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Article

High Prevalence of Pea Crab in a Mussel Population (Mytilus edulis) of the Atlantic French Coast

by
Sara Cobo-Arroyo
1,2,
Sofía Blanco
1,2,
Angel P. Diz
1,2 and
Paloma Morán
1,2,*
1
Department of Biochemistry, Genetics and Immunology, University of Vigo, 36310 Vigo, Spain
2
Centro de Investigación Mariña, Universidade de Vigo (CIM-UVigo), 36310 Vigo, Spain
*
Author to whom correspondence should be addressed.
Ecologies 2025, 6(3), 60; https://doi.org/10.3390/ecologies6030060
Submission received: 29 June 2025 / Revised: 18 August 2025 / Accepted: 1 September 2025 / Published: 4 September 2025
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Abstract

Pea crabs live in association with various marine invertebrates, particularly notable for their parasitic relationship with numerous bivalves of great economic importance. During a genetic study of mussel populations on the French Atlantic coast, these crabs were found in the mantle cavity of individuals in La Tranche-sur-Mer, predominantly affecting Mytilus edulis. Given the history of mass mortality events in this area, the apparent susceptibility of M. edulis to parasitism, and the negative impact of pea crabs on host physiology, this study focused on determining the crab species found and their effects on mussels. The prevalence of infestation in this population was high (64.6%), with 61.3% of infested mussels hosting one crab and 38.7% hosting two crabs. There were no significant differences in infestation rates between male and female bivalves. Genetic analysis (16S and H3) identified the crab as Pinnotheres pisum, of which two morphotypes were observed depending on sex and stage of development. Despite the high prevalence, infested mussels showed no evidence of impaired growth or reproductive development. However, long-term studies are recommended to explore whether subtle or cumulative physiological effects of crab infestations could indirectly contribute to the mass mortality events recorded in this area.

1. Introduction

Crabs of the family Pinnotheridae (Crustacea, Decapoda, Brachyura), commonly known as pea crabs due to their small size and rounded shape, live in association with a wide variety of marine invertebrates, including holothurians, ascidians, gastropods, echinoderms, and polychaetes [1]. However, their presence in economically valuable bivalves such as cockles [2,3,4], oysters [2,5], scallops [2,6], clams [2,7], and mussels [2,3,4,8,9,10] is of particular interest. Several studies have demonstrated that this relationship is parasitic, as pea crabs can negatively affect the physiology of their hosts by causing gill lesions, reducing filtration efficiency, and limiting oxygen availability, all of which together result in decreased growth rates, impaired gonadal development, and a reduction in condition index [4,10,11,12,13,14,15,16,17]. Additionally, the presence of pea crabs inside bivalves can lead to consumer rejection and may compromise international trade, as it could facilitate their spread to new farming areas. This crustacean can also act as a vector for other pathogens [18], potentially affecting the health of infested mussels, although it poses no risk to consumers. The combination of these factors can result in substantial economic losses. For instance, in New Zealand, pea crabs cause estimated annual losses of US$2.16 million by significantly reducing the size and weight of the mussel Perna canaliculus [15].
