One-Year-Old Clams: It’s Time For The Mud

Since my time here, I have learned just how unpredictable it is to raise soft-shell clams in an upweller. Right when you think you have things under control, a new problem arises. We have been working hard, attempting to solve issues ranging from identifying and controlling harmful species in our upwellers, to water flow inconsistencies, to slow growth rates. Throughout this article, I discuss one of these problems – unwanted filter feeders – and the ways in which we are trying to solve this problem.

Earlier this month, I wrote a brief article on the increasing number of filter-feeding tunicates found attached to our one-year-old clams. However, that article contained one important mistake. Although we did have tunicates growing on our clams, the most common organism growing on them (the brown fuzzy organism) was misidentified as another type of colonial tunicate. This “brown fuzz” was actually a bryozoan. Like clams and tunicates, bryozoans are filter feeders. They create colonies and are made up of individual bryozoans called zooids. Bryozoans use a lophophore which is a crown of tentacles to capture phytoplankton and other small particles. I wasn’t able to identify this organism correctly until I emailed Zac Tobias, a graduate student studying tunicates at Woods Hole Oceanographic Institute. He didn’t think the “brown fuzz” was a tunicate and suggested looking at it under a microscope for more identifying features. The suggestion came at the perfect time because we had just gained access to a microscope a few days before I received the email. Looking through the microscope, I was able to identify features like the lophophore and the branching colonies which helped me identify the organism as a bryozoan. Although most of the clams with bryozoans were still alive, I was still worried that, like the tunicates, the bryozoans were increasing competition and slowing the growth rate of our one-year-old clams. 

Last week, we were finally able to sort through each bucket of one-year-old clams. During this process, we separated shells and tunicate/bryozoan-covered clams from the rest of the live, clean clams. After sorting, we counted all of the live clams. We placed the live clams back in the bucket, the tunicate/bryozoan-covered live clams in the periwinkle bucket, and the shells in the trash. In the end, we counted a total of 10,264 live clams.

All of our tunicate/bryozoan-covered clams were now in one bucket, but the tunicates and bryozoans seemed to be spreading among the clams faster than the periwinkles could eat them. Our next solution was to set up an experiment to see whether puting the bryozoan-covered clams in the mud would kill the bryozoans. In the upwellers, the clams are exposed to the water allowing for tunicates and bryozoans to easily attach and grow. However, when the clams are in the mud, their shells are not exposed to the water column and tunicates and bryozoans are not able to attach themselves to the clams. The thinking here is that the bryozoans would “suffocate” under the mud, while the clams would survive due to their long siphons that can extend above the mud to feed.

I filled 8 planter pots with mud from the old lobster pound and placed seven bryozoan-covered clams an inch deep in the mud for each pot. I then wrapped a mesh netting over the planter pots to prevent green crabs from predating on the clams. I placed the planter pots in the mud out in the old lobster pound and let them sit for a week. After that week, I recovered the clams from the planter pots to see whether the mud had killed the bryozoans. Initially I noticed that most of the clams still had the fuzz on them, but it had turned black from the mud. When I returned to the clam lab, I looked at the bryozoans under the microscope. Even though the clams still had the fuzz on their shells, the bryozoans appeared to be dead and what was left behind was an exoskeleton of zooids. Now that we know bryozoans can’t survive long buried in the mud, it makes more sense to get as many one-year-old clams in the mud as quickly as possible. With the clams in the mud, we won’t have to worry about the constant spread of bryozoans in the upweller. When reseeding the flats with our one-year-old clams, we plan to cover them with a netting in hopes that this prevents green crab predation.

More articles on other challenges we are facing and the steps we are taking to find solutions will follow soon.