Star Tunicate

Welcome back to Ask a Naturalist, your own personal Google for information on all things natural on PEI.  I recently received this beautiful photo with the question: what is this? It’s the amazing (but invasive) Golden Star Tunicate (Botryllus schlosseri, photo used with permission).

Photo: A colony of Star Tunicates on PEI, contributed photo used with permission.

Tunicates are marine, filter-feeding invertebrates that typically spend their adult lives attached to something underwater: natural material like rocks, shells, and seaweed, or human-made objects such as rope, buoys, boats, and fishing gear. They belong to a Class of animals called Ascidians (also known as Sea Squirts). Nearly 60 different species of Ascidians are known from Eastern Canadian waters, of which 50 are native.

If you’ve heard of tunicates at all, it’s likely as invasive species. Although they comprise a small proportion of Ascidians in our area, those few invasives cause major issues. For example, the Clubbed Tunicate first appeared in Island waters in 1998 and has since become a serious problem for the mussel industry by growing in large groups that cover mussel lines, increase the cost of production, and decrease yield.

Since 1998, we’ve seen the arrival of additional invasive tunicates, including the Golden Star Tunicate in 2001. (Despite their name, Golden Star Tunicates come in a range of colours including black, grey, gold, yellow, and violet). Unlike Clubbed Tunicates which can grow to around 18 centimetres (about seven inches) long, Star Tunicates are tiny – just a couple of millimeters (about 1/16th of an inch) each – and live colonially. What you see in the photo isn’t a single tunicate, but hundreds. 

Notice that the stars in the photo have different numbers of arms, from five to about 20. Each of those arms is an individual tunicate, called a ‘zooid’. Zooids come together into star-like colonies, and colonies come together into a gelatinous mass that can cover marine plants and shellfish, smothering them. Because of that, and their ability to attach to fishing gear, they have negative impacts on both native marine biodiversity and commercial aquaculture operations.

Star Tunicates can reproduce from eggs that hatch into free-swimming larvae, but the larvae don’t usually travel very far. Longer-distance dispersal happens when colonies are unintentionally transported as hitchhikers on ships, which is likely how this invasive species made it to the Maritimes from its native home in the Mediterranean.

It doesn’t take a large vessel to spread invasive species: recreational boats, trailers, and fishing or diving gear can also do the trick. Anyone who enjoys our marine areas can play a part in preventing the spread of invasive species by remembering to clean, drain, and dry: clean all mud and plants from boats, trailers, boots, and equipment; drain all water from the boat, bilge, buckets, and wells; and allow everything to dry completely between trips. And, of course, never release any plants or animals – or rocks, shells, or sticks from elsewhere – into PEI waters.

If you have a question about PEI’s wild side, it’s likely others do too! So, follow me here or on Facebook and Instagram, join the conversation, and Ask a Naturalist about PEI untamed!