Recent taxonomic revisions, incorporating both morphological traits and DNA markers [19], have confirmed the presence of five pea crab species in Europe [20]: Nepinnotheres pinnotheres (Linnaeus, 1758), Pinnotheres pectunculi Hesse, 1872, Pinnotheres pisum (Linnaeus, 1767), Afropinnotheres monodi Manning, 1993, and Pinnotheres bicristatus Garcia Raso & Cuesta, 2019 in Cuesta, García Raso, Abelló, Marco-Herrero, Silva & Drake, 2019. Each species has a specific geographical distribution and hosts [19]. Reported prevalence rates also vary greatly depending on habitat, geographic location, host species, and the sex and stage of development of the crab [4,19,20,21].
Climate change is driving a significant increase in global water temperatures, along with other associated consequences such as ocean deoxygenation and acidification—a trend that is expected to continue in the coming decades [22]. These changes facilitate the spread of certain marine pathogens and the rise in diseases in a wide range of organisms [23]. A clear example is the African pea crab, A. monodi, whose larvae exhibit higher survival rates and faster development at temperatures close to 25 °C [24]. This thermal advantage has promoted its expansion toward European coasts, where water temperatures are becoming increasingly warmer, and it now shows a concerning prevalence in the south of the Iberian Peninsula [3,4,7]. These effects are further exacerbated by the negative impact of high temperatures on the immune response of bivalves, making them much more vulnerable to pathogens [25,26].
There is growing concern within the European aquaculture sector regarding the sustainability of marine mussel populations (Mytilus spp.) due to increased mortality rates and seed shortages that are disrupting production [27]. France, the second-largest mussel producer in Europe [28], is among the most affected countries. Since 2014, French mussel farms have experienced several events of mass mortality, known as Abnormal Mussel Mortality (AMM), resulting in the loss of between 30% and 100% of individuals depending on the year [29,30,31]. These events have occurred mainly along the French Atlantic coast and in the English Channel and may be linked to a combination of factors such as rising temperatures, metal pollution, and the influence of certain pathogens [31]. Additionally, the Atlantic coast of France includes areas where the mussel species Mytilus edulis and M. galloprovincialis occur in allopatry, interspersed with regions where both species coexist in sympatry and hybridize [32,33,34,35]. This distribution is particularly relevant for analyzing whether the ongoing mortality events in this area have varying impacts depending on the genotype of the individuals [36,37,38,39].
During research aimed at the genetic characterization of mussel populations along the French Atlantic coast, an opportunity arose to investigate the incidence of pea crabs when a high number of infested individuals were discovered during dissection. The prevalence of pea crabs was assessed, and the species was identified using mitochondrial and nuclear DNA markers, revealing a high incidence of Pinnotheres pisum in a single population. Two distinct morphotypes were observed in this species, which appear to correspond to different sexes and stages of development. The prevalence of pea crabs was analysed about host sex and genetic profile (M. edulis, M. galloprovincialis, and hybrids, including backcrosses). The only infested population was predominantly composed of M. edulis individuals, with no significant differences between sexes. Finally, it was evaluated whether this parasite could contribute to the recurrent mussel mortality and population decline events reported in France over the past decade by comparing shell length and gonadal histology between infected and non-infected mussels as proxies to determine if growth and reproductive development are impaired.

