PREVALENCE OF THE BOPYRID ISOPOD PROBOPYRUS PANDALICOLA IN DAGGERBLADE GRASS SHRIMP FROM SALT MARSH CREEKS AND PANNES OF CAPE COD, MASSACHUSETTS

Parasites are ubiquitous in nearly all ecosystems and phyla (Dobson et al., 2008), and parasitic plants and animals play a large role in structuring host populations, usually by negatively influencing host fitness as parasites rely on hosts for refuge, nutrition, and/or mobility (Pennings and Bertness, 2001; Hatcher and Dunn 2011; Welicky and Sikkel, 2014). In salt marshes, parasites play an important role in influencing population, community, and even ecosystem dynamics (Lafferty et al., 2006; Morton and Silliman, 2020). For instance, some parasitic salt marsh plants may negatively influence host plant abundance and distribution by depleting nutrients from their hosts (Pennings and Bertness, 2001). In one recent study, trematode parasites of marsh periwinkle snails (Littoraria irrorata) from North Carolina marshes were experimentally reduced and salt marsh plants subsequently declined, indicating that parasite prevalence may indirectly benefit salt marsh plants and potentially increase their resistance to overgrazing in the face of prolonged drought (Morton and Silliman, 2020). Additionally, parasites have been shown to affect community structure by infesting multiple species in salt marsh food webs, influencing the number of trophic links and food web connectance (Lafferty et al., 2006). Although parasites in salt marshes are abundant and play an important ecological role, prevalence data on parasites and host-parasite interactions generally remain lacking (Dobson et al., 2008; Welicky and Sikkel, 2014). This may partially be explained by the cryptic nature of parasites (Roche et al., 2013), although many ectoparasites are fairly conspicuous, even post-infection (Roche et al., 2013).

Parasitic isopods are a common group of crustaceans and can act as ectoparasites on several taxa (Leonardos and Trilles, 2003; Welicky and Sikkel, 2014) including salt marsh shrimp species, such as the ecologically important daggerblade grass shrimp (Palaemon [previously Palaemonetes] pugio; see De Grave and Ashelby, 2013). Daggerblade grass shrimp are endemic to salt marshes along the western Atlantic and throughout the Gulf of Mexico (Welsh, 1975; Collins, 1981; Anderson, 1985). Palaemon pugio, hereafter referred to as grass shrimp, plays a critical role in salt marsh food webs as prey for a variety of shorebirds, crabs, and finfish, including commercially important species (Heard and Lutz, 1982; Manderson et al., 2000). Depending on their life stage and food availability, grass shrimp act as opportunistic omnivores or detritivores, influencing multiple trophic levels (Welsh, 1975; McCall and Rakocinski, 2007). Additionally, grass shrimp influence salt marsh structure, functioning, and dynamics through their feeding behavior and movement. For instance, grass shrimp can indirectly increase macrophyte productivity by grazing on epiphytes that inhibit photosynthetically active radiation and can also increase nutrient availability to macrophytes and microbes via bioturbation and fecal deposition (Welsh, 1975; Sikora, 1977; McCall and Rakocinski, 2007).

Grass shrimp serve as definitive hosts of the bopyrid isopod Probopyrus pandalicola (Packard, 1879), an ectoparasite found in female-male pairs in the shrimp’s branchial chambers (Anderson, 1977). After Pr. pandalicola have parasitized their intermediate hosts (calanoid copepods), the first parasite to settle on the host grass shrimp will metamorphose into a female, and the second parasite to settle will become male (Baeza et al., 2018). Bopyrid isopod parasites remain on their hosts, growing with them even as their hosts molt (Cash and Bauer, 1993). The female isopod feeds on host hemolymph, consuming 1–10% of the total energy intake of the shrimp, reducing shrimp respiration, metabolism, and growth (Anderson, 1975, 1977; Ludwig, 2009). These parasites also castrate shrimp hosts and have been known to negatively influence host population size (Chaplin-Ebanks and Curran, 2007; Calado et al., 2008; Sherman and Curran, 2015). In addition to the direct effects that Pr. pandalicola have on grass shrimp, they also may influence shrimp behavior, resulting in altered movements and more conspicuous hosts, with possible implications for predator-prey dynamics (Bass and Weiss, 1999; Brinton and Curran, 2015a).

