Introduction
The digenetic trematode Proctoeces maculatus is a widely distributed parasite, recorded from nearly every major oceanic basin, including both sides of the Atlantic, the Mediterranean (its type region), and the Pacific. This species utilizes invertebrates, such as mollusks and polychaetes, as first or second intermediate hosts and reaches sexual maturity as adults in fish. Proctoeces maculatus is notably euryxenous, having been reported in 65 species of fish and 26 invertebrates – an unusual trait among marine trematodes (Vermaak et al. Reference Vermaak, Kudlai, RQ-Y and Smit2023). A recent morphological and molecular study by Vermaak et al. (Reference Vermaak, Kudlai, RQ-Y and Smit2023) suggested that P. maculatus is likely both cosmopolitan and composed of cryptic species, complicating its identification and the assessment of its connectivity across regions.
On the eastern coast of the United States, P. maculatus infects the native blue mussel, Mytilus edulis, a keystone species and an economically important bivalve for New England’s blue economy (Evensen et al. Reference Evensen, Figueroa, Goncalves, Chan, Vu, Hounain and Poynton2023; Fairbanks Reference Fairbanks2016). Studies indicate that P. maculatus imposes significant fitness costs on M. edulis, including reduced reproductive output (Valderrama et al. Reference Valderrama, Olivia, Campos and Brown2004) and, in rare cases, mortality (Costau et al. Reference Costau, Robbins, Delay, Renaud and Mathieu1993). High infestation rates are also associated with reduced mussel growth, diminished filtration capacity, and lower byssal thread production (Lauckner Reference Lauckner and Kinne1983; Thieltges Reference Thieltges2006; Stier et al. Reference Stier, Drent and Thieltges2015).
Despite its impact as a pathogen of an economically important species, the genetic characterization and connectivity of P. maculatus along the east coast of the United States remain unknown. This is particularly relevant as climate change is thought to be driving the range expansion of P. maculatus in this region. In 1983, Cape Cod, Massachusetts, a major phylogeographic boundary in the western Atlantic, marked its northernmost distribution (Pondick Reference Pondick1983). More than two decades later, it was reported in Great Bay, New Hampshire, approximately 180 km north of Cape Cod (Markowitz et al. Reference Markowitz, Williams and Krause2016). Most recently, P. maculatus was documented infecting M. edulis in Casco Bay, Maine, approximately 90 km north of Great Bay.
The purpose of this study was to provide the first genetic characterization of Proctoeces maculatus from the eastern United States, with a specific focus on its northeastern range (Maine to New York). Using the 28S ribosomal RNA genetic marker, we assessed genetic variation and connectivity among populations in New England and compared them to other global samples.
Materials and methods
More than 300 specimens of Mytilus edulis were collected from six localities in the northeastern United States in 2023 and 2024 (Figure 1, Table 1). At each site, prevalence was determined by observing the presence of the parasite in the mantle tissue of the mussels under a dissecting microscope. Sporocysts, and adult trematodes when present, were isolated from the mantle tissue and frozen at -30°C for 24 hours. Genomic DNA was extracted using the DNeasy Blood and Tissue Kit (Qiagen), following the manufacturer’s protocol. DNA concentrations ranged from 12 to 60 ng/μL per sample. The D1–D3 region of the 28S nuclear ribosomal RNA gene was selected for amplification due to its previous success in elucidating the genetic intra- and inter-relationships of Proctoeces (Antar and Gargouri Reference Antar and Gargour2016; Vermaak et al. Reference Vermaak, Kudlai, RQ-Y and Smit2023; Wee et al. Reference Wee, Cribb, Bray and Cutmore2017) and other digenetic trematodes (Choudhury et al. Reference Choudhury, Valdez, Johnson, Hoffmann and de Leon2007; Blasco-Costa et al. Reference Blasco-Costa, Cutmore, Miller and Nolan2016; Shylla et al. Reference Shylla, Ghatani and Tandon2013). A ~900 bp fragment was amplified using primers Digl2 (5′-AAGCATATCACTAAGCGG-3′) (Tkach et al. Reference Tkach, Pawlowski, Mariaux, Swiderski, Littlewood and Bray2001) and 1500R (5′-GCTATCCTGAGGGAAACTTCG-3′) (Snyder and Tkach Reference Snyder and Tkach2001), following the PCR protocol described by Tkach et al. (Reference Tkach, Littlewood, Olson, Kinsella and Swiderski2003). Amplicons were visualized on a 2% agarose gel, excised, and purified using the QIAquick Gel Purification Kit (Qiagen). Sequencing of the purified amplicons was performed using both forward and reverse primers with Big Dye Terminator Cycle sequencing chemistry at Azenta LLC (Plainfield, NJ).