2. Materials and Methods

As part of a research project on the genetic profiles of marine mussels, nine populations were sampled along the French Atlantic coast over one week in March 2022, proceeding sequentially from north to south (Table 1). At each location, an area of approximately 5 m2 was delineated within the rocky intertidal zone, and all mussels within this area were collected from 5 randomly selected points. Each mussel population was placed in an individual polyurethane box under refrigerated and humid conditions. Upon completion of sampling, the mussels were transported to a biosafety facility at the marine station of the University of Vigo (Spain) (ECIMAT, CIM-UVigo). There, each population was transferred to an individual box containing filtered seawater treated with ultraviolet light and maintained under recirculating and aerated conditions until processing, which began two days after the mussels arrived at the station. Temperature (13 ± 0.5 °C), salinity (35 ± 1 PSU), and pH (8.0 ± 0.2) were kept constant and monitored using multiparameter sensors.
From each population, 48 individuals with a shell length equal to or greater than 2 cm were randomly selected so that they were more likely to be sexually mature. To avoid bias, a subset of mussels from each population was dissected daily, and the process was repeated over 2–3 days until the target sample size was reached. During dissection, a high incidence of pea crab infestation was observed in a single population, La Tranche-sur-Mer, with individuals hosting one or two crabs. The infestation status (infested/not infested) and the number of crabs per host were recorded, along with data on the external morphology (size, consistency of the carapace, and colour) of each of these parasites. 17 crabs were preserved in 95% ethanol for DNA extraction and subsequent genetic analysis. Prevalence was calculated as the percentage of infested hosts relative to the total examined [40].
A fragment of the gonad was removed from 48 mussels from each population for histological examination. Each tissue sample was placed in a microcassette, fixed in Davidson’s solution, and embedded in paraffin. Sections 5 μm thick were obtained from each sample and stained with hematoxylin and eosin. All resulting histological sections were examined under a light microscope to determine sex and gonadal development stage, following the morphological classification proposed by Lubet (1959) [41]. For simplicity, mussels with gonadal development stages 0, I, and II were grouped as “immature”, while those at stages IIIA, IIIB, IIIC, or IIID were classified as “mature.” In addition, a piece of mantle edge from each mussel was preserved in 95% ethanol for DNA extraction and subsequent genetic analysis.
Genomic DNA was extracted from crab specimens and mussel mantle edge tissue using the E.Z.N.A. Mollusc DNA Kit (Omega Bio-Tek, Norcross, GA, USA), following the manufacturer’s instructions. For the crab DNA, PCR amplification of partial sequences of the mitochondrial 16S gene and the nuclear H3 gene was performed using the primers and conditions described by Perez-Miguel et al. (2019) [19]. PCR products were sent to the Genomics Service at the Scientific-Technological Support Center for Research (CACTI, UVigo, Spain) for purification with ExoSap (ThermoFisher, Santa Clara, USA) and bi-directional sequencing using the BigDye Terminator v3.1 Cycle Sequencing Kit (Life Technologies, Austin, TX, USA) on a SeqStudio Genetic Analyzer (Applied Biosystems, Foster City, CA, USA). Raw trace files were edited, and consensus sequences were aligned using the built-in Geneious Global Alignment algorithm (Geneious Prime, version 2019.2, Geneious, Biomatters, Auckland, New Zealand) [42]. To verify species identity, each sample was compared to Pinnotheridae sequences using the nucleotide Basic Local Alignment Search Tool (BLAST, National Library of Medicine, Rockville, MD, USA). BLASTn searches were performed against the nucleotide collection (nr/nt) [43], with a threshold identity set at 99%.
Genotyping of DNA extracted from the mussel mantle edge was carried out by LGC Genomics (Hoddesdon, UK) using the KASPTM (Kompetitive Allele Specific PCR) method [44] with 57 ancestry-informative SNPs that have high discriminant power between M. edulis and M. galloprovincialis [35,45,46,47], developed from the selective enrichment sequencing data of Fraïsse et al. (2016) [48]. Genotyping results (unpublished data) were analyzed with the program NewHybrids vs. 1.1 [49], following the methodology described by Diz and Skibinski (2024) [47]. This software assigns individuals to one of six genotype categories based on posterior probabilities: P0 (parental genotype M. edulis), P1 (parental genotype M. galloprovincialis), F1 (first generation hybrids), F2 (second generation hybrids), BC0 (backcrosses to M. edulis), and BC1 (backcrosses to M. galloprovincialis). For simplicity, individuals identified as F1, F2, BC0, and BC1 were grouped into a single category, Hybrids (HB), used throughout this article alongside the parental categories P0 (M. edulis) and P1 (M. galloprovincialis).
The shell length of mussels from the only population in which crabs were found, La Tranche-sur-Mer, was used as a proxy to assess the potential effects of infestation on host growth, assuming that all mussels in the sample were of comparable age or cohort and experienced similar environmental conditions. Three infestation categories were defined: non-infested, infested with one crab, and infested with two. The mean value and standard deviation (SD) of the shell length for each of these groups were calculated, and the normality and homogeneity of their variances were tested using the Shapiro–Wilk test and Levene’s test, respectively. Subsequently, a one-way ANOVA was performed to assess whether mean shell length—and by extension growth—differs among infestation categories. The host sex influence on prevalence was evaluated in mussels from La Tranche-sur-Mer using Fisher’s exact test with Monte Carlo simulation (10,000 simulations). All these statistical analyses were performed using the R v.4.2.2 program [50] and with a significance level of 0.05. The study of the gonadal maturity of the mussels was only qualitative, as it was not necessary to perform a statistical test since all the individuals were mature.

