When I began my self-imposed mission to observe and catalogue what was going on in our lagoons, one of the first curious things I saw that I didn’t recognise as being either fish or plant were a few rubbery flatish greeny-grey things growing across some rocks. There were just a few of them, in one spot of Emily Bay, and because, to me, they were so unusual they caught my eye.

I took a photo and went home to try and identify them, but I couldn’t find anything that quite fitted the bill. And so, on 2 July 2020, it became my first observation on the citizen-science platform iNaturalist. I uploaded my photo, the time and where I’d seen it, and when I had to suggest an ID I wrote, ‘Seaweed (?) in about 1 m of water inside a coral reef lagoon’.

I clearly had no idea at what I was looking at! It turns out, they are not an alga, but are, in fact, marine animals. And what’s more they were from the same branch as humans, which makes us distant cousins!

Apparently there are nearly 3,000 species of tunicates, in all shapes and colours, but they’d clearly been absent from my life!

That was my introduction to the tunicate fam – overgrowing tunicates, Family Didemnidae, commonly known as sea squirts. Someone responded to my post, kindly commenting with a virtual pat on the head: ‘It’s great to see observations from such an under-represented area. Hope you can post more …’

According to Stanford’s SeaNet:

‘Tunicates are chordates without backbones, the closest relatives to vertebrates (fishes, mammals, birds, etc). Their chordate features are evident only in their brief larval stage when a notochord, dorsal nerve cord, pharynx with gill slits, post-anal tail are all present. As adults, they retain only the pharynx with gill slits, which they use to filter feed.’

Their bodies are enclosed in a tough outer tunic made of cellulose (from which they get their name ‘tunicates’), which is what you can see in the photos here, peppered with tiny pores and some slightly larger holes through which they filter feed (inhalant syphon) and excrete (exhalant syphon).

And that was where I left them for the next couple of years – filed away in the recesses of my mind as a curiosity and nothing more.

By March 2022, I noticed these critters popping up all over our lagoons. They are known as colonial sea squirts because they grow in colonies and often share their tunic. At around that time we had some consultants here from Biofouling Solutions Pty Ltd undertaking a marine pest survey, so, in passing, I mentioned what I was seeing and asked them to have a look and tell me what they thought.

Dr Ashley Coutts, the principal scientist got back to me:

‘I just wanted to let you know that we also noted patches of what we think might be Diplosoma virens (Hartmeyer, 1909). We collected some samples and are looking to have it confirmed. To put your mind at rest, we did not witness any of the highly invasive colonial sea squirts such as Didemnum vexillum or D. perlucidum which we were targeting during our surveys. Nevertheless, we did note some heavy smotherings of the suspected D. virens you referred us to within Slaughter and Emily Bay, but we also witnessed similar growth forms on the Kingston and Cascade Jetties and elsewhere around the Island (e.g. Anson Bay). So this suggests that it is a native species, but think it is certainly worth keeping an eye on just in case it increases in distribution and abundance.’

Which was great to know.

Fast forward to late August, early September 2022, and the growth of these tunicates seemed to accelerate. I began seeing them on corals as well as on coral rubble, right along the length of the lagoons.

In tandem with their burgeoning numbers, I noticed that there were plenty of flatworms, Pseudobiceros sp. often around feeding on the tunicates.

These flatworms are leaf-shaped with highly ruffled edges. The tiny tentacles over their heads are pseudotentacles and look like tiny ears. These are made from the folded edges of their bodies.

In September, Ashley was giving interested locals a talk about his findings from the survey, so I took the opportunity to mention my latest observations to him. He has asked for some specimens so we can get a definite identity. Meanwhile, the coral reef health researchers from the Sydney Institute of Marine Sciences, who have been investigating the health of our reef for almost three years now, have offered to examine their most recent transects undertaken in September 2022 to compare them with their earlier ones in order to see if they can corroborate my casual observations.

I’m hoping that we can get some answers for some of my questions, such as:

• What has caused these tunicates to multiply?

• Do they concentrate toxins in their bodies, which have then been ingested by Doris (the green sea turtle who was rescued from the bay in poor health with suspected brevotoxin poisoning), as suggested by David Blyde, the turtle vet who was sponsored by Australian Marine Parks to look at her. Or was it the cyanobacteria that caused her to become sick? Apparently some species have poisonous flesh to deter predators.

• Will the flatworms be enough to create a natural handbrake on the spread of the tunicates and prevent them from spreading further and suffocating out other species?

• Have the tunicates (or the cyanobacteria for that matter) caused the seagrass to die back, particularly in the channel off the Salt House, were all the fresh water enters the bay?

• And my final questions: do I even need to worry? Are these critters just quietly doing their thing?

I’ll finish on a positive note, courtesy of Amy Nevala in Oceanus:

‘For all the headaches that invasive sea squirts can cause, native species are valuable. Some people use them for fishing baits. Scientists use them for genetics research and are exploring them for chemicals that could treat cancer. In nature, sea squirts filter water, keeping it clear. Sea squirt waste is a food source for other marine organisms.  

Below are some photos of the tunicates and just a few of the places I have observed them growing on corals and in the rubble.