Probopyrus pandalicola have been observed from New Hampshire to São Paulo, Brazil (Richardson, 1904, 1905; Beck, 1979). However, although Pr. pandalicola have been commonly found on grass shrimp from salt marsh systems throughout the southeastern United States (Markham, 1985), observations noting Pr. pandalicola grass shrimp infestation in salt marshes north of Maryland have been less frequent, leaving a paucity of data regarding Pr. pandalicola infestation of estuarine grass shrimp in the northeastern United States (but see Kunkel, 1918 for records of Pr. pandalicola from Connecticut and Fowler, 1912 for records of Pr. pandalicola from New Jersey). In Massachusetts, Pr. pandalicola have been reported from the Ashcunet River in the early 1900s (Sumner et al., 1913). Additionally, 66 individual specimens of Pr. pandalicola were taken from Woods Hole, Massachusetts, in August 1950, and 7 additional individual specimens were documented from Buzzards Bay, Massachusetts, from January 1882 (Smithsonian National Museum of Natural History, USNM 91223, 91224, 91225, 39428). Although these reports are notable, the limited number of observations documenting Pr. pandalicola over the past century suggests either a low abundance or broad underreporting of these parasites in the Northeast, although the known observations and distribution of Pr. pandalicola may be obscured by their taxonomic complexity, with specimens initially identified as Pr. pandalicola (Markham, 1985), later differentiated into unique species (Ribeiro et al., 2019).

In the spring of 2019, Pr. pandalicola were observed on daggerblade grass shrimp in salt marshes on Cape Cod, Massachusetts (E. Stoner, pers. comm.). As such, we sought to provide an updated report on the prevalence of Pr. pandalicola from grass shrimp in salt marsh habitats on Cape Cod.

In August 2021 we collected grass shrimp from 5 salt marsh sites dispersed across Cape Cod: Wellfleet (41°56′15″N, 70°2′0″W), Yarmouth Port (41°42′17″N, 70°13′15″W), East Sandwich (41°44′16″N, 70°25′23″W), West Dennis (41°39′42″N, 70°9′43″W), and Cummaquid (41°43′12″N, 70°15′0″W; Fig. 1). Each site was dominated by a combination of the foundational salt marsh cordgrass (Spartina alterniflora) and/or salt marsh hay (Spartina patens) and was accessible by foot at low tide. Except for the Cummaquid site, grass shrimp were collected at the marsh’s edge from a single location within a perennial first-order stream (hereafter referred to as a tidal creek). Because there was no tidal creek at the Cummaquid site, we instead sampled 13 individual pannes (tide-pool-like depressions in the marsh) to investigate the distribution of both grass shrimp and Pr. pandalicola within these salt marsh microhabitats. The August 2021 sampling revealed a relatively high prevalence of Pr. pandalicola at the Cummaquid site, and as such, pannes were resampled in the fall (October 2021) and spring (March 2022) to examine shifts in shrimp and parasite abundance and shrimp infestation prevalence. However, because of the dynamic and seasonal nature of salt marsh pannes at this site, resampling the same pannes or even the same number of pannes was not always feasible. Therefore, we sampled 9 pannes in summer 2021 (Fig. 2A, pannes 6–14), 7 pannes in fall 2021 (Fig. 2A, pannes 1–7), and 7 pannes in spring 2022 (Fig. 2A, pannes 2–8). This sampling allowed for general comparisons of parasite prevalence by panne and season.

All sampling was completed at low tide. At each tidal creek site, salinity and temperature were recorded at the bottom water column (∼1 m) of the tidal creek directly adjacent to the sampling area using a YSI handheld device (YSI ProQuatro®, Xylem Brand, Yellow Springs, Ohio). At each site, shrimp were sampled following the modified protocol outlined in Hammerschlag-Peyer et al. (2013) in which submerged dip nets were swept through the water until 3 consecutive sweeps yielded fewer than 3 additional grass shrimp.

