Egg sacs under a dock?

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tunicate colonytunicatesThe Question:
These seem like egg sacs. We found them clinging to the bottom of a dock on an island off South Freeport, Maine. One of the pictures contains my son’s thumb for scale. Any ideas what they are?
Submitted by: Becki, Maine, USA

(click on photos and graphics to expand)

The Short Answer: The dock Angie is referring to is normally floating and therefore the underside is typically in the water. When I sent the photos around to some marine experts, they all agreed that these are tunicates. Complicating the answer a bit, however, is that these are two very different species of tunicates. What look like upside down sand drip sculptures hanging under the dock are Didemnum vexillum, a tiny, colonial tunicate that is an invasive species in Maine and other places on the eastern shores of North America. The other ones, spread out on the top of the dock near Becki’s son’s thumb, are Ciona intestinalis, a much larger tunicate that is native and common in Maine.

diagram of tunicateWhat is a Tunicate: When you look at this diagram of an adult tunicate, showing its very simple structure, it might be difficult to see it as a relative. An adult tunicate is essentially a sac with an opening at the top and a second opening on the side. Water is drawn in at the top, filtered for microorganisms, and then expelled through the side opening. Food is moved down into the stomach, processed, and then waste is eliminated into the exiting stream of water. When touched, many tunicates expel water forcefully, which is why they are commonly called “sea squirts.” And they seem much like small sea cucumbers or filter feeding invertebrates like sponges. They certainly don’t seem much like you or any other four limbed animals.

tadpole - smallerBut when you look at the larval tunicate in this diagram, you might not immediately think, “Uncle Fred!” but you might note that it looks tadpole-ish. And since tadpoles are the juvenile stage of amphibians, all of a sudden we’re thinking about vertebrates and not marine invertebrates. And it turns out you’d be on the right track.

As unlike us as the animal in the photo that includes Becki’s son’s thumb seems, it is a member of the Chordata, the phylum that includes all vertebrates and therefore includes you and me. Clearly, it’s a much more distant relationship than between us and a chimpanzee, or even between us and an amphibian. But there are 35 or so phyla in the animal kingdom, spanning everything from rotifers to squid to ants and apes, so to say that the odd creatures in Becki’s photos are in the same phyla as us is no minor thing, either. We share far more similarities with tunicates than we do with insects or an octopus, for example.

various tunicatesThere are over 3,000 species of tunicates, all of which are found in marine environments. This small sampling shows a bit of the beauty of these often colorful animals. You can see more of that in the photo collection of the Encyclopedia of Life by clicking here or at the Ascidiacea World Database by clicking here. Tunicates can be colonial, with tiny individual zooids, like the Didemnum vexillum in Becki’s under the dock photo, or larger and solitary like the Ciona intestinalis in the thumb photo. The largest individual tunicates are about 30 cm (12 inches), but colonial tunicates can get much larger. In fact, odd free floating colonial tunicates called pyrosomes can be tens of meters long (20-30 feet), as seen in this video:

http://www.youtube.com/watch?v=5EQGA_4BZ5s

What to do about Didemnum vexillum:
The sponge-like colonial tunicate under Becki’s dock, Didemnum vexillum, is not native to the Maine coast. Recent genetic testing points to Japan as the original home of Didemnum vexillum, but it has been spread around the world and is a very worrisome invasive species. In some places, including off the eastern coast of the U.S. and Canada, it is spreading across the sea bottom covering everything – outcompeting all the native animals that would otherwise live on the rocks of the ocean floor. It’s not clear yet what this means for ocean ecosystems, but it doesn’t seem likely that this is good news. As we come to the end of 2013, there is no practical way to halt the spread of Didemnum vexillum or remove it from areas it has colonized. It’s pretty much the worst kind of invasive species situation: an organism that, once introduced, completely overwhelms an otherwise diverse ecosystem and we are pretty much powerless to stop it.

Sources:

Thanks to Kevin J. Eckelbarger, Professor of Marine Biology at the University of Maine School of Marine Sciences, and to Sara M. Lindsay, Associate Professor of Marine Sciences & Marine Biology at the University of Maine, for their help in identification of the tunicates in Becki’s photos. Thanks also to Gretchen Lambert of Ascidian News for her help in reviewing this article for accuracy.

Encyclopedia of Life – http://eol.org/pages/1486/overview x

Ascidian News: http://depts.washington.edu/ascidian/

Ascidiacea World Database – http://www.marinespecies.org/ascidiacea/index.php

Stefaniak, L., Zhang, H., Gittenberger, A., Smith, K., Holsinger, K., Lin, S. and Whitlatch, R. B. 2012. Determining the native region of the putatively invasive ascidian Didemnum vexillum Kott, 2002. J. Exp. Mar. Biol. Ecol. 422–423: 64–71.

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Cite this article as: Pelletier, TC. (November 18, 2013). Egg sacs under a dock? Retrieved from https://askanaturalist.com/egg-sacs-under-a-dock/ on March 27, 2017.

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