It’s Alive! Finding Tiny Clams Aboard Driftwood

Close-up view of wood with shavings around it, and pieces hollowed out

This is the eleventh in a series of posts by Associate Curator of Invertebrate Zoology Janet R. Voight following her expedition to Norway. There, she and colleagues look for a wood-boring clam, Xylophaga dorsalis, to study its, well, poo. Read the first post and stay tuned for upcoming posts.

In my previous post, I wrote about how all our hopes for success rested on the one piece of wood we trawled. When we finally got it back from the R/V Hans Brattström to the marine station, it wasn’t much to look at. Once in the aquarium, the mud dissolved in the water, making the whole aquarium nearly opaque; what we could see was coated with tube worms. Not the kind of glorious, huge ones of hydrothermal vents, but tiny ones first discovered near here, just about the diameter of thin, uncooked spaghetti. Like their larger, more famous cousins, these tube worms live by using bacteria to harvest energy from the conversion of sulfide to sulfate. Their presence meant that the whole thing was pretty toxic at one point and still could be.  

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A close-up view of a cratered piece of wood
A small, live Xylophaga dorsalis is poking out of the wood in the center of this photo.

Ken Halanych, who’s worked with these tube worms before, tried to sample some of them and couldn’t find any alive—just their tubes were left. Maybe they had used up the sulfide and died (meaning the wood isn’t toxic!). But it turns out they were not the first animals to colonize the wood. We could see boreholes from what we figured were shipworms that were bigger around than my thumb and forefinger combined. The animals were long gone, leaving no sign of their bodies or calcareous borehole lining. If the wood was buried in sediment, they would have died and their rotting bodies would have fed the tube worms. But the question remained: was there anything we wanted alive in there now?

We set up seawater to run through the aquarium and finally had dinner. I snuck over afterward to check the wood but didn’t see anything interesting. The next morning, I found three fecal chimneys! They had to be our target animal, Xylophaga dorsalis. What we had sought and been told wasn't likely, we now had in captivity! By that evening, I counted seven individuals. The only catch is that they are all tiny. Tiny isn’t bad, but it means that some of my carefully laid plans for manipulations are impossible. These animals routinely get to two centimeters (three-quarters of an inch) across; these are less than two millimeters (less than a tenth of an inch) across. They are all we have, and it is miraculous that we have these!

I planned to remove the fecal chimney from boreholes with clams in them, but these specimens were too small to work with, so I decided to flush away the chimneys every few hours. I planned to test how the animals responded to low salinity; with only one piece of wood and seven to 10 individuals, I would have to break the wood apart and risk killing them all if I released the sulfide I suspected was there. I reluctantly decided to not pursue that line of action.  

I could let them grow, collect fecal chimneys for isotope analysis and bacterial studies, and get the answer to why they build the chimneys—which is nearly all we could see of the animals besides their siphons with characteristic edges. The next few days will be quiet; I’ll spend them working on the specimens from the Bergen Museum and those from Henrik G., tending to the animals and as possible, watching them behave (breathe and poop). They are eating, of course, but they eat deep inside the wood. Their valves scrape against the wood, filing off tiny bits that they eat and digest. These animals live 75 percent below an impenetrable barrier, yet looking into the aquarium, for the first time I am within a foot of live members of the group I have studied for over a decade. I need to use this opportunity to learn as much as I can about them and how they live.

Read on in Part 12, the final post: Closing the Clam Case and Wrapping Up a Norwegian Expedition

Funding for this project was provided by the Robert A. Pritzker Center for Meteoritics and Polar Studies established by a grant from the Tawani Foundation.