Oyster farms in the northeastern US are plagued by biofouling caused by marine polychaetes belonging to the genus Polydora. Generally referred to as mud-worms, these ubiquitous polydorid worms can kill oysters, reduce oyster growth, fragment the oyster’s shells making them difficult to shuck, and cause internal shell blisters that decrease product appeal and marketability. Oyster farmers invest an extensive amount of time and resources to control infestations, significantly reducing farm profits.
The purpose of this project was to develop efficient and effective methods for control of Polydora infestation in eastern oysters. Three control methods, hypersaline dips, freshwater dips, and lime dips were evaluated for efficacy in controlling Polydora infestations at an oyster farm in Delaware Bay, NJ. Life history aspects of the worm, including season planktonic abundance and settlement patterns, were investigated to establish treatment regimes that target the worm at the onset of infestation.
The treatment methods did not yield significant results: dip treatments were no more effective at controlling P. cornuta biofouling than the standard washing method. Plankton samples were collected weekly from April through September and P. cornuta settlement and community succession were monitored. Fouling, larvae and worm abundance, and life history characteristics were measured and analyzed. Egg-bearing adult worms were observed in March, indicating the presence of overwintering infestations on the farm.
Fluctuations of the condition and intensity of fouling mud tube colonies occurred throughout the spring and summer months. Planktonic larvae abundances peaked every four weeks during the course of the study (May-September), coinciding with the full moon, indicating that spawning events occurred at regular intervals. This finding could point to employing targeted treatments prior to summer full moons. The largest peak in planktonic larvae occurred in mid-May and the heaviest fouling was observed in July.
Information gained on the life cycle of P. corunuta has be valuable in pointing out possible strategies of control:
Early spring or late winter dip treatments to kill any overwintering worms and eggs.
Early spring/winter wash treatments to dislodge and eggs and worms when their survival is not likely.
These results have been shared at aquaculture meetings in Delaware, Connecticut, Florida and New Jersey.
Two species of Polydorid worms which plague oyster farms are problematic in the northeast of the US, Polydora websteri and Polydora cornuta (formerly ligni). Generally referred to as mud-worms, these ubiquitous polydorid worms can kill oysters, reduce oyster growth, fragment the oyster’s shells making them difficult to shuck, and cause internal shell blisters that decrease product appeal and marketability. Oyster farm profitability is limited by mud-worm associated oyster mortalities and by the extensive time and effort invested in mud-worm control. The issue has existed as long as farmers have been cultivating oysters; however, mitigating treatments have been met with limited success. Location, local conditions, and cultivation methods play an important role in modulating infestation timing and severity, and the efficacy of control methods. New Jersey’s oyster farms are concentrated on the extensive intertidal sand flats of the lower Delaware Bay (Cape Shore) where they are exposed twice daily during low tide. The water with its moderately high salinity and rich food quality supports rapid oyster growth and yields exceptional quality oysters. Here hatchery reared oysters are grown in plastic mesh bags that are secured to rebar frames positioned on the bay bottom. The farms are accessed from the shore at low tide and the bags are tended for a 1-3 year production cycle. Oyster culture systems provide ideal substrate for Polydora settlement and growth. During the spring and summer oyster farmers must clean the worms and their thick mud tubes from the oyster bags in order to avoid oyster mortality caused by the mud smothering the oysters. These mud tubes were found to cause extensive oyster mortalities in Delaware Bay by Stauber and Nelson (1940) who found as many as 36 worms per cubic centimeter.
The dry weight of mud accumulated by these worms has been estimated to be 98 tons per acre (Orth,1971). Shellfish culturists around the world have employed a variety of methods to control Polydora spp.(cornuta and websteri) and other related Spionidae worms. These treatments have included mechanical cleaning using brushes, scraping, and water pump sprays; prolonged exposures to air and refrigeration; modulation of tidal exposure (rack height), and immersion in freshwater, saturated seawater, or chemical baths. Refrigeration of infested eastern oysters for 3 weeks was effective for killing adult P. websteri; however, it was unknown whether the same treatment would be effective at killing eggs. The need for cold storage precludes the usefulness of this method for many farmers (Morse, D. personal communication). Raising the elevation of intertidal oysters to increase aerial exposure during low tides offered some reduction in Polydora; however, this treatment also reduced oyster growth (Littlewood et al. 1992).
