Progress report for FNE25-114
Project Information
This will be the first study to address the function and heritability of purple shell color in this species. The objectives of this study are to 1) determine if the purple shell is harder (and therefore more resistant to crushing predators) than non-pigmented shell; 2) if selection for the purple shell color is in fact possible; and 3) if purple shell color increases survival on aquaculture sites. The goal of this research is to increase our understanding of the purple shell color found in some northern hard clams so that scientifically informed decisions can be applied to management, restoration, and selective breeding for aquaculture. The findings will also support the essential cultural and economic needs of Indigenous peoples such as the Wampanoag Tribe of Aquinnah, MA. Subsequent work has the potential to create an enticing marketing opportunity for the quahog industry by enabling the reliable production of a colorful and aesthetically exciting food product, in addition to increased survival of hard clam juveniles.
The hard clam or northern quahog, Mercenaria mercenaria, is a bivalve mollusc with a native range extending continuously from Nova Scotia to Florida. It has been farmed along the east coast of the United States for decades and is recognized as one of the nation’s most socioeconomically important marine resources (Cody, 2021).
Harvesting of wild stocks traditionally represented the majority of total landings, however aquaculture production has exceeded wild capture values for more than twenty years (FAO, 2022). The maintenance and continued growth of the hard clam aquaculture industry as well as efforts to restore wild populations now heavily rely upon hatchery produced seed. It is therefore of great importance to develop robust breeding programs for this species with the capacity to understand and incorporate varied desirable traits.
The proposed project will be a first step to inform the establishment of site-appropriate brood stocks for aquaculture by addressing the nature of the interior purple pigmentation found in the quahog shell. This trait warrants attention and consideration for incorporation into breeding programs as its enhancement and maintenance in populations is of cultural significance and it may provide benefits to clam farmers, the commercial market, and support population restoration efforts.
In the early 1980s, notata clams were introduced to regions, such as Martha’s Vineyard, MA, where they were not historically common. This variant, which has red/brown “zigzag” or “check mark” pigmented bands on the outside of the shell (Chanley, 1961; Humphrey and Walker, 1982), originated from mid-Atlantic broodstock and was bred into cultured clams selected for fast growth to visibly distinguish them from wild ones and more readily assess the success of seeding programs.
It is possible that this and other translocations has had unintended effects on shell color, hardness, and other characteristics of the local population of hard clams on Martha’s Vineyard and other areas. When wild and domesticated animals interbreed, impacts on wild populations can include the loss of genetic diversity within and between populations and to the disruption of patterns of local adaptation due to outbreeding depression.
In recent years, local harvesters, artisans, and members of indigenous tribes, and especially the Wampanoag Tribe of Gay Head/Aquinnah have reported seeing a reduction of purple shell frequency as well as weaker shells. Anecdotal evidence suggests that the purple pigmented area at the posterior end of the hard clam is harder than the unpigmented shells. Weaker shells make clams more difficult to shuck in raw bar markets, more difficult to work with for jewelry production, and presumably more vulnerable to predation.
This is concerning as the use of molluscan shells and their products is a tradition that has played an important role in Native American cultures (Hammell, 1983). Both the white and purple parts of shells are used by Wampanoag people to make traditional wampum, a shortened form of “wampumpeag,” an Algonquian word meaning “string of shell beads.” The cylindrical beads are either white (‘wa“pa”piag’) or the rarer purple (‘suka”piag’); the latter are made exclusively from pigmented quahog shells. For more than one thousand years, wampum beads were assembled into long belts in order to convey important messages and document pieces of Wampanoag history and culture. Wampum beads, strings, and belts were also traditionally used to summon meetings, as reminders of laws, for ritual exchange, as gifts, and for personal ornamentation (Bradley, 2011; Mackenzie et al., 2002; Slotkin and Schmitt, 1949).
This will be the first study to address the function and heritability of purple shell color in this species. The goal of this research is to increase our understanding of the purple shell color found in some northern hard clams so that scientifically informed decisions can be applied to management, restoration, and selective breeding for aquaculture.
Some mortality is expected to occur during the 3 or 4 year growing period that it takes clams to reach market size. Farmers can typically harvest about 75% of the clams that they deploy on their lease. However, farmers in New Jersey have reported that hard clam mortality has increased in recent years and that growth has slowed noticeably. Several factors may contribute to this decline, but it is critical for farms to use site-appropriate seed and incorporate traits that may be beneficial for aquaculture. If, in fact, the purple shell is harder and therefore more resistant (i.e., against crushing predators), selection for purple shells could increase survival on shellfish farms. Future work on clam pigmentation has the potential to create an enticing marketing opportunity for the quahog industry by enabling the reliable production of a colorful and aesthetically exciting food product.
Positive results of this study could increase the survival of juvenile hard clams on aquaculture sites, which would result in greater economic viability and profit. Positive results will also inform breeding programs and management plans for commercial aquaculture, public aquaculture and restoration aquaculture which may in turn help to preserve a culturally and economically important trait (shell color) for seaside and indiginous communities in the northeast.