At the Cummaquid site, temperature and salinity measurements were taken, and shrimp were collected within each of the salt pannes sampled. Additionally, the length, width, and depth of each panne were recorded. Shrimp were then collected from a 3 × 1 m area within each panne; the panne area and distance to the shoreline were measured in Google Maps. Collected shrimp were transferred to plastic bags, placed on ice, and taken back to the lab at Bentley University, where samples were stored in a freezer until processed.

Grass shrimp species were confirmed using available published keys (Anderson, 1985). The presence or absence of Pr. pandalicola was determined visually and confirmed morphologically by inspecting the shrimp’s branchial chambers. Female-male pairs of Pr. pandalicola were easily discernible under the carapace and confirmed in a small subset of shrimp using a dissecting microscope (Fig. 3A–D). The length of each shrimp was measured from the tip of the rostrum to the end of the uropods to the nearest 0.1 mm following protocols from Kirkham et al. (2021), and their width was measured below the gills at the first pereomere. Parasite length and width were also measured to the nearest millimeter using calipers by measuring the anterior margin of the head to the end of the pleotelson, and width was measured at the first pereomere. Female parasites were measured exclusively; parasite males were not measured because of their small size. Wet weight was recorded for all shrimp and bopyrid isopods to the nearest 0.001 g.

We used the ‘lmer4’ package (Bates et al., 2015) to construct a quasibinomial model to explore which parameters best explained the variation of Pr. pandalicola prevalence in infected shrimp. This model included salinity, shrimp density, and panne distance from shore as predictor variables. All statistical analyses were conducted using R v.4.20 (R Core Team, 2022). Similar to Johnson (2014), we evaluated near-surface ocean temperature data from 2016 to 2020 from Cape Cod, Massachusetts, and compared to it water temperature from 2021 (US National Data Buoy Center, Station 44018) to evaluate temperature-related drivers of Pr. pandalicola prevalence.

Five marsh sites across Cape Cod were sampled in August 2021 (Fig. 1). We collected 2,263 grass shrimp in total, 220 of which were parasitized by Pr. pandalicola (9.7% prevalence). Shrimp infested with Pr. pandalicola had a single female parasite or male-female parasite pair present in 1 branchial chamber and no cases of infestation of both branchial chambers. Prevalence varied between salt marshes and was far higher in Cummaquid, with 14.1% (213 of 1,515) of the grass shrimp infested. All other salt marshes had <5% Pr. pandalicola prevalence with the distribution as follows: Sandwich (1% parasite prevalence), Wellfleet (0.35% parasite prevalence), Yarmouth Port (4.1% parasite prevalence), and Dennis (0% parasite prevalence).

Across all seasons (summer, fall, and spring), 12.8% of the 2,665 grass shrimp collected across 13 pannes in Cummaquid were infested with Pr. pandalicola (n = 342). Shrimp were most abundant during the warmer months of the summer (n = 1,515 shrimp, mean temperature: 25.6 C), followed by fall (n = 741 shrimp, mean temperature: 16 C), and spring (n = 409 shrimp, mean temperature: 8.3 C; Fig. 2B). Every panne sampled in summer had shrimp infestation present, while 6 of 7 pannes sampled in fall and 3 of 7 pannes sampled in spring had shrimp infestation present. Overall, the highest prevalence of Pr. pandalicola was in spring (median ± standard deviation 21% ± 4.87%, n = 70 infected shrimp), followed by summer (15% ± 9.66%, n = 213 infected shrimp), then fall (6% ± 4.06%, n = 59 infected shrimp; Fig. 2B). Although the actual number of infected shrimp was variable, the highest prevalence of Pr. pandalicola infestation was recorded at individual pannes 6 (30.3%, n = 36 infected shrimp) and 9 (25.9%, n = 25 infected shrimp) in summer (Fig. 2C). All parasites were found on both male and female shrimp in male-female pairs, and summer was the only time in which ovigerous female Pr. pandalicola were present. There was a significant positive relationship between grass shrimp abundance and Pr. pandalicola abundance (parasite vs. shrimp counts, n = 9, adjusted R² = 0.35, P = 0.05; Fig. 4A), driven by the high abundance of Pr. pandalicola (n = 70) and grass shrimp (n = 324) observed in panne 12 (Fig. 5A). Measurements on all grass shrimp and Pr. pandalicola collected from Cummaquid across seasons and pannes revealed significant positive relationships between shrimp length (6–42 mm) and female parasite length (<1–8 mm) (n = 338, adjusted R² = 0.47, P < 0.001; Fig. 5A) and shrimp wet weight (0.4–60 mg) and parasite length (n = 338, adjusted R² = 0.45, P < 0.001; Fig. 5B).

