Summer Series 1: Jellyfish – not really fish, mostly jelly


Washed up like a jellyfish on the sand this summer? New Zealand has the moon jelly, spotted jellyfish, and lion's mane, and all three jellyfish are prevalent in our coastal waters all around the country, and the ocean, at this time of year. Jellyfish have weak powers of direction, they drift into bays, and tides and currents wash them up.

"It's a seasonal thing. Over summer there is more food available, and it fits with their lifecycle, so there are more around," says NIWA Principal Scientist Dr Dennis Gordon.

If you're in the water and there's a jellyfish beside you this summer, chances are you can out-swim it. They move by slow pulsing, and try to stay near the surface, as they follow food supplies.

They can't see, they don't have a brain, and if you are in the water with one, it can probably smell you, if not see you. Strangely, jellyfish do not have a digestive system, respiratory system or even a circulatory system. And are mostly made up of water and jelly!

The moon jellyfish

The moon jellyfish has a bell-shaped body with relatively short tentacles around the margin of the bell.

They are capable of only limited motion, primarily drifting with the current, even when swimming. Their slow pulsations weakly drive them forwards. The body of the moon jelly is whitish or translucent, about 25–40 cm across. It can be recognised by its four pinkish-purple crescent-shaped gonads, which are easily seen through the top of the bell.

What does the lion's mane look like?

The lion's mane jellyfish is the biggest species of jellyfish in New Zealand waters. Its bell can reach a diameter of up to two metres, and its tentacles can grow up to 36 metres in length. "It feeds by collecting other small organisms with its dense mop of tentacles that hang under the bell, and it then brings the prey into its body for digestion," says Dr Gordon.

The lion's mane is a pinky brown colour. It has about eight clusters of tentacles and each cluster has more than a hundred individual tentacles.

"When they are washed up on the sand, its safest to touch the top of the bell, but not the tentacles," says Dr Gordon. 

Spotted jellyfish

Spotted jellyfish have 30-centimetre-long tentacles and generally dark polka dots on the body. In the South Island they are reportedly responsible for deaths of farmed salmon, because of the mucus they emit.

People do eat jellyfish. In Asia they are steamed and fried – the bell is cut into strips, like noodles, and fried with sauces and spines. Dried jellyfish can be bought in New Zealand.

If you see a jellyfish next to you in the water ...

If you see a jellyfish in the water, avoid touching the tentacles, which have nettle cells that sting. The moon jelly has the weakest sting of the three New Zealand jellyfish. If you do get stung, it hurts, leaves a red mark, but probably won't kill you. Treat the sting with vinegar, or if you are at a beach, flush the sting with lots and lots of seawater.

Dr Gordon says: "Worldwide there are only about 250 species of jellyfish. Gene sequencing will probably determine that some of the species that we have under one name will prove to be species complexes."

About Dennis Gordon:

Dennis Gordon has been at the National Institute of Atmosphere and Water for 31 years, including the period when it was called the New Zealand Oceanographic Institute.

"New Zealand's jellyfish have been little studied and I'm interested to know how many species there are, especially the small stalked jellyfish that attach to seaweeds. Worldwide, there are only about 50 species. A class of stingers by themselves, we used to think that there was one species in New Zealand, but we now know of at least seven," says Dr Gordon. 

Lion's mane jellyfish. Credit: NIWA
Moon jellyfish near a mussel line. Credit: NIWA
Moon jellyfish. Credit: NIWA
Spotted jellyfish. Credit: NIWA
Research subject: Marine Invertebrates