3. Results

Data on shell length, sex, and gonadal maturity of mussels from each studied population are presented in Table 2. At all sampling sites, the sex ratio did not differ significantly from 1:1 (Binomial Exact test, p > 0.05 in all cases), and nearly all individuals were mature, with gonads containing large follicles practically filled with mature gametes and lacking connective tissue. Only one immature mussel was found at Granville, Gâvres, and Rochefort, while two immature mussels were found at Chémoulin and Dune du Pilat.
Among the nine sampled populations, pea crab infestation was exclusively observed at La Tranche-sur-Mer (TR). Of the 48 mussels examined at this location, 31 were infested (64.6%): 19 of them (61.3%) hosted a single crab, and the remaining 12 (38.7%) hosted two crabs, which was the maximum degree of infestation detected in an individual. In addition, two types of crabs with distinct morphological characteristics were observed (Figure 1). 31 crabs had a soft, large (~6 mm width) and translucent carapace, with an orange spot due to the accumulation of eggs. 12 crabs had a hard, small (~3 mm width) and dark carapace. According to these observations, soft crabs have external characteristics typical of adult females commonly referred to as soft females. Hard crabs match the description of juvenile females (known as hard females) and males, which are very similar in appearance [5,20,51]. In all cases where only one crab was present in the mantle cavity, it was a soft female; when two crabs were present, one was a soft female, and the other was either a hard female or a male. At no time during sampling, transport, and storage were crabs observed outside the mussels.
SNP genotyping (unpublished data) revealed the genetic profile of mussels from each sampled population (Figure 2). The location where the pea crab was detected, La Tranche-sur-Mer, is predominantly composed of pure M. edulis (P0) individuals, representing 93.7%. The rest of the mussels were classified as hybrids (HB): one second-generation hybrid individual (F2) and two backcrosses with M. edulis (BC0). Although this percentage is low, it is noteworthy that two hybrids were found to be infested. Furthermore, histological testing revealed that of the 48 mussels examined from La Tranche-sur-Mer, 21 were female and 27 were male. Of these, 12 females were infested: 8 with one crab, and 4 with two. Of the males, 19 were infested: 11 with one crab, and 8 with two. Analyses revealed no evidence that infestation prevalence varied according to host sex (Fisher’s exact test, p = 0.618), although infestation rates were 57.1% for females and 70.3% for males (Figure 3).
Molecular identification based on sequencing a fragment of the 16S and H3 genes of the ribosomal RNA of 17 pea crabs (10 soft females and 7 males or hard females) confirmed that the species found in the mussels at La Tranche-sur-Mer was Pinnotheres pisum. It was only possible to obtain the partial sequence of the 16S rRNA gene from 4 out of 17 pea crabs analyzed due to technical problems experienced during sequencing. In contrast, no issues were encountered during the sequencing of the H3 gene (the sequences are available in File S1). The alignment of all sequences obtained with the GenBank Pinnotheridae database showed an identity greater than 99% with the reference sequences of P. pisum (H3: KU679862/63; 16S ribosomal RNA: KU679724/25, MF069151, AM180694). This high degree of sequence identity provides strong evidence for species-level assignment.
To assess the potential impact of P. pisum on the growth of infested mussels from La Tranche-sur-Mer, their shell lengths were compared across the three established infestation categories. The mean ± SD values were 3.34 ± 0.19 cm for non-infested mussels, 3.27 ± 0.26 cm for those with one crab, and 3.23 ± 0.23 cm for those with two crabs (overall mean: 3.28 ± 0.23 cm). Data met ANOVA assumptions: normality (Shapiro–Wilk test, p = 0.765, 0.531, and 0.606, respectively) and homogeneity of variances (Levene’s test, p = 0.602). No significant shell length differences were detected among groups (ANOVA, F2,45 = 0.856, p = 0.432), indicating comparable sizes across infestation categories.
Histological tests also made it possible to determine whether the pea crab could be affecting the normal development of the gonads. Microscopic examination of infested females from La Tranche-sur-Mer (Figure 4, F.1.) revealed normal gonadal maturation with large gonadal follicles practically filled by mature gametes and no connective tissue. The gonadal tissue of these females showed no differences with non-infested females from La Tranche-sur-Mer (Figure 4, F.2.) or with females from populations not affected by pea crab (Figure 4, F.3.). The same was true for infested males from La Tranche-sur-Mer (Figure 4, M.1.), which showed no apparent differences when compared with non-infested males from the same population (Figure 4, M.2.) and those from other populations (Figure 4, M.3.). No apparent abnormalities or pathologies were detected in any case. Therefore, it appears that the pea crab does not affect gonadal development in these mussels either. The data for each mussel from La Tranche-sur-Mer examined (genotype, sex, gonadal maturity, and shell length) are available in File S2.