Figure 1. Map showing sampling localities of host mussels (Mytilus edulis). State abbreviations: ME – Maine, MA – Massachusetts, RI – Rhode Island, CT – Connecticut, NY – New York. CCBP – Cape Cod Phylogeographic Break.
Table 1. 28S rRNA sequences data used for phylogenetic and haplotype analysis of Procteces maculatus. South African site abbreviations: TNP - Tsitsikamma section of the Garden Route National Park, DHNR – De Hoop Nature Reserve
Returned sequences were initially identified using the NCBI BLASTn tool. Sequences of P. maculatus generated in this study were then combined with 28S sequences published by Vermaak et al. (Reference Vermaak, Kudlai, RQ-Y and Smit2023) and other 28S sequences of Proctoeces from the GenBank database (Table 1). The sequences were aligned and edited using the MUSCLE alignment tool in Biopython (Cock et al. Reference Cock, Antao, Chang, Chapman, Cox, Dalke, Friedberg, Hamelryck, Kauff, Wilczynski and de Hoon2009). After editing, a 790-bp fragment with 135 polymorphic sites remained for analysis. For phylogenetic analysis, an unrooted Bayesian tree was constructed in MrBayes ver 3.2 (Ronquist et al. Reference Ronquist, Teslenko, Van Der Mark, Ayres, Darling, Hohna, Larget, Liu, Suchard and Hulsenbeck2012) using the GTR + G nucleotide substitution model, as determined by AICc best model fit in jModelTest2 (Darriba et al. Reference Darriba, Taboada, Doallo and Posada2012). For tree-building parameters, Markov chain Monte Carlo chains were run for 5 million generations with the burn-in parameter set for 25% of sampled trees. The resulting phylogenetic tree was visualized in FigTree ver 1.4.3 (Rambaut 2009). Intra- and inter-specific kimura-2-parameter (K2P) genetic distances were also calculated using MEGA11 (Tamura et al. Reference Tamura, Stecher and Kumar2021) to determine levels of genetic relatedness. Finally, to determine population admixture and connectivity, a median-joining network was constructed in PoPART ver. 3.5 (Leigh and Bryant Reference Leigh and Bryant2015) to determine any geographic patterning of haplotypes.
Results and discussion
Overall prevalence of P. maculatus within blue mussels ranged from 4.1% to 90.8% (Maine: 4.1%, N = 98; Massachusetts: 49.8%, N = 66; Rhode Island: 65.5%, N=101; Connecticut: 60.4%, N=45; New York: 90.8%, N = 119). Progenesis was also observed in both Rhode Island and Connecticut populations, where both sporocysts and adults co-occurred in the same host.
Sequence data from the northeastern United States showed a 99.6–99.9% identity match to Proctoeces maculatus based on BLASTn analysis. A Bayesian phylogenetic tree provided strong posterior support for the monophyly of P. maculatus, encompassing sequences from the northeastern U.S., South Africa, and the Mediterranean (Figure 2). Within the northeastern U.S. clade, samples from Maine exhibited genetic divergence from other New England specimens, with a K2P distance of 0.3%. South African P. maculatus formed a distinct clade, separate from U.S. populations, indicating limited connectivity between these regions (K2P distances: 0.4–0.5%). Additionally, two P. maculatus specimens from Mississippi (USA) and Chile did not cluster within the primary P. maculatus clade. The Mississippi specimen showed K2P divergence distances of 4.7–5% from New England and New York specimens, while the Chilean specimen exhibited K2P distances of 5.8–6.1% when compared to specimens from New England, New York, South Africa, and the Mediterranean. These two specimens likely represent either misidentified trematode species or cryptic species.
