United States Atlantic Coast Research Articles

Surf zone hazards before and after a beach nourishment in Virginia, USA

Beach nourishment is the leading coastal protection technique in the United States to combat erosion, enhance resilience to storm surge, and maintain recreational value. Despite these benefits, anecdotal reports suggest that beach nourishments elevate the surf zone hazard to beach patrons by steepening the beach face and altering the shoreface morphology such that conditions are more favorable for rip current formation. This study analyzes lifeguard rescue reports collected on the United States Atlantic Coast before and after a 2019 beach nourishment in Virginia Beach, Virginia, to assess whether the nourishment was correlated with an increased hazard to beach patrons. The data indicate that regardless of nourishment status, rescues were most probable during periods of high rip current probability (moderate to large wave heights and low-obliquity wave angles), along with low water level. To formally quantify pre-versus post-nourishment hazards, the proportion of rescues observed in nourished versus unnourished beach zones was compared with bootstrapped distributions of the pre-nourishment rescue proportions. Although the proportion of rescues in the nourished section of the beach exceeds the pre-nourishment average, it is not outside the overall range of pre-nourishment values obtained by random resampling. Consequently, there is insufficient evidence to conclude that the existing coastal management beach nourishment strategy increased the hazard to beach patrons at Virginia Beach.

Phylogeography of a Widely Distributed Atlantic Species: The Case of the Ghost Crab Ocypode quadrata (Fabricius, 1787) (Decapoda: Brachyura: Ocypodidae)

Ocypode Weber, 1795 (Brachyura: Ocypodidae) is popularly known as ghost crab, and encompasses 21 valid species, including Ocypode quadrata (Fabricius, 1787). This species has wide distribution along the Atlantic coast of America, from the USA (Massachusetts) to Brazil (Rio Grande do Sul), Central America, and Antilles. Such distribution, along with some biological characteristics of its life cycle and the presence of geographic barriers, could lead to genetic structuring. Herein, we evaluate the hypothesis of the presence of geographic barriers using COI and 16S partial gene fragments. The Maximum Likelihood tree suggests the monophyly of O. quadrata, while the values of intraspecific genetic distance along with the star-shaped haplotype network suggested a lack of genetic structure in Brazilian, Panama, and French Guiana populations, probably caused by larval dispersion. USA and Mexico populations may be a new lineage, but we cannot say it with few sequences and with no morphological characters.

Microplastics in Atlantic Ribbed Mussels (Geukensia demissa) from the Delaware Inland Bays, USA

Due to the prevalence of plastic pollution in coastal ecosystems, aquatic organisms are at high risk for accumulating microplastics (MPs). Filter-feeding bivalves, such as mussels and oysters, may be exposed to, and subsequently accumulate, MPs due to the high volume of water they pass through their bodies. This study assessed the levels of MPs within Atlantic ribbed mussels (Geukensia demissa), a common filter feeder found along the United States Atlantic Coast, from 12 sites within Rehoboth Bay, Indian River Bay, and Little Assawoman Bay, collectively known as the Delaware Inland Bays. Composited mussels from each site were digested using potassium hydroxide and filtered. Microplastics were physically identified, sorted based on color, and counted using a digital microscope. Microplastics, almost entirely dominated by synthetic microfibers, were found in all mussels well above laboratory blanks. Across all sites, 40% of microfibers were black, and 27% of fibers were clear. The composite concentrations of MPs ranged from 0.25 to 2.06 particles/g wet tissue, with a mean of 0.08 ± 0.06. In general, higher concentrations were found in mussels collected at sites that were adjacent to more urbanized land use versus those from rural sites. At two sites, individual mussels, in addition to composites, were analyzed and had MP concentrations ranging from 11 to 69 particles/mussel. This study represents the first evaluation of MPs in this ecologically important coastal species and suggests its viability as a biomonitoring species for microplastic pollution.