4. Discussion

Of the nine mussel populations sampled on the French Atlantic coast, the presence of the pea crab was only detected in La Tranche-sur-Mer, with a prevalence of 64.6%, with no significant differences observed between male and female Mytilus. The infestation rate observed in this French population can be considered notably high compared to, for example, the value recorded in intertidal mussels of the Bay of Cádiz in the Iberian Peninsula (30.5%) [4]. However, the percentage of infested individuals can vary considerably depending on the host species, habitat, time of year, and parasite characteristics (species, sex, or developmental stage), which complicates comparison between studies [4,14,20,21,51]. This variability is well illustrated in the research carried out by Becker and Türkay (2017), who documented very different prevalence levels within the same population of Modiolus modiolus on the island of Helgoland (Germany) throughout the year, with minimum values of 23% and maximum values of 86% [2]. Cuesta et al. (2020) [21] also reported variations in the prevalence of pea crab in a M. galloprovincialis population depending on its proximity to the coast, with values of 17% in offshore areas and 75% in inshore areas. No recent research has been found on the prevalence and influence of this parasite in mussel populations on the French Atlantic coast.
Among the infested mussels, 61.3% hosted a single pea crab in the mantle cavity, while 38.7% contained two, the maximum number recorded in an individual in this study. In cases of infestation with a single crab, it was always a mature female (soft female) with a great number of eggs accumulated under the pleon, giving them a characteristic orange spot. In cases of double infestation in the same individual, one of the crabs was always a mature female and the other was a hard crab that could be a juvenile female (hard female) or a male. In this case, it was not possible to distinguish between these two types of crabs, as they are very similar, and only a more detailed morphological analysis would allow them to be differentiated [51]. This pattern is consistent with that observed in other mussel populations, where the proportion of individuals infested by a single crab is usually higher, probably due to the complex life cycle of pinnotherids [4,14,20]. Males and juvenile females (hard females) are small facultative pelagic parasites with a high swimming capacity. After copulation, which normally takes place in the mantle cavity of the bivalve, the female undergoes a metamorphosis, increasing in size and developing a translucent, soft shell (soft female), in which she accumulates a significant number of fertilised eggs. At this stage, the soft female cannot leave the host and becomes an obligate parasite. The male, on the other hand, remains free and can temporarily lodge in other bivalves [5,8,51,52,53,54]. This lifestyle would explain why the most abundant cases in the present study are mussels infested by a soft female, as the latter is forced to remain inside the hosts, while cases of double infestation would be temporary phenomena due to the ability of juvenile females and males to live in the water column in search of new hosts. In addition, mating usually occurs in late winter and early spring [54], which would have favored an increase in the frequency of soft females loaded with eggs at the time of sampling (March).
Genetic marker analysis confirmed Pinnotheres pisum as the species infesting mussels at La Tranche-sur-Mer. According to data compiled by Pérez-Miguel et al. (2019) [19], of the five pinnotherid species currently present in Europe, three are present in France: P. pectunculi, P. pisum, and Nepinnotheres pinnotheres. P. pectunculi seems to be restricted to the coast of French Brittany, whereas P. pisum and N. pinnotheres are distributed along the coasts of French Brittany, the Atlantic, and the Mediterranean, a pattern that aligns with our detection of P. pisum at La Tranche-sur-Mer. Nevertheless, among the nine populations sampled along the French Atlantic coast, only crabs were found at this site. This makes La Tranche-sur-Mer a point of special interest for the study of the incidence of this parasite in Mytilus, since a series of biological and environmental factors could be converging that have favored the infestation only in this population studied. Local ecological factors—such as mussel population density, proximity to other infested bivalves, and hydrodynamics of the area—may create optimal conditions for a high prevalence of this parasite. For instance, the presence of A. monodi in intertidal and subtidal mussels appears to be considerably higher when they are found near other host species. In these cases, cockles and clams act as primary hosts (mainly males and hard females, facultative parasites), while mussels serve as secondary hosts (mainly soft females, obligate parasites) [3,21]. At La Tranche-sur-Mer, these dynamics could be key factors explaining the observed infestation rates, as affected mussels were found in an area with a high abundance of oysters, another host of the pea crab P. pisum [19]. Furthermore, this population is in the northern part of the Pertuis Breton strait, off the island of Ré, in a partially protected environment that can promote larval retention and consequently increase infestation rates [21]. However, at other sites such as Arcachon (AR), where these factors also converge, infested mussels have not been detected, at least in this study.