Several chemical treatments have been evaluated including: copper sulphate (CuSO4 ) (Quayle and Newkirk, 1989), chlorine solutions at 25 g mL–1 or 2% formol (Robles-Mungaray and Salinas-Ordaz, 1993), and marine dipterenes obtained from algal extracts (Takikawa et al., 1998). Though some of these products may be efficient in the control of mud worms, they are not widely used because they are toxic, expensive, or require careful handling. Hypersaline dips have been used with some success (Nel et al 1996, Nell 2007, Mackenzie, C., and Shearer 1995) and Gallo-Garcia et al (2004) found weekly immersion baths in 0.2% solution of calcium hydroxide (lime) to significantly reduce polychaete worm infestations in Pacific oyster, C. gigas grown in a Mexican lagoon. Currently New Jersey oyster farmers control Polydora cornuta biofouling by using gas powered water pumps to spray ambient seawater on the oyster bags and remove the mud worm and its dense tube colony structure from the oyster bags. During the hot summer months it is impossible to keep all the oysters clean and the resulting mortality can be significant.
Labor costs associated with Polydora mitigation for a midsize farm (250,000 market oysters) are approximately 700 man-hours, and equipment and supplies cost about 2000 dollars annually.
Control measures when applied are typically administered once infestations have developed with little attention to the biology of the fouling organism.
I operate a small-scale oyster farm with annual production of 40,000 market oysters sold under the brand name Betsy’s Cape Shore Salts. My farm has been operating since 2006 and employs rack and bag culture systems on a riparian grant on the intertidal sand flats of the lower Delaware Bay, NJ in Green Creek. I am a founding member of the Cape May Oyster Cooperative. Central to the Cooperative’s mission is the adoption of stringent production standards for the sustainable and profitable production of safe, high quality cultivated oysters for the upscale half shell (raw consumption) market. Member farmers appreciate the importance of science in guiding management strategies and practices. This work serves to support profitability on my farm as well as on the farms of my fellow Co-op members by providing important information on biofouling control, one of the most important issues facing the industry as a whole. By improving the efficiency and effectiveness of treatment methods labor costs will be reduced and oyster survival and quality will be improved increasing farm profit margins.
This project has been conducted in collaboration with Rutgers University, Haskin Shellfish Research Laboratory and New Jersey Sea Grant extension specialist, Lisa Calvo and Rutgers University, Institute of Marine and Coastal Sciences researcher, Rose Petrecca. Allegheny College undergraduate, William Schroer provided technical support for the project.
The purpose of this project was to develop a better understanding of the life history and biology of the polycheate worm Polydora cornuta and to develop efficient and effective methods for control of its infestation to the oysters. Specific objectives were:
1. Conduct plankton and settlement surveys to determine temporal variability in abundances of Polydora cornuta larvae and peak settlement periods to inform and improve the efficacy of treatment timing and options.
2. To evaluate the effectiveness of time targeted hypersaline, lime dip, and freshwater treatments as a control for P. cornuta.
The study was conducted at my farm located in on the Cape Shore flats of the lower Delaware Bay. Weekly larvae samples were collected with a plankton net (100 micron mesh) from April 2013 through August 2013. Thereafter, monthly samples were collected through October 2013. Surface tows were collected on falling tide approximately two hours before low tide. Tow time was standardized to 2.5 minutes. Plankton samples were preserved in 90% ethanol, stained with Rose Bengal and processed as follows. Samples were subsampled with a plankton splitter Polydora cornuta larvae were identified and counted. Setiger counts of 25 larvae were recorded.
In order to assess the timing of larval settlement, triplicate shell bags containing 8 liters of clean oyster shells were placed on rebar racks at the farm in March 2013. Five shells from each bag were removed weekly from May through October and examined for Polydora in order to determine settlement periods. Additionally shell strings (n=3) containing 5 shells each were deployed for one-week intervals from March through October and similarly examined for newly recruited worms.
Examinations were made using a dissecting microscope. Mud was removed from all the shells and mud pack wet weight was recorded. The five shells from strings were pooled and the three shells from bags were replicates. When necessary worms were preserved in formalin solution prior to quantification. From May through mid July a sub-sample of mud pack was weighed and the number of Polydora worms in the sub-sample was recorded.
Water temperature at the site was continuously monitored at 15 min intervals through the course of the study using an Onset data logger.
Four treatments were evaluated for efficacy in controlling P. cornuta infestations:
(1) high pressure wash with ambient water,
(2) hypersaline dip,
(3) lime dip, and
(4) freshwater dip.
The experiment began with only three of the treatments (ambient water wash, hypersaline and lime dips); however, early in the experiment it became clear that lime was not effective, so freshwater dips were substituted for lime for the remainder of the study. Fifteen oyster bags containing an eight-liter volume of one-year old oysters were randomly assigned to one of the three treatments. The three replicate bags for each treatment were labeled and deployed on the farm in a block design with 1-replicate from each secured on each of three racks whose location within the farm were distributed on inshore, mid and off shore farm locations. Treatment bag placement on each of the racks was assigned randomly.
