Gregarious Clam Settlement Experiments Thwarted by Milky Ribbon Worm Predation (2014, 2017, 2018)

In 2014, 2017 and 2018 DEI scientists, in partnership with the Maine Clammers Association, conducted field experiments to determine if wild clams settle and recruit to the mudflats gregariously. These experiments were part of the investigation into the cause of southern Maine’s clam decline. Many marine invertebrates settle gregariously near their own kind, especially adults.  This has been shown in barnacles, ascidians (sea squirts), tubeworms, oysters and other bivalves, but has not been shown definitively in soft-shell clams.

If it is found that adult clams do attract settling juveniles, planting adult clams could be a way to locally enhance clam populations.

The studies were designed to examine the combined effects of adult clams and predator deterrent netting on the number of juvenile wild clams.

We conducted the experiments at the same two locations in Freeport, Maine – Spar Cove and Recompense, in the lower intertidal gradient, in each of the three years.

We were unable to determine if soft-shell clams actually settle gregariously in any of the field experiments because milky ribbon worms infiltrated our experimental units (all of which were covered with protective mesh netting or screening) making it impossible to draw unambiguous conclusions about whether adult clams attract newly settled juvenile clams.

2014 Experiments

In early May thirty 10-ft x 10-ft plots were set out in the two coves in a 6 x 5 matrix. There were six treatments:

1) Plots with no adult clams;
2) Plots with no clams plus netting (flexible, 4.2 mm aperture) to discourage predators;
3) Plots with 1 bushel of live, commercial size clams that were hand-planted throughout the 100-square foot plot;
4) Plots with 1 bushel of commercial size clams plus netting;
5) Plots with 2 bushels of commercial size clams;
6) Plots with 2 bushels of commercial size clams plus netting.

Treatments were replicated 5 times, with 20-ft between rows and columns.

The experiment’s design also tested the possibility that wild spat are not gregarious, but are enhanced by predator protective netting.

Five benthic core samples were taken from each of the 30 plots at both sites in November (N = 300 core samples, each of which was 0.02 ft²) and their contents washed through 1mm mesh sieves to determine the number of wild clams that settled in a particular plot.

When sampling occurred, it was found that most of the adult clams had not survived, having been eaten by milky ribbon worms (we found milky ribbon worms in most plots and none of the clam shells had been damaged, which is typical of ribbon worm mortality). Since the adult clams did not survive we were unable to draw any conclusions about the effect of planting adult clams on juvenile clam enhancement.

2017 Experiments

In 2017 we repeated the experiment with protections in place that we thought would protect clams from milky ribbon worms.

One of the more important findings from the investigation into the cause of the clam decline in Freeport was that in order for clams to survive they need to be protected from milky ribbon worms. Many of our experiments in 2014 relied on using nets to protect clams, as DEI has done in many coastal communities since the 1980s. Milky ribbon worms live in the sediments so nets that cover the top of the flat do not stop their predatory activities.

With low survival under nets, we moved to wooden boxes that protect clams from the top and bottom, as well as the sides. For this experiment in 2017 we used 8-in diameter x 6-in deep plastic plant pots as experimental units (EU). These were dug into the mud so that they provided bottom and side protection to the adult clams inside.

We deployed 240 plant pots in May, with half being placed in Spar Cove and half at Recompense Flat. At each site, half of the plant pots (n=60) had a piece of window screening placed on the bottom of the plant pots to keep out the milky ribbon worms (because plant pots have rather large drainage holes), half had no protection against milky ribbon worms. Ambient sediment was placed in the pots. Each pot was protected from the top with a piece of mesh netting. Adult clams were planted in six different densities: 0, 1, 2, 4, 8, 16. We used 1,240 adult clams, sized 50-65 mm SL for this experiment.

After planting, half of the plant pots were protected across the top by a piece of Pet Screening®, half were protected by a 3.2mm flexible screening. All pieces of top mesh were 20-inches x 20- inches. The top mesh was held in place by a rubber band.

 

The experiment was set up prior to clam spawning season, and continued November well after clam larvae had settled to the flats.

In November, the 120 pots from each site (N = 240) were removed, their contents washed through 1 mm sieves, and all adult clams (live and dead) and clam juveniles (recruits) counted and measured. Upon processing the samples, we discovered that the window screening inserts we had placed in the bottom of the plant pots were not effective in stopping milky ribbon worms from reaching the adult clams inside. The milky ribbon worms were able to push and slide through the small spaces between the holes in the bottom of the plant pot and the window screening. Again, most of the adult clams were dead making it impossible to assess whether their presence enhanced juvenile clam recruitment.

2018 Experiments

In 2018, we deployed the same experiment we used in 2017 with the same number and size EU, and the same field layout. The only difference was that the window screen was hot glued to the bottom of half the plant pots to stop the milky ribbon worms from penetrating the EU.

The contents of all 240 EU were washed through 1 mm sieves in November of 2018, and the data are only now being analyzed. While the hot-glued screens did deter milky ribbon worm attack, we know it was not 100% effective. Definitive results of this study from the two locations will be known soon.