Sea turtle density surface models along the United States Atlantic coast

Spatially explicit estimates of marine species distribution and abundance are required to quantify potential impacts from human activities such as military training and testing, fisheries interactions, and offshore energy development. There are 4 protected species of sea turtle (loggerhead, green, Kemp’s ridley, and leatherback) commonly found along the east coast of the USA, our study area, and which require impact assessments. Data from 7 different survey organizations were used to create density surface models for the 4 sea turtle species utilizing 1.2 million km of line-transect surveys. A substantial portion (29.7%) of available sightings were not identified to the species level. Not including these sightings would underestimate density, so a conditional random forest model was used to assign unidentified sightings to species. Higher densities of loggerhead, green, and Kemp’s ridley sea turtles were predicted south of the Outer Banks in cool months, transitioning northwards in late spring to occupy seasonal neritic habitats. The highest leatherback densities were predicted off the coasts of Georgia and Florida. Leatherbacks were also predicted throughout offshore areas. The predicted distribution patterns generally matched satellite tracking and strandings data, indicating the models reproduced established seasonal movements. Surveys rarely detect sea turtles smaller than 40 cm, so these age classes are not represented. The models are the first for the study area to apply availability bias estimates developed in or near the study area and attempt to classify unidentified sightings to the species level, providing an updated, critical tool for conservation management along the eastern seaboard.

Plumage microorganism communities of tidal marsh sparrows

Microorganism communities can shape host phenotype evolution but are often comprised of thousands of taxa with varied impact on hosts. Identification of taxa influencing host evolution relies on first describing microorganism communities and acquisition routes. Keratinolytic (keratin-degrading) microorganisms are hypothesized to be abundant in saltmarsh sediments and to contribute to plumage evolution in saltmarsh-adapted sparrows. Metabarcoding was used to describe plumage bacterial (16S rRNA) and fungal (ITS) communities in three sparrow species endemic to North America's Atlantic coast saltmarshes. Results describe limited within-species variability and moderate host species-level patterns in microorganism diversity and community composition. A small percentage of overall microorganism diversity was comprised of potentially keratinolytic microorganisms, warranting further functional studies. Distinctions between plumage and saltmarsh sediment bacteria, but not fungal, communities were detected, suggesting multiple bacterial acquisition routes and/or vertebrate host specialization. This research lays groundwork for future testing of causal links between microorganisms and avian host evolution.

Low recruitment drives the decline of red porgy (Pagrus pagrus) along the southeast USA Atlantic coast: Inferences from fishery-independent trap and video monitoring.

Red porgy (Pagrus pagrus) is a reef-associated, economically-important, winter-spawning, protogynous Sparidae species that appears to have declined in abundance in recent years along the southeast United States Atlantic coast. We used spatially-explicit generalized additive models built with fishery-independent chevron trap (1990-2021) and video data (2011-2021) to quantify the ways in which red porgy relative abundance and mean size varied across temporal, spatial, environmental, and habitat variables. Mean red porgy relative abundance from traps declined by 77% between 1992 and 2021, and declines were similarly large (69%) on video between 2011 and 2021. The largest two-year decline in relative abundance occurred early in the COVID-19 pandemic (2019-2021)- 32% in traps and 45% on video-despite already low abundance. Highest red porgy relative abundance from traps and video occurred in deep areas (i.e., 60-100 m) between southern North Carolina and north Georgia, and red porgy preferred low relief but continuous hardbottom habitats (i.e., pavement). We confirmed recent low recruitment of red porgy in the region based on the large increase in mean length (29%) and severe (~99%) declines of juvenile red porgy caught over the 32-year trap survey. Evidence suggests that recruitment failure is partially or mostly responsible for red porgy abundance declines, and, moreover, the regulation of harvest is unlikely to achieve sustainable management goals until recruitment increases.