Figure 2. Bayesian phylogenetic tree of Proctoeces maculatus and related species obtained from analysis of the 28S rRNA marker. Values above branch nodes represent posterior probability support values derived from Bayesian inference analyses. The P. maculatus clade is highlighted, and arrows indicate highly divergent individuals of P. maculatus that have likely been misidentified.
The 28S haplotype network revealed a clear geographic pattern of genetic diversity and connectivity among Proctoeces maculatus populations, consistent with the phylogenetic tree findings (Figure 3). The most common haplotype, referred to as ‘US1’, was widely shared among specimens from New York and all New England localities except Maine. Interestingly, US1 was also detected in a single specimen from Tunisia. This could indicate historical gene flow between the northeastern Atlantic and the Mediterranean, facilitated by host species movement. This finding aligns with the biology of the host species, as P. maculatus is known to parasitize fish hosts with broad geographic ranges and migratory behavior. Such host mobility could promote long-distance dispersal of the parasite, contributing to the observed genetic connectivity between distant populations. The Tunisian specimen was sampled from Bizerte Lagoon, which harbors Mytilus galloprovincialis, a species closely related to M. edulis (the two hybridize where their ranges overlap) (Barhoumi et al. Reference Barhoumi, Le Menach, Clerandeau, Ameur, Budzinski, Driss and Cachot2014; Skibinski and Ahmad Reference Skibinski, Ahmad and Beardmore1978). Mediterranean populations of Mytilus galloprovincialis are also known hosts of P. maculatus (Robledo et al. Reference Robledo, Caceres-Martinez and Figueras Huerta1994). Transoceanic movement of mussels, whether intentional (e.g., through the aquaculture trade) or unintentional (e.g., hull fouling of infected mussels in shipping), likely facilitates occasional connectivity between populations in the western Atlantic and the Mediterranean.
Figure 3. Haplotype network for Proctoeces maculatus based on 28S rRNA sequence data. The smallest circles represent a haplotype frequency of one. Each solid connecting line between haplotypes represents one mutational step, while values above lines represent additional mutational steps. Broken connecting lines represent more than 20 mutational steps.
The ME1 haplotype was exclusively shared by P. maculatus sampled from Maine mussels. This haplotype’s isolation reflects significant differentiation from both U.S. and Mediterranean populations, reinforcing the idea of regional genetic structuring. Maine represents the northernmost extent of P. maculatus’s range, and mussels in this region are subjected to environmental and oceanographic conditions distinct from those further south, specifically colder waters. In fact, Cape Cod is a well-documented phylogeographic barrier on the U.S. east coast, delineating distinct biogeographic regions and restricting gene flow for numerous marine species, including M. edulis (Altman et al. Reference Altman, Robinson, Pringle, Byers and Wares2013; Jennings et al. Reference Jennings, Shank, Mullineaux and Halanych2009; Riginos and Henzler Reference Riginos and Henzler2008). Although P. maculatus resides within the mantle tissue of its mussel host, which provides some insulation from environmental fluctuations, water temperature may still play an indirect role in shaping its distribution. Mussels, as ectothermic hosts, are directly affected by environmental temperature, which influences their health, growth, and reproduction – factors that could affect the parasite’s establishment and persistence. Additionally, colder water temperatures in Maine may slow the developmental rates of P. maculatus larvae or alter host-parasite interactions, potentially favoring the fixation of a distinct haplotype in this region. For example, studies have found that higher temperatures can enhance transmission rates of trematode parasites in marine snails by increasing the activity and reproduction of both hosts and parasites, whereas colder temperature may reduce transmission efficiency (Friesen et al. Reference Friesen, Poulin and Lagrue2021). One consequence of this is that in cooler climates, such reduced transmissions could lead to isolated parasite populations, promoting genetic divergence and the emergence of unique haplotypes. Alternatively, the isolated ME1 haplotype could reflect P. maculatus’s recent arrival to Maine (the species was first reported in Casco Bay, approximately 60 km north of our Maine sample site, in 2021). The northward range expansion of P. maculatus, likely driven by warming ocean temperatures or anthropogenic transport via aquaculture or shipping, could have introduced a small founder population with limited genetic diversity. This founder effect, coupled with the Gulf of Maine’s distinct environmental conditions at the northern edge of Mytilus edulis’s North American range, may have facilitated the establishment of the unique ME1 haplotype. Similar patterns of reduced genetic diversity and haplotype isolation have been observed in other range-expanding parasites (Knapp et al. Reference Knapp, Bart, Giraudoux, Glowatzki, Breyer, Raoul, Deplazes, Duscher, Martinek, Dubinsky, Guislain, Cliquet, Romig, Malczewski, Gottstein and Piarroux2009; Wielgoss et al. Reference Wielgoss, Taraschewski, Meyer and Wirth2008). Unfortunately, despite two years of sampling mussel hosts in the region (Davinack, unpubl. data), we were unable to obtain trematode samples from sites in New Hampshire, which are also located north of the Cape Cod barrier but south of Maine. We recommend that future studies focus on recovering additional samples from New Hampshire to better understand the connectivity of Proctoeces maculatus populations north of Cape Cod, which may provide additional genetic diversity information on the species in the northeast United States.