Probopyrus pandalicola prevalence increased with panne distance from the marsh edge (n = 9, adjusted R² = 0.43, P = 0.05; Fig. 4B). Panne area, shrimp counts, and panne distance from the edge of the marsh all significantly correlated with parasite prevalence (Table I). Salinity varied widely across pannes in Cummaquid across all seasons (5–35 ppt) and was not correlated with grass shrimp infestations in Cummaquid in August. Based on year-over-year averages, the water temperature from Cape Cod in 2021 was 0.68 C higher (13 C) than yearly averages from 2016 to 2020 (12.3 C).

This research documented the prevalence of the bopyrid isopod (Pr. pandalicola) in an abundant and ecologically influential host species (grass shrimp) in salt marsh habitats of New England. Our results describe a relatively high prevalence of bopyrid parasitism of grass shrimp in Cape Cod, Massachusetts. Although in extreme cases Pr. pandalicola have been reported to infest up to 25% of a grass shrimp population, it is typical for them to infest <1% of grass shrimp in many locations, and in general, studies report low prevalence of infestations by bopyrid isopods on carideans (Heard and Lutz, 1982; Sheehan et al., 2011; Briggs et al., 2017; Finn and Buck, 2024). Here the prevalence of Pr. pandalicola in grass shrimp was higher than other published reports (for instance, 12.8% infestation across 3 seasons in Cummaquid and a single panne yielding a 30.3% infestation in summer; Table II), although bopyrid prevalence in grass shrimp in the present study was comparable to other species of grass shrimp (17.7% of a population of Palaemon ritteri were parasitized in Baja, California; Campos and Campos, 1989), and generally lower than brown shrimp hosts (up to 34% of Crangon spp were parasitized in Humboldt Bay, California; Jay, 1989). Although the high prevalence of Pr. pandalicola is noteworthy, this study also provides the first documentation of Pr. pandalicola in this region in almost 75 years (the last documentation of Pr. pandalicola in New England was from 1950). This suggests a lack of observations of this species in New England, which to our knowledge have not been reported on in the literature in more than 70 years.

In this study we describe the presence, and high prevalence, of a host-parasite association poorly described in its northern range. The seasonal and geographic context of host-parasite relationships is important to note as we attempt to quantify climate-driven changes. For example, in marine systems, shifts in water characteristics (i.e., temperature, salinity, oxygen, nutrients) can influence host susceptibility, parasite intensity, and pathology, which may impact host-parasite relationships, while large-scale climate shifts may alter the range of their occurrence (Byers, 2021). Furthermore, shifts in host-parasite relationships have the potential to influence salt marsh community dynamics. Here we explore relationships between parasite prevalence in grass shrimp and spatial/environmental parameters, which indicate the combination of panne area, shrimp abundance, and distance from the marsh edge all contribute to the high parasite prevalence measured in this study.

Bopyrid parasite abundance and size were correlated with grass shrimp abundance and size in this study (Figs. 4, 5). Similar relationships between definitive shrimp host size and bopyrid infestation have been observed in several other systems, although it should be mentioned that these studies determined shrimp size differently from our study, ranging from measuring shrimp carapace length (CL; Smith et al., 2008) to total length (TL; Bortolini Rosales et al., 2021, to total length converted to CL (Whalen et al., 2020). Nonetheless, mature, reproductively viable grass shrimp are more likely to be infested by bopyrids than young shrimp. Taken together, these findings suggest that on a small scale, the high prevalence of shrimp infestation on mature individuals, and subsequent sexual sterilization of shrimp (also called “reproductive death”; Calado et al., 2008), may influence shrimp mortality and, ultimately, population dynamics. For instance, in one study evaluating the effects of bopyrid isopods and parasitic trematodes on grass shrimp, shrimp had decreased survivorship over the course of the experiment compared to uninfected shrimp, although no significant difference was seen in shrimp mortality infected with bopyrid isopods or a parasitic trematode (Finn and Buck, 2024).