Calibration of fish counts in video surveys: a case study from the Southeast Reef Fish Survey

Changes to sampling gears or vessels can influence the catchability or detectability of fish, leading to biased trends in abundance. Despite the widespread use of underwater video cameras to index fish abundance and the rapid advances in video technology, few studies have focused on calibrating data from different cameras used in underwater video surveys. We describe a side-by-side calibration study (N = 143 paired videos) undertaken in 2014 to account for a camera change in the Southeast Reef Fish Survey, a regional-scale, multi-species reef fish survey along the southeast United States Atlantic coast. Slope estimates from linear regression for the 16 species included in the analyses ranged from 0.21 to 0.98, with an overall mean of 0.57, suggesting that original cameras (Canon Vixia HF-S200) observed an average of 43% fewer fish than newer cameras (GoPro Hero 3+). Some reef fish species had limited calibration sample sizes, such that borrowing calibration information from related or unrelated species was justified in some cases. We also applied calibrations to 11-year video time series of relative abundance of scamp Mycteroperca phenax and red snapper Lutjanus campechanus (N = 13,072 videos), showing that calibrations were critical to separating changes in camera sightability from true changes in abundance. We recommend calibrating data from video cameras anytime changes occur, and pairing video cameras to the extent possible to control for the spatial and temporal variability inherent in fish populations and environmental conditions. Following these guidelines, researchers will be able to maintain the integrity of valuable long-term video datasets despite intentional or unavoidable changes to video cameras over time.

Intraspecific competition in common coastal dune grasses overshadows facilitation on the dune face

Restoration of degraded dunes through revegetation of native dune building species is critical to preserve multiple ecosystem services (i.e., coastal protection, habitat). As dunes exhibit high levels of abiotic stress, it is likely that facilitative interactions among plants occur, reducing stress caused by harsh abiotic conditions, and benefiting restoration efforts. The research was to determine how plant spatial arrangement (clumped vs. dispersed) at the dune slope affects the growth of two dominant dune grasses along the United States Atlantic coast, Ammophila breviligulata (American beachgrass) and Uniola paniculata (sea oats). We determined the local distribution and density of each species through field surveys to inform a manipulative study of planting arrangement and to assess plant growth and survival under different spatial arrangements. Planting arrangement did not influence the survival of U. paniculata, but dispersed plantings had a positive effect on growth parameters (height, stem number, ramet number), compared to the clumped arrangement. No plantings of A. breviligulata survived by the end of the second season, regardless of planting arrangement. Clumped arrangements did not improve growth of U. paniculata or growth and survival of A. breviligulata over the two growing seasons studied; thus, creating clumped arrangements may not be necessary for revegetation efforts. However, observing natural spatial patterns and using this information to guide restoration planting may be beneficial as climate change may impact revegetation results due to increased abiotic stress. Further research focusing on biotic interactions and density of plantings at multiple locations along dunes will contribute to dune restoration planning.

Distributing transmitters to maximize population-level representativeness in automated radio telemetry studies of animal movement

Telemetry is a powerful and indispensable tool for evaluating wildlife movement and distribution patterns, particularly in systems where opportunities for direct observation are limited. However, the effort and expense required to track individuals often results in small sample sizes, which can lead to biased results if the sample of tracked individuals does not fully capture spatial, temporal, and individual variability within the target population. To better understand the influence of sampling design on results of automated radio telemetry studies, we conducted a retrospective power analysis of very high frequency (VHF) radio telemetry data from the Motus Wildlife Tracking System for two species of birds along the United States Atlantic coast: a shorebird, the piping plover (Charadrius melodus), and a nearshore seabird, the common tern (Sterna hirundo). We found that ~ 100–150 tracked individuals were required to identify 90% of locations known to be used by the tracked population, with 40–50 additional individuals required to include 95% of used locations. For any number of individuals, the percentage of stations included in the sample was higher for common terns than for piping plovers when tags were deployed within a single site and year. Percentages of stations included increased for piping plovers when birds were tagged over multiple sites and, to a lesser extent, years, and increased with average length of the tracking period. The probability that any given receiver station used by the population would be included in a subsample increased with the number of birds tracked, station proximity to a migratory stopover or staging site, number of receiving antennas per station, and percentage of the tracked population present. Our results provide general guidance for the number and distribution of tagged birds required to obtain representative VHF telemetry data, while also highlighting the importance of accounting for station network configuration and species-specific differences in behavior when designing automated radio telemetry studies to address specific research questions. Our results have broad applications to remotely track movements of small-bodied migratory wildlife in inaccessible habitats, including predicting and monitoring effects of offshore wind energy development.

