Pacific Adventuer invasive tunicate page

Divers have been reporting a species of tunicate by the name of Botrylloides violaceous, thinking it could be Didemnum sp. While Botrylloides is a non-native colonial tunicate, it has been in our waters for at least 25 years and does not worry scientists as much as Didemnum. These two can look a lot alike, especially from several feet away. They grow in similar habitat (often right next to each other) and take on similar shapes. If you do see Botrylloides, still please let me know.

Port Townsend diver, Amy Leitman, reported what looked like Didemnum growing up and over eel grass and oyster shells near Pleasant Harbor Marina. This is Don's backyard (he runs Pacific Adventures Charters) and he was kind enough to take photos and send them to me for confirmation. Turns out that it was actually Botrylloides, though is was growing in a way that appeared very similar to how we often see Didemnum. Thanks to both Amy and Don for making this identification.

If you compare Don's photos to Frank Poole's of Didemnum (http://www.psdivers.com/photopost/showgallery.php/cat/557), you notice some differences. Each little hole in the tunicate that you see in both sets of photos is the intake syphon of a tiny tunicate zooid (these animals are like corals, lots of little critters sharing a common structure). In Didemnum, these little holes are organized in a random fashion all over the shared strucuture of the animal (called its "tunic"). In Botrylloides, they tend to be well defined and organized in a linera or circular fashion, each right up against its neighbor, so that you get patterns of zooids, often in rings or half loops. Another feature that Botrylloides has that Didemnum doesn't are little thin tenticle-like structures growing along its edge margins (I don't know exactly what these are). These are pretty clear in photos 6663 and 6658 on Don's site.

Didemnum has two clear diagnostic features. The first are small white dots (pin-prick size) between the zooids just beneath the surface of the tunic (calcium carbonate spicules that stiffen the animal). These are the number-one ID feature for all genus Didemnid. The other key ID feature that you often see on mature colonies are dark-colored veins running through and between zooids which link each with larger excurrent openings used to vent filtered water.

So in summary
Botrylloides (can be many different colors):
1) zooids well defined and organized in linear fashion, often forming circular or half loop patterns
2) thin tendrils along the side margins
3) spaces between zooids lack real structure or any kind of white dots.
Didemnum:
1) Zooids less well defined, appearing as small holes randomly distributed
2) small white dots in tunic between zooids, which often give the tunicate a white or cream colored appearance.
3) darker channels running between groups of zooids and common excurrent openings.
Pictures are worth a thousand of my words, please look and compare. If you have any doubt about what you're seeing in the field, I'm always happy to answer questions. Frank's website is also a great place for information or to chat about tunicates (compelling conversation topic, I assure you!).

Simon Geerlofs
School of Marine Affairs

Also, check out Frank Pool's web site for photos and a forum he's set up on this topic. Click Here to go There!

These are photos taken by Don Coleman near a private dock in Pleasant Harbor on Hood Canal on 28 Oct 2005

Depth was 10 - 20 feet. These photos were taken in a sparse eel grass bed and on the dock pilings.

GPS location is 47 39.726N 122 54.796W