Secret sex lives of Jellyfish

How do jellyfish find each other to have sex and make more jellyfish? And are these prolific beasties really on the rise around the oceans today?
11 February 2011

Interview with 

Cathy Lucas, National Oceanography Centre, Southampton, University of Southampton


Helen Scales hears how jellyfish reproduce from Cathy Lucas, National Oceanography Centre, University of Southampton...

Cathy - They breed in 2 different ways. They're life cycle consists of the jellyfish or the medusa that we're familiar with, swimming in the water. And they also have a polyp, a small things that looks like a sea anemone that lives on hard substances on the sea bed. There's not separate males and females but what they do is they clone or they bud off new polyps then at certain times of the year, mainly this time of the year certainly around the UK, they produce new baby jellyfish. And that's a process that we can strobilation. 

Aurelia aurita polypsBaby jellyfish grow up through the spring taking advantage of the plentiful food. They grow very rapidly. And then in the late spring, early summer, these reproduce sexually. So we have separate sexes, a male and a female and these will produce eggs and sperm. And they release the eggs and sperm into the water column, the female will take up the sperm from the water column, the eggs will turn into a little larvae that swims around for a few days and then that settles back down onto the seabed and that metamorphoses into a polyp. And so the life cycle starts all over again.

Helen - And we all know that the oceans are absolutely enormous. So, how do those free-swimming jellyfish, how do they actually find each other to reproduce sexually?

Cathy - Most jellyfish that we see, or by far the biggest abundances of jellyfish that we see tend to be found in estuaries and bays and enclosed areas. And so just the confined areas of those different sort of environments will tend to keep the jellyfish together in a physical way. They tend to be aggregated with tides and currents. And sometimes you find if there's very strong winds they tend to get blown into a certain area and that's quite often why we find jellyfish stranded on our beaches.

So the physically processes are quite a major factor in helping jellyfish populations to keep together. But there's also some aspects of behaviour that they can use. They're able to migrate up and down the water column, usually in response to light. And as a group of jellyfish they might migrate up to the surface and then migrate back down. Although they're very simple organisms they do have some sort of sensory capacity and so, it's a sort of a mixture of the physical properties of the water and maybe what the weather's doing, and also there's some sort of behavioural adaptations as well.

Helen - Over the past few years, we're heard more and more about the rise of slime, with the explosion of jellyfish in particular parts of the ocean. Where are we seeing these blooms and what sorts of problems are they causing?

Cathy - Yes, there has been a big increase in both the scientific and media reports Aurelia auritaconcerning the increase in jellyfish numbers. And, there are some particular hotspots that are being quite well documented. And these include the Mediterranean and there have been quite a few incidences in the last 5 to 10 years where a lot of jellyfish have washed up on beaches and they cause nasty stings so they have to close the beaches there.

In the 1980s there was a classic example of a proliferation of not a true jellyfish but a thing called a sea gooseberry or a ctenophore that had been transported over from North America into the Black Sea and it exploded in numbers and caused real problems there.

There's also problems in Southeast Asia around Japan and China with the giant jellyfish that can be absolutely vast, two metres in diameter. And that's causing a lot of problems for their fisheries there.

And then also, the Benguela upwelling, which is on the Southwest African coast, that's a very important area for fisheries, and partly because of overfishing there's been an explosion of jellyfish that have appeared in that sort of environment.

Helen - We see that jellyfish have this ability to increase their numbers pretty rapidly. Presumably their mode of reproduction has a part to play there, does it?

Cathy - Jellyfish in general are very opportunistic. They have enormous appetites, so they can consume large amounts of plankton and they grow extremely rapidly. And also their life cycle is relatively short. So, within 4-6 months you can go from a newly released jellyfish up to a fully grown sexually mature adult jellyfish. You know each polyp can produce many baby jellyfish, and in the polyp population there might be thousands, or tens of thousands of polyps living on the seabed. 

And also the jellyfish themselves are producing thousands of eggs and sperm. And so, they have the potential in both phases of the life cycle, to produce vast amounts of new jellyfish.

And so definitely reproduction is a big part of their success but it's just part of their biological make up, there's the growth, and there's the feeding rate as well.

Helen - But they can definitely get those swarms going if they can to.

Cathy - They certainly can.

Helen - And in terms of the swarms of jellyfish that we're seeing at the moment, happening around the world, what is the feeling about that - is it a global phenomenon, a sign of long term change in the oceans? Is that really playing out in what you're seeing?

Cathy - I'm actually part on an international project that's examining in a way, these Jellywatchvery questions of is the global expansion of jellyfish blooms really happening? And if so, what is the magnitude of that? What are the causes and what are the consequences of that?

Our primary goal that we've been working on over the last 12 months is actually to collate all the data that we can get our hands on of jellyfish abundance and appearance from as many regions of the world as we possibly can. So then we can start to say, okay, are these natural cyclic phenomena, or is it actually genuinely expanding?

And so we've been compiling a global database of jellyfish abundance, it's called Jedi and we've been using published data, extracting data from long term monitoring programmes, going through historical records, cruise logs, and also using fisheries bycatch data and as well as public sightings, so if people have been to the beach they can enter their sightings in the database and we can use that as well.


Find out more




Jellywatch website

Cathy Lucas,


National Oceanography Centre, Southampton


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