Immersion solutions, lime (0.2% calcium hydroxide, 2 g L-1) and hypersaline (90 g L-1rock rock salt) were prepared in 40-gallon cans using ambient seawater. Freshwater was obtained from a well. Oyster bags were removed from the rebar racks and placed in the bath for 5 minutes for lime treatment and 15 minutes for hypersaline and freshwater treatments. After immersion the bags were allowed to dry on the rebar racks for a minimum of 30 minutes. Drying time and temperature of treatment baths were recorded. At the same time bags assigned to seawater spray washing were treated using a Honda gasoline powered water pump and hose at 160 gal/min. Treatment bags were sprayed thoroughly while laying on rack, flipped 1-3 times and washed until visual inspection showed minimal mud tube colonies.
Treatments were applied beginning on May 14. Subsequent treatments were administered 14 days after the initial treatment and as needed through the remainder of the summer, generally every 7 days.
Polydora infestation of treatment bags was measured via visual inspection of the entire bag and ranked as negative- no mud apparent, light- approximately <50% of oysters fouled with mud, moderate- greater than 50%-80% of oysters fouled, heavy- 80-100% oysters fouled but patches of clean shell still evident, severe- all oysters coated in mud but are not stuck together, severe + – all oysters are fouled and the mud tube colony has made them clump together.
Oyster mortality was assessed in July and August. On each date all dead and live oysters in each treatment bag were counted. Statistical analyses of fouling ranks, oyster mortality, and worm abundance included ANOVA tests.
This project greatly enhanced our understanding of Polydora cornuta population dynamics in the lower Delaware Bay and represents one of the first efforts to systematically study P. cornuta recruitment dynamics in relationship to oyster aquaculture practices. Additionally, the investigation evaluated the efficacy of three dip treatments for controlling P. cornuta associated biofouling in comparison to the industry’s standard wash method.
One of the most interesting and important findings was the presence of viable egg-bearing adult worms in late winter suggesting the presence of overwintering infestations (Figure 1). Planktonic larvae were first detected on April 24, four weeks after sample collection commenced. Abundances peaked every four weeks during the course of the study (April-September) with peaks occurring at regular intervals coinciding with the full moon (Figure 2). The largest peak in planktonic larvae occurred in mid-May as water temperature first exceeded 20°C (Figure 3). Subsequent peaks in larvae abundance from June through August were uniform in magnitude, approximately half the abundance of the mid-May peak. Abundance was lower in September with the peak being about half as high as in the previous month.
Sentinel deployments of shell bags and shell strings, respectively, provided an examination of cumulative and weekly variation in mud-tube colony fouling in an untreated condition. Mud tube colony fouling intensity increased from spring to mid-summer with a peak during the second week of August (Figure 4), as indicated by the mean weight of mud tube colonies on sentinel shells that were deployed for the duration of the study in shell bags. The mean weight of mud on the shells declined from mid-August through October. Biofouling on the weekly-deployed shell strings demonstrated three distinct peaks in late June, late July and early September with lower levels of fouling occurring on intermittent dates. The weekly-deployed sentinels indicate that P. cornuta recruits to the farm oysters and structures continuously through the summer months, though the magnitude of recruitment varied. This is consistent with the observed planktonic larvae abundances. During the course of the summer season, mud tube colonies on single shells weighed as much as 60 grams per shell and 1-2 cm in thickness. The number of worms within the colonies was as high as 300 worms per g in the shell bags and up to 250 worms per gram on the weekly-deployed shell strings (Figure 5). The total number of worms on individual shells was calculated (mud wt.x #worms/g) to be as high as 6000 on the shell bag shells and 2000 on the shell string shells (Figure 6).
Evaluated treatments (brine, lime, and freshwater dips) for the control of P. cornuta biofouling were not as effective as the pump method. A comparison of biofouling between treatments is shown in Figure 7. Logistic regression analysis indicated significant treatment effects relative to the ambient water pump control method. Oysters treated with brine experienced 1.55 times more fouling that the control (pump) treatment (p = 0.032, odds ratio 1.55). Oysters treated with lime had significantly more fouling than those treated with the pump method. Lime treated oysters experienced 8.49 times more fouling than those oysters treated by pumping ambient water (p=0.06, odds ratio 8.49). The effect of freshwater treatment was marginally significant (p=0.056, odds ratio 1.36) with oysters having 1.36 times more fouling than the pump method control.