Similarly, the parasitic isopod Tachaea chinensis has been found to infect the shrimp Palaemonetes sinensis in rice fields of Lianing Province, China, leading to shrimp mortality (Li et al., 2018). Population declines have been observed with mud shrimp (Upogebia pugettensis) infested with the bopyrid isopod (Orthione griffenis) in the Pacific Northwest, although it should be noted that this species of bopyrid is non-native, likely exerting stronger controls on shrimp populations as there has not been co-evolution between parasite and host (Dumbauld et al., 2011). Additionally, grass shrimp have high site fidelity, residing in individual pools within a salt marsh or intertidal creek basin (Allen et al., 2015). Thus, once parasites are present, localized shrimp infestation and sterilization may become much higher.

Alternatively, some literature indicates that bopyrid isopods may not drive non-predative host mortality of grass shrimp, as the survival of the isopod is dependent on the survivorship of the shrimp (Finn and Buck, 2024). Bopyrid isopods appear to decrease the risk of their hosts being predated, possibly through shifts in shrimp behavior (Bass and Weis 1999; Brinton and Curran, 2015a; Finn and Buck, 2024). Additionally, there are systems in which the prevalence of parasitic castrators is high, and yet host populations are not particularly affected because hosts have co-evolved with their parasites. For example, trematode prevalence in California horn snails can be >90%, but the parasites do not measurably influence recruitment because the hosts have evolved a high reproductive output as compensation (Buck et al., 2017). Further work investigating host-parasite relationships is required to properly evaluate how parasite prevalence will impact shrimp populations over time.

Grass shrimp directly influence the processing of detrital material and meiofauna abundances, and as such, a high prevalence of parasitism in a relatively confined panne environment may also indirectly influence other levels of biological organization. The loss of grass shrimp as key detritivores and abundant food source may negatively affect salt marsh community and ecosystem dynamics, where high parasite prevalence at individual pannes, and female shrimp sterilization, would result in the loss of these species processing organic matter (also suggested by Brinton and Curran, 2015b). Similarly, altered behavior exhibited by infected grass shrimp may influence predator-prey dynamics, and thus ecological communities within salt marshes. For instance, parasitized grass shrimp have been found to exhibit decreased backward thrusting in the presence of a predator, mummichogs (Fundulus heteroclitus) while also making hosts more conspicuous to predators (visually the parasites diminish shrimp camouflage; Brinton and Curran, 2015a). It is important to note that although we explored the relationship between Pr. pandalicola parasite prevalence and grass shrimp size, we did not examine the possible relationship between the sex of grass shrimp and the developmental stage of the bopyrid parasite. Documenting parasite developmental stage alongside host size would provide specific host-parasite associations, such as indicating if parasitized male hosts are larger than unparasitized hosts or non-ovigerous parasitized female hosts. These interactions are necessary to explore, as they may influence the population structure of the host grass shrimp population.

Many potential mechanisms may be driving the high prevalence of Pr. pandalicola infestation in grass shrimp, and although in this study we were ultimately unable to identify drivers of Pr. pandalicola prevalence, we sought to describe possible relationships between key abiotic factors and parasite presence. Dagglerblade grass shrimp are euryhaline and tolerant to a range of salinities (<1–30 ppt; Heard and Lutz, 1982); however, salinity tolerance of Pr. pandalicola remains largely undocumented. Initial reports of Pr. pandalicola prevalence in grass shrimp in the southern United States describe higher parasite prevalence with higher salinities and temperatures, although habitat type and land use were confounding factors (Key et al., 2011; Briggs et al., 2017). In Hartnoll (1982), temperature was found to have a stronger influence on crustaceans (in the intermolt stage) with minimal effects by salinity. In Cummaquid, Massachusetts, there was no apparent relationship between salinity and grass shrimp abundance or parasite abundance. Although no strong relationship was seen between salinity and parasites, Bass and Weiss (1999) found that parasitized grass shrimp showed significantly reduced activity levels at higher salinities (15 ppt and 26 ppt) than unparasitized shrimp, suggesting that salinity may be an important driver of infected grass shrimp behavior, rather than Pr. pandalicola prevalence itself. Further research is needed to investigate how salinity may influence grass shrimp behavior because possible consequences hold for trophic dynamics between predators of infected grass shrimp in situ. For instance, increased predation on slower infected grass shrimp in salty water may lead to lower densities of grass shrimp and, as such, reflect higher densities of parasitized grass shrimp in less salty water.

