DEI’s research to find the cause of the clam decline identified milky ribbons worms to be a major predator of soft-shell clams, especially in southern Maine.
In an effort to research methods to reduce predation on clams, in 2016 and 2017 we tested a novel bioremediation approach to protect clams. Upon reviewing the literature, DEI’s scientists found that Canadian researchers conducted a laboratory experiment to find ways to reduce milky ribbon worm predation on clams in the late 1990s (Bourque et al., 2001). These researchers 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 decided to design a field experiment based on results of the Canadian study to determine if results of laboratory experiment could be duplicated successfully in the wild. That is, the field trial was carried out to examine the efficacy of a natural method to protect clams from predation.
The experiment was deployed at Collins Cove/Yorkies in Freeport at the beginning of May in the mid intertidal in partnership with the Maine Clammers Association.
We used 80 experimental units (EU), which were wooden boxes with dimensions of 4-ft x 2.5-ft x 6-inch deep. One-half (40) of the boxes had a Pet Screen® bottom that effectively exclude 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 of bottoms 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 an 1/8th 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 each box, which contained 400 juvenile clams added (a total of 32,000 clams). As part of the investigation into the cause of the clam decline, clammers grew these juvenile clams in an upweller the previous year, and overwintered them using methods invented in the 1990’s at DEI. 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 were washed through 1 mm sieves and the number and size of live and dead cultured and wild soft-shell clam seed was measured.
We were unable to determine if bloodworms do stop milky ribbon worm predation on clams due to the presence of predators inside the boxes. We did, however, make two key discoveries. The first was that milky ribbon worms are able to enter boxes with bottom mesh of 4.2mm aperture, making this size netting too large to protect clams from these predators. The second was that green crabs are so 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, their tiny size made them unable to detect and we inadvertently added some to the boxes. Most of the soft-shell clams were recovered dead with undamaged shells (from milky ribbon worms) or crushed/chipped shells (from green crab attack).
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 due to 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).
In order not to introduce worm and crab predators to the boxes by filling the boxes with ambient sediments from the mudflat, we filled the boxes with terrestrial sand from a local (North Yarmouth) gravel pit. Approximately fifty pounds of terrestrial sand were added to each box by MCA clammers and DEI scientists. The sand was wetted with seawater before planting clams. MCA clammers dug 120 6-inch deep x 1-ft x 2-ft depressions in the mud so that each box was able to sit nearly flush on the mudflat in order to provide as natural conditions 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). 14,400 juvenile clams were used for this experiment along with 540 commercial sized bloodworms.
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 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 has a top that is made of a rigid (extruded) mesh with a 2.1 mm (0.083-inch) aperture.
All boxes were sampled in November 2017, and the entire contents of each washed through 2 mm sieves. This size sieve allowed most of the sand to pass through the sieve, leaving only clams and worms. Number and size of live and dead cultured and wild soft-shell clam seed was measured, along with the number of living worms to determine the success of the bioremediation technique.
The samples from each box were frozen, and some of the 120 samples have been processed. We will be completing the analysis of the data during summer 2020.