First Time's a Charm? Loggerhead Neophyte Mothers Have Higher Hatch Success

Caretta caretta (Loggerhead Sea Turtle) is a globally threatened sea turtle species that nests on United States Atlantic coast beaches. While several environmental factors influence egg incubation and hatching success, maternal identity may also play an important role and has been overlooked. New molecular advances, such as ongoing genetic capture–recapture using eggshells, allow the identification of individual females using fresh eggshells, which can then be used to track nesting and hatching success across individual females. Across 2 consecutive nesting seasons, we monitored 170 nests for environmental conditions as part of a larger Georgia Department of Natural Resources monitoring program. Mothers had been identified using molecular data and classified as either neophytes or remigrants. We identified 34 of the nests as belonging to neophyte Loggerhead mothers, which exhibited a 61% hatching success rate, compared to only 46% success for remigrant mothers (P < 0.01). Further analysis suggested no differences in environmental parameters between neophyte and remigrant nests. Thus, while it is possible that different-aged mothers make different nesting-site choices that could affect the nest environment, it is likely that Loggerhead reproductive biology, such as maternal investment, may change over time. More data are required to fully investigate maternal effects on hatching success and to elucidate the mechanisms, both within this species and across multiple species of sea turtles.

Genomic evidence for ancient human migration routes along South America's Atlantic coast.

An increasing body of archaeological and genomic evidence has hinted at a complex settlement process of the Americas by humans. This is especially true for South America, where unexpected ancestral signals have raised perplexing scenarios for the early migrations into different regions of the continent. Here, we present ancient human genomes from the archaeologically rich Northeast Brazil and compare them to ancient and present-day genomic data. We find a distinct relationship between ancient genomes from Northeast Brazil, Lagoa Santa, Uruguay and Panama, representing evidence for ancient migration routes along South America's Atlantic coast. To further add to the existing complexity, we also detect greater Denisovan than Neanderthal ancestry in ancient Uruguay and Panama individuals. Moreover, we find a strong Australasian signal in an ancient genome from Panama. This work sheds light on the deep demographic history of eastern South America and presents a starting point for future fine-scale investigations on the regional level.

AIS-based characterization of navigation conflicts along the US Atlantic Coast prior to development of wind energy

This study characterizes navigation conflicts in a region with a large traffic volume along the United States (US) Atlantic Coast, using an empirical approach that analyzes Automated Identification System (AIS) data for a full year (2010). The region includes areas proposed for wind energy development, and this characterization of vessel conflicts could be useful as a baseline to evaluating the effect of offshore wind areas on navigation conflicts and vessel collisions. There have been no collisions reported within the study area. This study analyzes the one year of AIS data to obtain vessel tracks and makes simultaneous pairwise comparisons to determine the number and rate of vessel encounters and near-misses (navigation conflicts). For vessel pairs in an encounter (3 nm distance; crossing, head-on, overtaking), a ‘blind vessel’ assumption was used to calculate the number of potential near misses and potential collisions. Graphical analysis of the frequency of vessels involved in potential near misses produced vessel response curves that provided the time of peak frequencies prior to the time of closest approach for each encounter type. These peak times were up to 600 s prior to the time of closest approach. The analyses of potential near misses and collision used the data during the period between the time of peak frequency and the time of closest approach to filter the data used in the calculation of the probability of potential collisions. The period included collsion avoidance maneuvering. Statistical analyses are made to estimate the point values and uncertainty of near miss rates and the probability of potential collisions for each type of encounter from which bounding estimates of the collision rates are made. These bounding collision rates are found to be comparable with estimates from previous studies. Causation probabilities were also estimated and compared with previous studies. The response curves may also have utility in the simulation of navigation conflicts. The use of surrogate data to evaluate the effect of wind energy areas is considered, though their usefulness depends on comparability of traffic density and the level and type of traffic control systems implemented.