Although La Tranche-sur-Mer is composed mainly of M. edulis (P0), three hybrid individuals (HB) were found in this population, two of which were infested by pea crabs: a second-generation hybrid individual (F2) and a backcross to M. edulis (BC0). Understanding the genetic background of infested individuals can provide valuable insight into the potential impact of pea crabs on mussel populations, depending on host genotype. This is particularly relevant on the French Atlantic coast, where recurrent mass mortality events have occurred since 2014, with M. edulis being the most affected species [36,37,38]. This species appears to be more sensitive to both biotic stress factors (e.g., parasites) and abiotic stress factors (e.g., temperature fluctuations) [14,32,37,38,55], with tolerance ranges in some cases even lower than those of F1 hybrids [56,57,58,59]. Regarding the pea crab, a study conducted by Seed (1969) in Padstow (southwest England) demonstrated that M. edulis is significantly more susceptible to infestation than M. galloprovincialis, possibly due to anatomical or physiological differences [14]. Notably, although both species have historically coexisted in sympatry in this area of England [60,61], recent studies have not found any M. edulis individuals, suggesting that it may have been replaced by M. galloprovincialis due to the latter’s greater resistance to ongoing environmental changes [47]. Nevertheless, other populations along the French Atlantic coast with genetic profiles similar to that of La Tranche-sur-Mer, such as Arcachon (AR), or even those composed exclusively of M. edulis, such as Rochefort (RF), showed no evidence of pea crab presence. Overall, the degree of infestation appears to be determined by a complex interplay of genetic, biological, and ecological factors.
Pea crabs can seriously damage the physiology of the bivalves they infest. The most affected tissue is usually the gills, which limit the host’s filtration capacity and oxygen availability. This results in a decrease in condition index, a decrease in growth rate, and an alteration in normal gonadal development [4,6,8,9,10,11,12,13,14,15,16,17]. La Tranche-sur-Mer is located near one of the areas (Pertuis Charentais) most affected by the events of mass mortality of Mytilus that have occurred over a decade in France [29,30,62]. Although pinnotherid infestation has not been directly linked to host mortality, their physiological activity is significantly reduced, which could make infested mussels even more vulnerable to other stressors involved in these mass mortality events. Despite the high prevalence of the crab in La Tranche-sur-Mer, data on shell length and sexual maturity do not support the conclusion that P. pisum has affected mussel growth or gonadal development, as no differences were detected between infested and non-infested individuals. In other cases with extremely high prevalence of pea crabs, no effects on the reproductive development of mussels were detected either, although a positive correlation was observed between host shell length and infestation intensity [14]. Moreover, soft females, being larger than males and hard females, appear to require more space and thus preferentially infest longer mussels [54]. In the present study, however, no relationship was found between infestation intensity and mussel length, and soft females were recorded in all infested individuals, regardless of their size. It is also important to note that it was only possible to evaluate gonadal growth and development, but the crab could be affecting other physiological parameters for which we have no data, since the gills are usually the main tissue damaged by the pea crab [4,10,11,12,13,14,15,16,17]. Therefore, it would be advisable to conduct more comprehensive and long-term monitoring of P. pisum prevalence and its potential effects on other relevant physiological aspects, as these could increase the vulnerability of mussels, hinder their ability to cope with adverse conditions, and ultimately lead to events of mass mortality. This monitoring is important in the current context of climate change, especially for species that appear to be more susceptible, such as M. edulis.

