The ‘jelly soup’ that many New Zealanders experienced at the beach last summer was caused by blooms of salps, responding to warmer-than-average water temperatures.
A chain of baby blastozooids in the salp Salpa thompsoni before being released into the world. Photo: Mike Stukel / Florida State University
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It was like swimming through lumpy jelly soup.
That was the experience for many New Zealanders at the beach last summer, when high water temperatures resulted in swarms of salps around the coast.
New Zealand beaches turned into 'salp' soup' in the summer of 2018. The water in this photo is not out of focus - it is full of salp jelly (tiny black dots are the stomachs of the see-through creatures). Photo: RNZ / Alison Ballance
For many, this was their first introduction to these small see-through gelatinous blobs which NIWA plankton expert Dr Moira Decima says are more closely related to us than they are to jellyfish.
“They are urochordates,” says Moira. “They have a primitive ‘spine’, except it’s not really a spine. It’s a notochord and is made out of cartilage.”
Over long evolutionary time, that primitive notochord evolved to become the bony spine that defines all vertebrates.
That’s not the only surprising thing about salps. They have an unexpectedly gender fluid life cycle and play an important role in the ocean.
Moira Decima with salps preserved in formalin. The salps are part of NIWA's National Invertebrate Collection. Photo: RNZ / Alison Ballance
Take a close look at a single salp, and you can see it is shaped like a small barrel. Muscle bands around the salp squeeze to move the animal along and pump water through its feeding filter.
But as well as the solitary barrels, you’ll also see chains of salps. These look for all the world like a costume jewellery necklace made from linked glass beads. I’ll explain more about these two different life stages of salps later on in this story.
But first, the most visible part of the animal is usually a lump of food in its see-through stomach, and it’s this aspect of salps that most interests Moira.
The giant salp Thetys vagina can exceed 30cm in length. Plankton expert Moira Decima decribes it as the 'super pooper' of the salp world. Photo: Mike Stukel / Florida State University
Moira recently led a scientific expedition aboard the NIWA research ship Tangaroa, which was nicknamed the “SalpPOOP expedition (Salp and Particle expOrt Ocean Production).
Moira says the research team, which came from around the world, was interested in the role of large blooms of salps on biogeochemical flows in the ocean, especially in carbon cycling in the ocean.
Salps are near the bottom of the marine food web. They graze on tiny phytoplankton, and produce lots of waste that can quickly sink to the deep ocean, effectively removing carbon from the sea surface.
“We study a lot of poop,” is how Moira laughingly describes her work.
Before the team could study salps they needed to find some. As salps are only a few centimetres long and drift at sea, there was more than an element of luck involved.
However, Moira says they knew that fish such as oreo and warehou eat lots of salps, so they initially targeted areas where previous research had shown these fish were often found.
Moira explains that they ‘fished’ for salps by towing a special kind of plankton net behind the ship. They went salp hunting in cold subantarctic waters first and then in warmer subtropical waters.
In the cooler water they caught plenty of a large salp called Salpa thompsoni, which is about 10 centimetres long. Moira says they are one of the most well-studied salps, but have mostly been studied in cold Antarctic waters. She was impressed to find them equally at home in water that was at least 10ºC warmer.
The gender flipping sex life of a salp
This is the solitary asexual oozooid stage of a salp (Salpa thompsoni). The long white stolon will develop into a chain of salps. Photo: Mike Stukel / Florida State University
Salps have an intriguing way of reproducing that flips between sexual and asexual stages.
The large solitary stage is what is known scientifically as an oozooid. It is an asexual stage, which is neither male or female. It produces a long chain of clones that “will be the organisms that reproduce sexually,” says Moira.
Stage 2 of the salp life cycle. A Salpa thompsoni oozooid releasing a chain of blastozooids.This process occurs over many hours, and a chain can have about 120 zooids 'buds'. Photo: Mike Stukel / Florida State University
Once the chain is released, the immature buds separate off. At this stage they are all female.
In the third stage of the salp life cycle the blastozooids mature and mate. There are several fertilized female blastozooids in this photo, each of which contains a growing embryo. The whitish sacs correspond to the placenta, which feed the embryo. Photo: Mike Stukel / Florida State University
After maturing and mating with a male these smaller blastozooids each produce a single embryo, which they nourish via a placenta (another feature, like internal fertilisation, that they share with vertebrates). They eventually give birth to an embryo that is a new asexual oozooid.
A newly released embryo which will grow into a solitary asexual oozooid. It still has a remnant of its placenta. Photo: Mike Stukel / Florida State University
Now it’s time for the gender fluid part of their life cycle. After giving birth, the female blastozooids change sex to male, ready to fertilise the next generation of females.
Moira says that during the Tangaroa research voyage, the scientists saw all stages of the salp life cycle. The photgraphs in this story were all taken during the voyage.
To find out more about salps and Moira’s research, listen to the full podcast.
The complex life cycle of a salp. Photo: AUT
