It’s definitely summer and the sea has seasons, too. With summer comes the dread of Hong Kong’s beach-goers, the season of the lion’s mane jellyfish Cyanea nozakii.
This large, flat-topped jellyfish can grow to 50 cm in width with 8 bunches of 70 to 120 stinging tentacles that can trail up to 10 m. These tentacles can still sting even when they break off or are torn from the main body (though they lose potency over time).
It is cream to pale yellow with a darker centre and transparent on the outside edge.
You start to see these guys arrive from late March onwards and they reach their peak in mid-summer and decline in numbers after that.
Although it occurs all over the Indo-Pacific, seasonal outbreaks of the lion’s mane occur in the Yellow and East China seas and the Gulf of Bohai where fisherman regard them as a nuisance : first because they clog up nets and even damage them, second because their presence is linked to the decline of the edible jellyfish Rhopilema esculentum. Although it used to appear sporadically in those areas, it seems to have become more and more of a problem. One Chinese researcher even said that more harm and damage was caused to the coastal waters of China by this species than by red tides. Personally I have not noticed any change in Hong Kong over the years, but maybe I am wrong. When I was a kid (in the 80’s) I remember trying to count the number of jellyfish we passed off Lamma Island in our boat and lost count after 100 in 5 minutes…on average adults weigh about 1.2-1.3 kg so we passed literally a tonne of jellyfish in 5 minutes (and it was a slow boat).
So what about the barnacle and the suicidal fish?
Well, remember symbiosis from high school biology? The jellyfish are sometimes accompanies by other creatures taking advantage of the bad-ass protection offered by the stinging tentacles. One interesting guy is a transparent, soft barnacle called Alepas pacifica that hitches a ride by attaching itself to the outer rim of the jelly’s bell with a stalk and spends the rest of its life in care free existence filter feeding particles out of the water – not a care in the world. Until the jellyfish dies, then it will, too.
And the other two creatures are two commercial fish: the Razorbelly Scad (Alepes kleinii) and the Malabar Trevally (Caranx malabaricus). Both fish as juveniles like to seek out a lion’s mane soon after they hatch from eggs which – with perfect timing – seems to coincide with the arrival of the jellyfish. Hanging around the jellyfish gives them protection from predators. Think this is perfect adaptation? You would be wrong. The fish might gain an advantage from hiding by the jellyfish but they have not evolved any mechanism to avoid or deal with the stinging tentacles! Think of a child hiding from the bogeyman on top of a high voltage electricity pylon and you get a good idea of the risk to the fish. They have to swim very, very carefully! And on top of all that they are very loyal or attached to this dangerous fortress, too. When a jellyfish are caught and handled the fish stay with the jellyfish until inevitably they are forced into contact with the tentacles and are repeatedly stung until the die. Suicidal fish.
You can find more information on Hong Kong marine symbiosis in the book “Partnerships in the Sea: Hong Kong’s Marine Symbiosis” by Brian Morton.
And now – sex:
Jellyfish can reproduce sexually and asexually. And when they chose sex it’s between male and female jellyfish – not that you can tell the difference unless you have a microscope and a lot of time. But like most invertebrates they simply try to time their spontaneous ejection of sperm and eggs into the water and hope that some meet, fertilise and grow into larvae. The microscopic larvae, called planula, are tiny, hairy, slipper-shaped things that use their tiny hairs (called cilia) to propel themselves. The planula larvae of the lion’s mane only last a day and a half before they settle on the seabed and turn into small polyps like coral except solitary instead of in big colonies. After that they bud off little 2-3 mm wide jellyfish which then grow into the adult 20-30 cm wide jellyfish we all fear coming into contact with while swimming.
Some time ago the below image circulated online of a gigantic lions mane jellyfish dwarfing a diver. This is actually a different species (Cyanea capillata) to our lions mane (Cyanea nozakii) even though it shares the same common name. Thanks to Google’s reverse image search the image was quickly exposed as a photoshopped fake as you can read in this Forbes article .
And finally some interesting recent research on the lion’s mane jellyfish:
– Researchers in Qingdao investigating the venom of the stinging tentacles found it was most stable for storage at -80 degrees Celsius and that it was more effective at killing human colon cancer cells compared with human liver cancer cells.
– Researchers in Lianyungang attempting to find some sort of use for the enormous unwanted by catch of lions manes in fisheries found that it is a good source of collagen and that compared to other sources of collagen it had lower levels of arsenic, mercury and lead.
– Researchers in Japan have tried to isolate the gene responsible for the lions manes production of an enzyme called endoglycoceramidase which is useful in synthesising some other molecules. Also in Japan researchers worked on the acoustic signature of different jellyfish species to develop an avoidance system for fishing boats. The acoustic signature is essentially the characteristics of the echo received from a pinger when the sound waves hit the jellyfish. Unfortunately they were not able to tell jellyfish species apart , which would have been a wonderful way to quickly measure their populations by just towing a pinger over a vast area and then estimating the population from the echo data.