5. Conclusions

This study reveals a high prevalence of the pea crab Pinnotheres pisum in the Mytilus edulis-dominated population at La Tranche-sur-Mer on the French Atlantic coast. Despite this significant prevalence (64.6%), no significant effects of infestation on the growth or gonadal maturity of the mussels analyzed were detected. These results suggest that, at least in the short term, the presence of P. pisum does not appear to compromise the physical condition of hosts (growth and reproductive capacity) in this region. However, it is recommended that the study be expanded to assess how prevalence, infestation intensity, and the sex and developmental stage of the crabs vary throughout the year, and how these factors affect host physiology. Clarifying these aspects would allow for a more comprehensive evaluation of the potential role of this parasite in the recurrent mass mortality events occurring in France, which threaten both the production and sustainability of the aquaculture sector.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/ecologies6030060/s1: File S1: partial sequences of the H3 and 16S rRNA genes of the pea crab from La Tranche-sur-Mer; File S2: raw data mussels from La Tranche-sur-Mer.

Author Contributions

Conceptualization, A.P.D. and P.M.; Methodology, S.C.-A., S.B. and P.M.; Validation, A.P.D. and P.M.; Formal analysis, S.C.-A., S.B., A.P.D. and P.M.; Investigation, S.C.-A., A.P.D. and P.M.; Writing—original draft preparation, S.C.-A. and P.M.; Writing—review and editing, S.C.-A., S.B., A.P.D. and P.M.; Supervision, A.P.D. and P.M.; Funding acquisition, A.P.D. and P.M. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by grant PID2019-107611RB-I00 funded by MICIU/AEI/10.13039/501100011033, Xunta de Galicia (ED431C 2024/22), “Centro singular de investigación de Galicia” accreditation 2024–2027 (ED431G 2023/07), and ERDF A way of making Europe. Sara Cobo-Arroyo and Sofía Blanco were supported by the Spanish “Ministerio de Ciencia e Innovación” through FPI grant (PRE2020-094138) and FPU grant (FPU20/01370), respectively.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Partial sequences of the H3 and 16S genes of the studied pea crabs are available in File S1. The data for the mussels examined from La Tranche-sur-Mer can be found in File S2. The rest of the data contained in the work is available from the authors upon reasonable request.