The configuration of the pannes in Cummaquid also appeared to influence both grass shrimp and parasite prevalence, irrespective of salinity or any other abiotic factor. For instance, some pannes were farther inland in the high marsh, which likely had less tidal flushing than pannes that were closer to shore (thus retaining more grass shrimp and their parasites). This was observed as an increase in both parasites and shrimp with increasing distance from the shoreline, with no apparent difference in salinity associated with distance to the shoreline. Aquatic vegetation present within pannes facilitating habitat for grass shrimp and their parasites, in addition to other habitat characteristics (e.g., panne topography and/or flushing), could be drivers of Pr. pandalicola prevalence. As such, further research is required to establish the full extent to which abiotic factors (e.g., temperature and salinity) as well as biotic factors, including the larval transport and abundance of the intermediate copepod hosts of Pr. Pandalicola, drive parasite prevalence (Sasaski et al., 2023). For instance, dos Santos Periera et al. (2022) suggest that bopyrid parasites in certain regions of Brazil are limited by the distribution of estuarine copepods. To this end, it is necessary to disentangle drivers of parasite distribution from potentially confounding factors (i.e., panne placement within a marsh).

In 2021 we observed an increased abundance of grass shrimp in the warmer months of the summer and fall, and Pr. pandalicola was also abundant in the warmer summer months, although it is notable that Pr. pandalicola had the highest prevalence in the spring. Although it is unclear why parasites were most prevalent in cooler spring temperatures, it is not surprising that Pr. pandalicola were exclusively ovigerous in the summer when grass shrimp have been found to molt, thus continuing the parasitic life cycle (Brinton and Curran, 2015b). Warming ocean temperatures may play an important role in influencing host-parasite interactions and thus shrimp survivorship. For instance, Sherman and Curran (2013) found decreased survivorship of grass shrimp that were parasitized by Pr. pandalicola in warmer water (25 ± 0.10 C) than uninfected shrimp. In this study, temperatures on Cape Cod, Massachusetts, in 2021 were warmer compared to previous years (2016–2020), and so future work should consider the relationship between water temperature and Pr. pandalicola prevalence and define possible temperature-driven shifts in grass shrimp and Pr. pandalicola populations.

In temperate coastal marshes of the United States, the loss of total salt marsh area combined with the transient nature of salt pannes is resulting in shifts in salt marsh populations, with cascading effects on salt marsh communities and ecosystem dynamics. In this study the high prevalence of parasites on daggerblade grass shrimp, a key species in salt marshes, may result in important shifts in all levels of biological organization. Specifically, we must have a preliminary understanding of the distribution of Pr. pandalicola, as they may play a role in structuring grass shrimp populations in salt marsh ecosystems. Further, the ranges of host species and the ranges and prevalence of parasites may be shifting as climate change intensifies and associated abiotic characteristics shift. Since changes in the range and distribution of many species are known to have meaningful impacts on ecosystem structure, functioning, and stability, understanding the prevalence and range of parasites and their hosts is essential.

The authors wish to thank A. Boyd for sample guidance and processing, R. Jenkins, B. Powers, A. Granzier, and L. West for sample processing, I. Ives and M. Faherty for logistical guidance and support, and the Massachusetts Audubon Long Pasture Wildlife Sanctuary for the use of their land and facilities. Comments by S. Archer and 2 anonymous reviewers greatly improved the manuscript.