In the haplotype network, the two extreme divergent haplotypes recovered were from single specimens originating from Chile and Mississippi, USA. These required more than 45 mutational steps to connect to the main network, which mirrors their high levels of genetic divergence from other P. maculatus specimens and also provides additional evidence that they were either incorrectly identified or represent cryptic species.
Altogether, our results broadly support the hypothesis proposed by Vermaak et al. (Reference Vermaak, Kudlai, RQ-Y and Smit2023), who argued that Proctoeces maculatus is indeed a cosmopolitan species, albeit one composed of unresolved cryptic lineages. Unraveling these lineages remains challenging for two primary reasons: (1) the morphology of isolates within Proctoeces is highly conserved, making it difficult to distinguish between taxonomically informative traits and phenotypic variation due to local adaptation, and (2) genetic data from the type locality (Trieste, Italy) and the type host (Labrus merula) is currently unavailable. Only with the acquisition of genetic data from topotypic material can definitive conclusions about cryptic species complexes within P. maculatus be drawn. Finally, while the identity of the Tunisian P. maculatus specimens from the Antar and Gargouri (Reference Antar and Gargour2016) study was initially questioned due to their geographic separation from the type locality and association with a sparid fish host rather than the labrid host characteristic of P. maculatus sensu stricto (Vermaak et al. Reference Vermaak, Kudlai, RQ-Y and Smit2023), subsequent analysis concluded that these specimens are best considered P. maculatus when all available evidence is taken into account. This conclusion is further supported by the shared haplotype between the Tunisian specimen and the northeastern U.S. populations of P. maculatus, providing strong molecular evidence for their conspecificity. Such genetic connectivity is inconsistent with the hypothesis that the Tunisian specimen represents a different species, as interspecific genetic divergence would preclude haplotype sharing.
In conclusion, the genetic diversity and connectivity of Proctoeces maculatus populations, including evidence of haplotype sharing between the northeastern U.S. and Tunisia, underscore the complex interplay of biogeographic and anthropogenic factors driving parasite dispersal. The presence of the isolated ME1 haplotype in Maine suggests that environmental factors and recent range expansions both contribute to shaping parasite populations at the edge of their distribution. This is particularly concerning in light of the ongoing decline of blue mussel populations in the northeastern U.S., driven by overharvesting, climate change, and disease. Parasites such as P. maculatus may exacerbate these declines by imposing additional fitness costs on mussels, further stressing populations already at their environmental limits. Molecular characterization of parasites remains critical for understanding the broader ecological impacts of environmental change on marine ecosystems. Future population studies using multiple mitochondrial markers and high-resolution SNP datasets (e.g., microsatellites or RAD-Seq) will be needed to provide more fine-scale insights into the connectivity and population dynamics of P. maculatus.
Acknowledgements
We would like to thank Jason Williams (Hofstra University) for providing trematode samples from New York. We would also like to thank Jeremy Miller and Jason Goldstein (Wells National Estuarine Research Reserve) for providing assistance and lab space in Maine.
Financial support
This research received no specific grant from any funding agency, commercial, or not-for-profit sectors.
Competing interest
The authors declare none.
Ethical standard
Not applicable. The host is a marine invertebrate species; therefore, ethical approval under the laws of the United States (IACUC) was not required.
