Dock fouling organisms at Jack London Square Marina

By Constance Taylor

First of all, what the heck is a fouling organism?

Essentially it’s all the stuff, native or introduced, that attaches itself to human-built structures to the point that it prevents those structures from functioning properly. At Jack London Square Marina, for example, the marine fouling organisms that glue themselves to the floating docks can build up so much that the docks will start sinking! Periodically the docks are scraped off to remove the barnacles, mussels, sponges, tunicates, and everything else that’s taken up residence on the submerged surfaces. They re-colonize fast enough though, and when Ken-ichi Ueda, founder of iNaturalist and fan of fouling organisms, took us to examine the docks on July 12 there were plants and critters aplenty to poke at and examine.

“Make sure to check out the differences between what’s growing on the floating docks versus the stationary structures,” Ken-ichi instructed at the beginning. Organisms on the floating docks are constantly submerged, while the stationary substrate such as the pilings driven into the harbor bed have dry periods when the tide is out. Looking at the floating docks is like being able to see what lives yards down in the depths of the water; anything that needs to be underwater all the time won’t be able to survive drying out during a low tide. Barnacles, mussels, and some algae encrusted the stationary pilings, while the floating docks had a far greater abundance of species underneath.

Adult Caprella mutica.  Photo: National Institute of Water and Atmospheric Research

Adult Japanese skeleton shrimp (Caprella mutica). Photo: National Institute of Water and Atmospheric Research

Laying on our stomachs and trying to not lose sunglasses and hats in the water, we found native and non-native species alike as we pulled up gobs of sponges and seaweed. Some of the most abundant critters were Japanese skeleton shrimp (Caprella mutica), a transplant that most likely came to Oakland via ballast water dumped in the harbor from shipping traffic. These strange looking crustaceans are omnivores that eat everything from decaying plant matter to each other; in turn, they’re eaten by fish and can act as an important link in the food web between microscopic plankton and larger animals. I pulled up a piece of red seaweed the size of my palm and took a close look… it was crawling with hundreds of itty bitty little skeleton shrimp! Lacking a larval stage, hatchlings come out of their eggs looking like tiny versions of the adults.

Unlike the skeleton shrimp, the many tunicates (also known as sea squirts) we found that day change significantly as they mature from larvae to adult. Hatching from eggs as free swimming, tadpole-like larvae, young tunicates have a nerve cord and rudimentary brain. But once they find a good spot to settle down they plonk headfirst onto their substrate of choice, absorb their tail, nerve cord, and “brain” back into their body, and never move again. Those absorbed parts are recycled into the digestive, circulatory, and reproductive organs the adult tunicates need to spend the rest of their lives as stationary filter feeders. The brain is an expensive luxury the larvae need to find a good home, but is unnecessary when the adult gives up the trappings of a moveable life.

From left to right: vase tunicates (Ciona intestinalis), golden star tunicates (Botryllus schlosseri), and solitary tunicate (Styela clava). Photos: Ken-ichi Ueda

From left to right: Vase tunicates (Ciona intestinalis), Golden Star tunicates (Botryllus schlosseri), and Solitary tunicate (Styela clava). Photos: Ken-ichi Ueda (Vase & Golden Star), Constance Taylor (Solitary)

Red beard sponges, brittle sea stars, spaghetti worms, and dozens of other organisms caked the sides of the dock as I squinted through the sunscreen residue stinging my eyes. Schools of small silvery fish too fast to catch (we tried), darted in front of us as we peered into the water. One crowd-pleasing find was Aplysiopsis enteromorphae, a sea slug that can be found munching algae in intertidal areas.

Aplysiopsis enteromorphae. Photo: Ken-ichi Ueda

Aplysiopsis enteromorphae. Photo: Ken-ichi Ueda


“We found an Elysia!” I heard Ken-ichi call. Oooh! It was the animal I had been hoping to see. Elysia hedgpethi is a frilly, elegant, dark green sea slug flecked with spots of iridescent turquoise that can be found on Bryopsis, a type of algae it likes to eat. E. hedgpethi, also known as Hedgpeth’s Sapsucker, has managed a rare evolutionary feat; it possesses chlorophyll-synthesizing genes that can photosynthesize sunlight into supplemental sugars for itself!

That… is… awesome.

It breaks open algal cells with its radula and sucks out the contents. The ingested chloroplasts move through digestive glands that branch into the parapodia, the ruffles on the sides of its body, and will continue to photosynthesize for up to ten days!

Elysia hedgpethi. Photo: Ken-ichi Ueda

Elysia hedgpethi. Photo: Ken-ichi Ueda


Wild Oaklanders on the dock!  Photo: Constance Taylor

Wild Oaklanders on the dock! Photo: Constance Taylor

So often, hearing a value-laden term like “fouling” can inspire instant antagonism towards entire communities of species. It can cloud our sense of amazement, and we forget to take a closer look at how fantastic these other lives are. I’m certainly not advocating for monocultures of organisms that elbow everything else out of house and home, but studying the dock fouling organisms at Jack London Square Marina and learning some of the different ways they survive in the world is a good reminder that every plant and critter has its own captivating magic, and introducing ourselves to introduced species can be a pretty marvelous way to spend a Saturday.

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