Milky ribbons worms were identified through DEI’s research as a major predator of soft-shell clams, contributing to the decline of soft-shell clam populations, especially in southern Maine.
IIn 2016 and 2017, In an effort to discover methods to reduce predation on clams, DEI researchers tested a novel bioremediation approach targeting milky ribbon worms. Upon reviewing published reports, DEI’s scientists found that Canadian researchers conducted a laboratory experiment in the late 1990s to find ways to reduce milky ribbon worm predation (Bourque et al., 2001). They found that the addition of bloodworms (Glycera dibranchiata) to aquaria that contained one milky ribbon worm and 20 soft-shell clams of various sizes significantly reduced clam mortality. Specifically, their results indicated that the addition of bloodworms to aquaria containing soft-shell clams and milky ribbon worms reduced soft-shell clam mortality by 80% compared to aquaria that contained only clams and ribbon worms.
DEI scientists designed a field experiment based on the results of the Canadian study to determine if the results of the laboratory experiment could be replicated successfully in the wild. The field trial examined the efficacy of a natural method to protect clams from predation.
The experiment was deployed in the mid-intertidal of Collins Cove/Yorkies in Freeport at the beginning of May, in partnership with the Maine Clammers Association.
We used 80 experimental units (EU), which were 4-ft x 2.5-ft x 6-inch deep wooden boxes. One-half (40) of the boxes had a PetScreen® bottom that effectively excludes milky ribbon worms while the other half had a 4.2-mm flexible netting bottom that does not inhibit worms. To reinforce the bottoms, both types were backed by a 4-ft x 2.5-ft piece of vinyl-coated trap wire. The top of each box was covered with a piece of flexible netting with a 1/8-inch (3.2-mm) aperture.
Boxes were filled with ambient sediments from the study site. The ambient sediments were carefully checked for small green crabs and milky ribbon worms before being placed in the boxes. 400 juvenile clams were added to each box, requiring atotal of 32,000 clams. These juvenile clams had been raised by commercial clammers in an upweller the previous year as part of the same set of experiments. Each box received either 0, 20, 30, or 40 bloodworms (a total of 1,800 bloodworms).
The eighty boxes were sampled in September 2016 by taking five sediment cores (A = 0.02 ft2) from each box. The contents of each core sample were washed through 1-mm sieves and the number and size of live and dead, cultured and wild, soft-shell clam seed were measured.
We were unable to determine if bloodworms do stop milky ribbon worm predation on clams due to the presence of other predators inside the boxes. We did, however, make two key discoveries. First, milky ribbon worms were able to enter boxes with a bottom mesh of 4.2-mm aperture, making this size netting ineffective in protecting clams from these predators. Second, green crabs are pervasive and persist at very small sizes in the intertidal mud. Even though we were careful not to add any visible green crabs to the boxes, some were too tiny to detect and were inadvertently added to some of the boxes. Most of the soft-shell clams were recovered dead with undamaged shells (from milky ribbon worms) or crushed/chipped shells (from green crabs).
In 2017 we refined our study design based on what we learned in 2016 and continued to test the same hypothesis regarding whether the presence of bloodworms would result in a decrease of clam deaths from milky ribbon worms. Similar to the previous year, clammers from the Maine Clammers Association (MCA) assisted with deployment and sampling.
The experiment was set up in the same location (Collins Cove) and tidal height as 2016.
In 2017, we used one hundred and twenty protective wooden boxes (40 more than the previous year). The boxes were smaller than those used in 2016 (1 ft x 2 ft x 6 in deep).
To avoid introducing worm and crab predators to the boxes by filling the boxes with ambient sediments from the mudflat, MCA clammers and DEI scientists-. filled the boxes with approximately 50 pounds of terrestrial sand from a local (North Yarmouth) gravel pit. The sand was moistened with seawater before planting clams. MCA clammers dug 120 6-inch deep x 1-ft x 2-ft depressions in the mud, enabling each box to sit nearly flush on the mudflat, creating conditions as close to natural as possible.
Three different densities of cultured juvenile soft-shell clams (60, 120, or 180) were placed in each box along with four different densities of bloodworms (0, 4, 6, or 8), resulting in a total of 14,400 juvenile clams and 540 commercial-sized bloodworms for this experiment.
The boxes were predator-protected with various mesh bottoms designed to allow or deter ribbon worms, and to retain the blood worms. One-half (60) of the boxes had a PetScreen® bottom (that excludes milky ribbon worms), and a half (60) of the boxes had a 4.2-mm flexible netting bottom (that allows milky ribbon worms to enter the box). Both types of bottoms were reinforced with a 1-ft x 2-ft piece of vinyl-coated trap wire. Each box had a top made of a rigid mesh with a 2.1-mm (0.083-inch) aperture.
All boxes were sampled in November 2017, and the entire contents of each were washed through 2-mm sieves, allowing most of the sand to pass through , leaving only clams and worms. To determine the success of the bioremediation technique, number and size of live and dead, cultured and wild, soft-shell clam seed were recorded.
The samples from each box were frozen, and some of the 120 samples have been processed. Analysis of the data will be completed during summer 2021.