An ancillary experiment was conducted in which worms were separated from mudpacks and exposed to brine, lime, and freshwater solutions in the laboratory. Under both the brine and freshwater exposures worms appeared to die within 5 minutes. No worm mortality was observed with the lime exposure. These results suggest that during field treatments, the worms may have been protected by the mudpacks. The pump method offered the advantage of physically removing mudpacks from the oysters, while as conducted, the dip treatments offered little physical removal of the mudpacks, hence the worms were afforded some refuge from the treatment solution and the degree of fouling was not reduced following treatment. It may be that under a slightly different regime, which offered longer post treatment drying periods, agitation to physically remove mud, and or a combination of pump and dip treatments may provide enhanced effectiveness of the treatments.
Mean oyster mortality for four months from March to July was: 13.07%,pump, 10.25%,lime/fresh, 9.95%,brine. Mean oyster mortality during the month from mid July to mid August and was respectively 3.4%. 3.3% and 1.9% for pump, lime/fresh, and brine treatments. Monthly mortalities were essentially constant throughout the study period. Within each mortality sampling there was no significant difference among treatments. (March-July p=.14, August p=.58)
The effect of treatment on growth (shell height) was not significant (p = 0.18).
While none of the treatments that we evaluated performed better than the standard method of pump washing oysters currently used by farmers in Delaware Bay we still found interesting results which will lead to other ways of approaching the Polydora problem. We sampled the wash water coming off the oyster bags and determined that no Polydora were killed by the pressure wash but were swimming intact and presumably could resettle easily and promptly. While the brine and freshwater dips were observed to kill Polydora, the sheer number of worms and larvae resettling on the oysters presumably swamped the effects of killing a relatively small number of those worms. Additionally, when the mud tube colonies were very thick in midsummer, the dips probably did not penetrate all the way through the mud.
The natural history portion of this project showed some interesting possibilities. Plankton surveys showed spikes in larval Polydora every four weeks coinciding with full moon. Even the standard wash treatment which would dislodge eggs just before hatching could reduce worm populations if those washes were targeted to the right time. Overwintering adults and eggs were found in samples taken in March. Early season plankton peaks were quite high which suggests that overwintering populations and eggs contribute significantly to an early surge in the worm population. Treatment during the early winter or very early spring might be most effective at reducing overwintering Polydora populations. This could possibly slow or reduce the onset of heavy summer fouling.
Education & Outreach Activities and Participation Summary
This work was presented by Lisa Calvo at the joint meeting of the 34th Milford Aquaculture Seminar, which was held Feb24-26 in Shelton CT.
The work was also presented by William Schroer at the 106th Annual Meeting of the National Shellfisheries Association in Jacksonville, FL in March 2014.
Both talks were titled: The Natural History and Control Of the Mud Worm Polydora cornuta on a Delaware Bay, NJ Oyster Farm.
An oral presentation was made to fellow industry members at a Delaware Bay Oyster Growers Forum also in March 2014.
This project has demonstrated that, in the middle of the summer, weekly treatments which can clean oysters in the short term are not sufficient to eliminate bio-fouling from Polydorid worms. Presumably there are such large numbers of worms and larvae in the bay that a treatments which kill the immediate fouling organisms (brine or fresh water)is overwhelmed by the recruitment of other worms. Wash treatments which do not kill worms may contribute to the fouling by creating continual disturbance.
The plankton survey yielded results which, coupled with the visual examination of oysters at the end of winter, provides some interesting possibilities for treatments. We saw a large early spring peak of larvae in the water column. An oyster shell pulled from the farm in early March was found to be harboring many Polydorid eggs and adults in small crevices, even though it appeared to be clean. Together, these pieces of information suggest that early spring dip treatments which kill overwintering eggs and adults might be very effective at reducing the early peak in larvae. This work also suggests that the wash treatment in early winter or early spring could remove the overwintering eggs and adults when the cold bay water would decrease their chances of survival. A wash time could be chosen when the bay is cold but water is warm enough (50 degees F) that predators would be present in the bay to feed on dislodged worms and eggs.
Any reduction in the over-wintering population could potentially delay the onset of heavy summer infestations. This, in turn, would reduce summer labor costs.
How do we use this information to improve bio-fouling control?
The most obvious place to begin is to target overwinter infestations and thereby reduce reservoir population. Several methods can be evaluated:
Early spring or late winter dip treatments to kill any overwintering worms and eggs.
Early spring/winter wash treatments to dislodge and eggs and worms when their survival is not likely.
Targeted treatments prior to summer full moons. Combine pump washes with dip treatments.
Coordinate treatment efforts with neighboring farmers to enhance the effectiveness of control.
Continue the collection of data on the natural history of Polydora: collect plankton samples, settlement data and evaluate annual variability of Polydora populations.
Rotate oyster bags directly onto the bottom to evaluate possible control by bottom feeding predators.
Other questions to ask:
Are Polydora larvae from local/farm source?
Do current farm practices exacerbate Polydora bio-fouling (increased worm fecundity as result of continuous disturbance)?