Using stationary video data to infer relative abundance and distribution of four Seriola species along the southeast United States Atlantic coast

Seriola spp. (i.e., amberjacks) are highly mobile, widespread, and heavily sought-after species in the family Carangidae. Juvenile amberjacks are mostly pelagic and associate with floating items such as Sargassum spp., but adults tend to be more demersal and often associate with natural hardbottom or artificial reefs. We used 9 years (2011–2019) of baited videos (N = 11,691) in a delta-generalized additive modeling framework to elucidate temporal and spatial trends in relative abundance, habitat use patterns, and environmental influences for four amberjack species along the southeast United States Atlantic coast. S. rivoliana was observed most frequently (25.9% of videos examined), followed by S. dumerili (17.9%), S. zonata (4.6%), and S. fasciata (0.5%), but models only explained a modest amount (5–15%) of variability in video counts among species. S. rivoliana relative abundance increased substantially (~450%) over the study period, while the other species displayed no obvious temporal trends. S. zonata was the most inshore and heavily aggregating species, whereas S. fasciata occurred in the deepest water and S. rivoliana was the least aggregating species. Relative abundance of S. rivoliana and S. dumerili was positively related to substrate relief and bottom water temperature, whereas S. zonata displayed a dome-shaped relationship with substrate relief and water temperature and a negative relationship with the amount of hardbottom. Continued video monitoring is needed to assess the potential distributional shifts of amberjacks due to climate change and improve stock assessments of these important reef-associated species.

Reef fish community structure along the southeastern US Atlantic continental shelf break and upper slope appears resistant to increasing lionfish (Pterois volitans/miles) density

Temperate reefs host diverse fish communities along the southeast United States Atlantic coast (SEUS), yet the sustainable management of reef fishes faces myriad challenges. One challenge has been the introduction of Indo-Pacific lionfish (Pterois volitans/miles; hereafter "lionfish"), which have spread quickly throughout the SEUS since their introduction in the late 1900s. We analyzed long- term (2001–2019) video data along the continental shelf break and upper slope (45–125 m deep) of the SEUS to assess changes in lionfish densities over time, characterize reef fish community structure, and determine if native reef fish community structure has changed due to lionfish. Lionfish densities increased substantially during the study, from zero individuals observed in 2001 to approximately 1.2 individuals observed per 100 m 3 (and present in all included transects) by 2019, yet no fish community metrics were negatively related to lionfish density. Demersal habitat influenced fish community structure more than any other variable examined, with more individuals and different fish communities occurring on high-relief compared to low-relief hardbottom habitats. The effects of latitude, depth, and bottom temperature on reef fish community structure were generally weak or nonexistent. Although previous empirical work has found that lionfish negatively affect native fishes at small scales (&lt;30 km2), it is unclear why we did not find similar results in our larger- scale study. It may be related to vagaries of the spatial scale of observation, lionfish effects being primarily limited to high- relief habitats, time lags, or lionfish densities not being high enough yet to cause observable ecological effects.

Systematics of Hypnea (Cystocloniaceae, Rhodophyta) from coastal North Carolina, with a first report of Calliblepharis saidana from the United States Atlantic Coast

Abstract Complete and/or partial DNA sequences for the plastid-encoded rbcL gene and the 5′ end of the mitochondrial cytochrome oxidase I (COI-5P) gene were used to re-examine the systematics of Hypnea species (Cystocloniaceae, Rhodophyta) from North Carolina, USA. These data, combined with light microscopic observations, indicate that two species (Hypnea cryptica and H. musciformis sensu stricto) are present in nearshore waters of coastal North Carolina. Molecular and morphological analyses with topotype material of Hypnea volubilis from North Carolina offshore waters revealed that it and Calliblepharis saidana are conspecific. Hypnea volubilis is proposed as a heterotypic synonym of C. saidana. This is the first report of Calliblepharis from the United States Atlantic coast and only the second report from the western hemisphere.