Cooperators
- - Producer
- - Technical Advisor
Research
The Martha's Vineyard Shellfish Group (MVSG) chose northern quahog (Mercenaria mercenaria) broodstock for two separate spawns in spring 2025. All broodstock chosen to spawn ranged from littleneck to topneck size. As a pre-emptive exercise to accurately select broodstock for two designated "all purple" or "all white" spawning groups, a test group of six individuals were submerged in 10% Epsom salt solution in glass dishes, and were allowed time to gape open to better observe inner coloration. Gaping individuals were observed by viewing them from the sides of the dishes to allow minimal disturbance. Predictions of inner coloration were made prior to the Epsom salt bath and were assessed after individuals sat in the solution for two days. After confirming initial predictions, context clues of outer characteristics of quahog individuals were taken into account during the greater broodstock selection process. Individuals with supposedly heavier purple coloration on the inside exhibited a thin, darker band along the edges of where both valves meet along their bills, as opposed to the white group chosen where individuals showed a total lack of any pigmentation where both valves came together along their bills. The first group of quahogs, selected for their heavier inner purple coloration, were spawned on April 28th, 2025. Out of 27 individuals, 15 expelled sperm or egg matter that contributed to development of larvae; 7 individuals were female and 8 were male. This first group of quahogs born became the designated "purple" cohort. On May 12, 2025, another 27 individuals that were chosen to have predominantly white coloration on the inside were spawned; 12 individuals that spawned were female and 10 were male. This second group was designated as the "white" cohort. Both purple and white-designated groups were kept separately. All quahog seed was held on downweller shellfish aquaculture systems until they reached 0.5mm in size on average, then graduated onto upweller systems, as per usual propagation methods at MVSG.
Upon reaching 2mm in size, quahog seed from both groups were planted into floating quahog rafts filled with sand in Menemsha Pond (Aquinnah) on July 29th, 2025. Growing quahog seed in floating sand rafts is a typical and well-favored method for quahog propagation on Martha's Vineyard among the various towns' Shellfish Departments, as opposed to using the bottom environment with mesh predator netting overtop quahog seed. Rafts allow shellfish more access to food as they're suspended higher in the water column, while dually protecting the small seed from benthic predators that dwell below. Both groups were held in rafts and allowed to grow in the field through October 10, 2025 (2 months and 11 days). When harvested from rafts at this time, quahogs averaged 11.45mm (n= 111. Min 4.5mm; Max 20.82mm) in size.
In addition to two quahog groups created at MVSG, a third cohort of quahog seed was obtained from the Aquacultural Research Corporation (ARC) commercial hatchery in Dennis, Massachusetts to compare future progress of purple and white groups in project locations and to better infer future results of genetic versus environmental influence of inner coloration. Average quahog seed from ARC was 13.25mm (n=30. Min 9.67mm; max 16.05mm). For this project, two grow-out locations were selected to propagate quahog groups for the duration of this project: An aquaculture lease site on the Aquinnah side of Menemsha Pond on Martha's Vineyard, and another aquaculture lease site in Stone Harbor, New Jersey. 4,000 quahog seed per group (purple, white, ARC) will be overwintered in the Stone Harbor site, and 2,000 per group (purple, white, ARC) will be overwintered in the Menemsha Pond site.
The three groups of quahog seed will be kept separated in each site for the duration of this project. All groups per site are being overwintered at higher densities, and will be thinned to the same number of quahogs per group come springtime.
Pigmentation analysis of broodstock quahogs in year one is ongoing. To date, both cohorts of quahogs ("purple" and "white") were cataloged and shucked post-spawn. Muscle tissue was completely removed and shells were scrubbed using a soft-bristle brush to provide an optimal surface for pigmentation analysis. A custom camera stand was assembled to provide consistent lighting, orientation, and object distance. Digital images were taken of both the inner dorsal and ventral halves of individual quahogs in both cohorts. These images have been cataloged and matriculated into a digital database. Percent area of purple pigmentation of each individual will be measured using the open-source ImageJ software, which can calculate total area of each shell half and the relative proportion of "purple" coloration in mm^2. Total area of shell and pigmentation area will be calculated as a two dimensional object for simplification. Relative thresholds that distinguish between "purple" versus "white" coloration will be established based on the results of this analysis. Binning of percent area of purple pigmentation data from this sample group will form the basis for the creation of a "purple scoring" table (i.e. 0-5) that will be used in future field sampling and assessment efforts. This scoring method can also be used as a baseline during stakeholder outreach efforts to determine the relative "value" of varying amounts of pigmentation in quahog shells for jewelry making purposes. All shells will be saved and stored for future analyses. Collection of pigmentation data from the 2025 cohort is expected to be completed by April of 2026, and methods will be replicated for the 2026 cohort.
Age of broodstock cohorts will be determined using the acetate peel method or other equivalent technique. Age data will be correlated with shell dimension and pigmentation measurements to assess the relationship between these factors. Shell hardness of broodstock individuals will also be tested with a simple crush test or with a device such as a durometer, to test for a possible relationship between the amount of pigmentation in shells versus shell durability. These data collection efforts are also expected to be completed by April of 2026.
Selecting broodstock for certain inner coloration was a worthwhile portion of this project that we plan to elaborate on and specify in year two. The inner purple hue that quahogs can express is highly variable in both intensity and area within the shell. Both outer and inner characteristics were taken into consideration when developing a process for inner color indication.
The 10% Epsom salt bath solution worked on some individuals, but was not an efficient way to select an entire cohort of broodstock. It was not guaranteed that quahogs submerged would open, and after sitting in the solution for multiple days we began to question if the solution was compromising quality of the broodstock. Even so, we determined that the quahogs that did end up gaping open were not open enough to visualize the entire inner coloration. This is because adductor muscles in quahogs are short and don't allow for a wide gape when these muscles are relaxed.
We determined that inner coloration can be strongly suggested from context clues of the outer animal.