There’s no doubt it’s cold in the water now. My dive watch told me that today’s temperature hovered between 11.1–11.3°C. After the last few days of wind it was a bit stirred up too. When the visibility is less than satisfactory, I tend to focus on the smaller things. So instead of swimming too far today I stayed shallow and spent my time discovering the yellow cunjevoi reefs. Cunjevoi are members of the Pyura family, otherwise known as sea squirts of tunicates.
Tunicates are incredibly abundant but there is also quite a diversity of forms. These short, squat examples, are more typical of the type of solitary species you see. Like most animals, they two main holes – an ‘in’ and an ‘out’ situated each end of the body.
Extraordinary Pyura Reefs
There is a lot that’s extraordinary about sea squirts. We could talk about the fact humans share an ancestor with them. We’re both in the phylum Chordata. When born, our embryos look similar and have a notochord, which in humans, turns into a spine. The one pictured above started its life as a tiny free-swimming larva. Almost immediately it sunk its head into the substrate, absorbed its brain and notochord and transformed into the sedentary organism you can see here.
What’s more remarkable is what they do. Perhaps as a result of swimming after recent rainfall, you become aware of the importance these animals play in filtration of the sediment. They physically pump water all of the time. Inside their body is a series of filtering membranes and a digestive system. Without them our water wouldn’t be anywhere near as clear.
The following is a map of Port Phillip Bay. The areas marked red in the map below are indicative of where these beds lie. As you can see, there are a lot of them just off the bayside area.
Level 3). The State of Victoria Department of Environment, Land, Water and Planning 2021
To give you an idea how much filtration these brown blobs might do, here is a little back-of-envelope calculation I did of the amount of water similar sea squirts were filtering in Indonesia, last year. There, each individual Golden Sea Squirt is capable of filtering about 400 litres of water per day. One study (Ruli & Yosmina, 2020) found densities of one to every three square metres. Komodo National Park is about 220,000 ha, with about 10% coral reef. That’s a population of 73 million Golden Sea Squirts, filtering 30 million tonnes of sea water through the park every day.
The Yellow Cunjevoi
I took the following photo off Black Rock where there are carpets of these creatures. At first you think you’re swimming over bare rock. If you were to walk on the seafloor, it would feel spongy underfoot. Like something out of a sci-fi film, you’d suddenly remark ‘the floor is alive’ and expect a creature to rise from beneath it. In actual fact, if you gently touch them, they just temporarily close up.
The Pyura species I photographed today was the Yellow Cunjevoi. These were only described in 2011 as before then, it was presumed all similar species were the same. There are others in the bay too. Another Pyura praeputialis is more common in the subtidal zone. If you happen to tread on one of these as you exit the sea, it’ll ‘squirt’ water … hence the name sea squirt. If you hang out on rocks at low tide, they do this anyway, without prompting. It can look quite amusing as the rocks seem to almost urinate in all directions.