Age of Tarpon (Megalops atlanticus) Leptocephali Recruiting to Salt Marsh Pools in a South Carolina Estuary

To increase our knowledge of the early life history of Megalops atlanticus (Tarpon) along the United States Atlantic coast north of Florida, we estimated the age and hatch dates of Tarpon larvae (i.e., leptocephali) recruiting to salt marsh pools in the North Inlet estuary, SC. We collected leptocephali (n = 36) from marsh pools during June through October 2016 and kept a subset (n = 24) of these individuals for aging. We were able to estimate ages for 18 individuals by counting presumed daily rings on their otoliths. We estimated hatch date by subtracting estimated age from the capture date. Mean age was 25 days post hatch (dph) and varied from 17 to 30 dph. Hatch dates spanned from 25 July to 9 September 2016. These are the first age and hatch date estimates for Tarpon leptocephali from estuaries north of Florida and provide insight into the biology and possible spawning behavior of Tarpon in this area of the US Atlantic coast.

The Path to an Ecosystem Approach for Forage Fish Management: A Case Study of Atlantic Menhaden

Atlantic menhaden (Brevoortia tyrannus) support the largest fishery by volume on the United States East Coast, while also playing an important role as a forage species. Managers’ and stakeholders’ increasing concerns about the impact of Atlantic menhaden harvest on ecosystem processes led to an evolution in the assessment and management of this species from a purely single-species approach to an ecosystem approach. The first coastwide stock assessment of Atlantic menhaden for management used a single-species virtual population analysis (VPA). Subsequent assessments used a forward projecting statistical catch-at-age framework that incorporated estimates of predation mortality from a multispecies VPA while analytical efforts continued toward the development of ecosystem models and explicit ecological reference points (ERPs) for Atlantic menhaden. As an interim step while ecosystem models were being developed, a series of ad hoc measures to preserve Atlantic menhaden biomass for predators were used by managers. In August 2020, the Atlantic States Marine Fisheries Commission formally adopted an ecological modeling framework as a tool to set reference points and harvest limits for the Atlantic menhaden that considers their role as a forage fish. This is the first example of a quantitative ecosystem approach to setting reference points on the United States Atlantic Coast and it represents a significant advance for forage fish management. This case study reviews the history of Atlantic menhaden stock assessments and management, outlines the progress on the current implementation of ERPs for this species, and highlights future research and management needs to improve and expand ecosystem-based fisheries management.

No effect of marine protected areas on managed reef fish species in the southeastern United States Atlantic Ocean

Marine protected areas (MPAs) have been recommended as an essential conservation tool for ecosystems and fisheries, with MPA coverage increasing four-fold globally since 2000. Despite the increased usage of MPAs, empirical results and scientists are divided on the effectiveness of MPAs to reach their conservation and management goals. In 2009, the South Atlantic Fishery Management Council established eight deep-water partially-protected MPAs off the southeast United States (SEUS) Atlantic coast with the goal of protecting long-lived, deep-water, reef-associated fishery species. Data collected during fish trapping from 2000 to 2018 as part of a fisheries-independent survey were used to evaluate the impacts of protection on a subset of fish species in three of these MPAs. There was a modest positive shift to larger fish observed in two of the MPAs and to older fish for one species, Pagrus pagrus (Linnaeus, 1758), in one of the MPAs relative to adjacent fished areas; however, there was either no change or a decrease in managed reef fish abundance in each MPA relative to adjacent fished areas. Based on these metrics, it does not appear that the SEUS MPAs have yet been effective at protecting managed reef fish species. Given that these MPAs have low enforcement, future assessments should examine compliance within the SEUS MPAs to determine if lack of success is due to illegal fishing, species examined, or MPA design before making a final determination if deep-water MPAs are an effective strategy for fisheries managers in the SEUS.

