the root mouthed sea jelly
Tentacles of the root mouthed sea jelly

Here’s a short video, with Tony for scale.

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Clare

Lapsed mathematician, creator of order, formulator of hypotheses. Lover of the ocean, being outdoors, the bush, reading, photography, travelling (especially in Africa) and road trips.

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