BAASICS.4: Watershed — Field Trip #3: Nets, Stars, & Vomit


Last week, BAASICS had the pleasure of a long and rambling conversation with Dr. Sarah Cohen at the Romberg Tiburon Center for Environmental Studies. Her lab is working on a number of exciting projects, including the ecology of invasive species and the population genetics of native marine flora and fauna.


The Center itself was a fun surprise, gorgeously (if a bit awkwardly) nested against an otherwise empty coastline. At the end of a steep, narrow road, the property opens onto an expansive tarmac, lorded over by an enormous renovated and repurposed net depot. The nets in question were those that stretched across the 7-mile long Golden Gate, keeping out Japanese submarines as World War II loomed.Before that, the building was a coal station, and the extensive system of gantry cranes built for that purpose were used in the 1930s to spin the suspended cables for the Golden Gate Bridge.


One of the reasons Christopher and I were particularly interested in meeting Dr. Cohen, other than to ask her to be a part of BAASICS.4:Watershed, was to talk about tunicates, or sea squirts, a love of which we share. Fast forward to now. The cranes are gone, and in their place are barrels of baby eelgrass and eager students from San Francisco State University.

Tunicates are marine invertebrates, famous in certain circles for their unusual life cycle. Many of them begin life as a free-swimming larvae, exhibiting all the characteristics of a chordate: they have a notochord (a semi-flexible rod running the length of the body), a dorsal nerve cord, pharyngeal slits (like gills), and a tail. However, later in life they attach to something hard, like a rock, lose their tail as well as their nervous system, and spend the rest of their lives as a sack with two openings. As if that weren’t remarkable enough, some kinds of tunicate choose to live life alone, while others replicate by budding into colonies.

We were introduced to these creatures during BAASICS.1: A Live Animal, when artist/scientist Brian Null performed the dance of the tunicate.  We are still in search of the video!

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Didemnum vexillum (D. vex, for short) is one of the species of tunicate Dr. Cohen studies. Due to a brilliant pr strategy designed to drum up disgust for this adaptable species, D. vex is now generally referred to as “sea vomit.” This tunicate is native to waters around Japan, but is now reported as an invasive species at multiple sites in both Europe and the United States, most famously in the waters off Sitka, Alaska, where it was originally dubbed sea vomit. D. vex currently threatens the San Francisco Bay. On the bright side, and somewhat ironically, since the bay has long been filled in with muddy sediment from hydraulic mining during the gold rush, there aren’t a whole lot of hard surfaces for the vomit to attach to. Chew on that conundrum.

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The recent rise of filter feeders such as sea vomit is largely due to the loss of their predators, including fish, turtles, nudibranchs, and sea stars.

Leptasterias_hexactis6DLC2005Speaking of sea stars, Dr. Cohen is also working with her students on a particular kind of sea star by the name of Leptasterias, characterized by their small size, six “legs,” and unique brooding behavior (as in growing up baby, not listening to Radiohead on repeat). These stars sit on their many, many young while they mature into tiny, tiny stars and finally crawl away, about as fast as the universe expands (assuming that isn’t very fast). One family of Leptasterias has been found living in the mouth of the Golden Gate, perhaps taking advantage of the unique habitat provided by the meeting of ocean and bay.

Toward the end of our conversation, only a tiny piece of which I’m recounting here, we got around to talking about the problem of ballast water. In case you aren’t familiar with the concept, this from Wikipedia:

“Cruise ships, large tankers, and bulk cargo carriers use a huge amount of ballast water, which is often taken on in the coastal waters in one region after ships discharge wastewater or unload cargo, and discharged at the next port of call, wherever more cargo is loaded. Ballast water discharge typically contains a variety of biological materials, including plants, animals,viruses, and bacteria. These materials often include non-native, nuisance, exotic species that can cause extensive ecological and economic damage to aquatic ecosystems.”

These discharges are believed to be the leading source of invasive species in US marine waters. Here, in the San Francisco Bay, ships from all over the world bring in Green crabs, the Asian clam, many types of copepods, worms, and other species. These days, regulation requires ships to perform a mid-ocean ballast water exchange, but this is often dangerous and/or impossible due to weather or stability issues. As you might expect, many different types of water treatment options are being investigated.

Needless to say, we are excited to have Dr. Cohen presenting as part of BAASICS.4!

Photo credits: courtesy Dr. Sarah Cohen and the Romberg Tiburon Center

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