New Insights Into the Seasonal Movement Patterns of Shortfin Mako Sharks in the Gulf of Mexico

Highly mobile apex predators such as the shortfin mako shark (mako shark;Isurus oxyrinchus) serve an important role in the marine ecosystem, and despite their declining populations and vulnerability to overexploitation, this species is frequently harvested in high abundance in both commercial and recreational fisheries. In 2017, the North Atlantic stock was deemed overfished and to be undergoing overfishing and was recently listed in CITES Appendix II. Effective management of this species can benefit from detailed information on their movements and habitat use, which is lacking, especially in the Gulf of Mexico, a potential mating and parturition ground. In this study, we used satellite telemetry to track the movements of mako sharks in the western Gulf of Mexico between 2016 and 2020. In contrast to previous studies that have primarily tagged juvenile mako sharks (&amp;gt;80% juveniles), ∼80% of sharks tagged in this study (7 of 9) were presumed to be mature based on published size-at-maturity data. Sharks were tracked for durations ranging from 10 to 887 days (mean = 359 days; median = 239 days) with three mature individuals tracked for &amp;gt;2 years. Mako sharks tagged in this study used more of the northwestern Gulf of Mexico than reported in previous movement studies on juveniles, suggesting potential evidence of size segregation. While one mature female remained in the Gulf of Mexico over a &amp;gt;2-year period, predominantly on the continental shelf, two mature males demonstrated seasonal migrations ∼2,500 km from the tagging location off the Texas coast to the Caribbean Sea and northeastern United States Atlantic coast, respectively. During these migrations, mako sharks traversed at least 12 jurisdictional boundaries, which also exposed individuals to varying levels of fishing pressure and harvest regulations. Movement ecology of this species, especially for mature individuals in the western North Atlantic, has been largely unknown until recently. These data included here supplement existing information on mako shark movement ecology and potential stock structure that could help improve management of the species.

Molecular data reshape our understanding of the life cycles of three digeneans (Monorchiidae and Gymnophallidae) infecting the bivalve, Donax variabilis: it's just a facultative host!

The coquina, Donax variabilis, is a known intermediate host of monorchiid and gymnophallid digeneans. Limited morphological criteria for the host and the digeneans’ larval stages have caused confusion in records. Herein, identities of coquinas from the United States (US) Atlantic coast were verified molecularly. We demonstrate that the current GenBank sequences for D. variabilis are erroneous, with the US sequence referring to D. fossor. Two cercariae and three metacercariae previously described in the Gulf of Mexico and one new cercaria were identified morphologically and molecularly, with only metacercariae occurring in both hosts. On the Southeast Atlantic coast, D. variabilis’ role is limited to being a facultative second intermediate host, and D. fossor, an older species, acts as both first and second intermediate hosts. Sequencing demonstrated 100% similarities between larval stages for each of the three digeneans. Sporocysts, single tail cercariae, and metacercariae in the incurrent siphon had sequences identical to those of monorchiid Lasiotocus trachinoti, for which we provide the complete life cycle. Adults are not known for the other two digeneans, and sequences from their larval stages were not identical to any in GenBank. Large sporocysts, cercariae (Cercaria choanura), and metacercariae in the coquinas’ foot were identified as Lasiotocus choanura (Hopkins, 1958) n. comb. Small sporocysts, furcocercous cercariae, and metacercariae in the mantle were identified as gymnophallid Parvatrema cf. donacis. We clarify records wherein authors recognized the three digenean species but confused their life stages, and probably the hosts, as D. variabilis is sympatric with cryptic D. texasianus in the Gulf of Mexico.