Acknowledgments

The authors thank the staff at the marine station (ECIMAT, CIM-UVigo) for maintaining and processing the mussels histologically, and the Genomics Service at the Scientific and Technological Research Support Centre (CACTI, University of Vigo, Spain) for the molecular analysis.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Pea crabs found in the mantle cavity of infested mussels from the La Tranche-sur-Mer population preserved in ethanol. (A) Dorsal view of an adult female (soft female). (B) Dorsal view of a male or juvenile female (hard female). (C) Ventral view of a male or juvenile female (right) and an adult female (left), showing the accumulation of fertilized eggs beneath the pleon (black arrow). Scale bar: 2 mm.
Figure 1. Pea crabs found in the mantle cavity of infested mussels from the La Tranche-sur-Mer population preserved in ethanol. (A) Dorsal view of an adult female (soft female). (B) Dorsal view of a male or juvenile female (hard female). (C) Ventral view of a male or juvenile female (right) and an adult female (left), showing the accumulation of fertilized eggs beneath the pleon (black arrow). Scale bar: 2 mm.
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Figure 2. Proportion of genotypic categories P0 (Mytilus edulis parental genotype, blue color), P1 (M. galloprovincialis parental genotype, red color), and HB (first- and second-generation hybrids and backcrosses, green color) of mussels from the nine French Atlantic coast populations analyzed in this study (GV: Granville; GR: Gâvres; KB: Kerbihan; CH: Chémoulin; TR: La Tranche-sur-Mer; RF: Rochefort; AR: Arcachon; DP: Dune du Pilat; BR: Biarritz). Genotyping was performed from mussel mantle edge DNA using 57 ancestry-informative SNPs with high discriminant power between M. edulis and M. galloprovincialis species, the results of which were analyzed with the NewHybrids vs. 1.1. program.
Figure 2. Proportion of genotypic categories P0 (Mytilus edulis parental genotype, blue color), P1 (M. galloprovincialis parental genotype, red color), and HB (first- and second-generation hybrids and backcrosses, green color) of mussels from the nine French Atlantic coast populations analyzed in this study (GV: Granville; GR: Gâvres; KB: Kerbihan; CH: Chémoulin; TR: La Tranche-sur-Mer; RF: Rochefort; AR: Arcachon; DP: Dune du Pilat; BR: Biarritz). Genotyping was performed from mussel mantle edge DNA using 57 ancestry-informative SNPs with high discriminant power between M. edulis and M. galloprovincialis species, the results of which were analyzed with the NewHybrids vs. 1.1. program.
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Figure 3. Frequency of female and male mussels from La Tranche-sur-Mer that were not infested or infested with one or two crabs. Numbers of mussels in each category are shown within the bars.
Figure 3. Frequency of female and male mussels from La Tranche-sur-Mer that were not infested or infested with one or two crabs. Numbers of mussels in each category are shown within the bars.
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Figure 4. Representative photographs of histological sections of gonadal tissue from female mussels (code F) and male mussels (code M) from La Tranche-sur-Mer (French Atlantic coast) infested with the pea crab (code 1), from not infested individuals from La Tranche-sur-Mer (code 2) and from other populations on the French Atlantic coast where no infestation was detected (code 3). In all cases, normal gonadal development was observed, with minimal connective tissue and large follicles filled with mature gametes. No signs of pathology or abnormalities were detected. Scale bar: 200 μm.
Figure 4. Representative photographs of histological sections of gonadal tissue from female mussels (code F) and male mussels (code M) from La Tranche-sur-Mer (French Atlantic coast) infested with the pea crab (code 1), from not infested individuals from La Tranche-sur-Mer (code 2) and from other populations on the French Atlantic coast where no infestation was detected (code 3). In all cases, normal gonadal development was observed, with minimal connective tissue and large follicles filled with mature gametes. No signs of pathology or abnormalities were detected. Scale bar: 200 μm.
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Table 1. Codes and coordinates of the nine populations sampled along the French Atlantic coast.
Table 1. Codes and coordinates of the nine populations sampled along the French Atlantic coast.
PopulationCodeCoordinates
GranvilleGV48.8413, −1.5972
GâvresGR47.6915, −3.3520
KerbihanKB47.5670, −3.0197
ChémoulinCH47.2347, −2.2957
La Tranche-sur-MerTR46.3395, −1.4379
RochefortRF45.9740, −1.0833
ArcachonAR44.6648, −1.1636
Dune du PilatDP44.6014, −1.2117
BiarritzBR43.4633, −1.5772
Table 2. Data from the 48 mussels examined from each of the populations on the French Atlantic coast in March 2022. The maximum and minimum shell length (cm), the number of males and females, and the mature and immature individuals from each location are indicated.
Table 2. Data from the 48 mussels examined from each of the populations on the French Atlantic coast in March 2022. The maximum and minimum shell length (cm), the number of males and females, and the mature and immature individuals from each location are indicated.
PopulationShell Length Range (cm)FemalesMalesMaturesImmatures
GV4.0–2.32523471
GR3.6–2.42028471
KB4.0–2.22325480
CH3.8–2.11929462
TR3.7–2.82127480
RF3.7–2.12325471
AR3.9–2.02622480
DP3.9–2.72226462
BR3.8–2.52721480
GV: Granville; GR: Gâvres; KB: Kerbihan; CH: Chémoulin; TR: La Tranche-sur-Mer; RF: Rochefort; AR: Arcachon; DP: Dune du Pilat; BR: Biarritz.
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Cobo-Arroyo, S.; Blanco, S.; Diz, A.P.; Morán, P. High Prevalence of Pea Crab in a Mussel Population (Mytilus edulis) of the Atlantic French Coast. Ecologies 2025, 6, 60. https://doi.org/10.3390/ecologies6030060

AMA Style

Cobo-Arroyo S, Blanco S, Diz AP, Morán P. High Prevalence of Pea Crab in a Mussel Population (Mytilus edulis) of the Atlantic French Coast. Ecologies. 2025; 6(3):60. https://doi.org/10.3390/ecologies6030060

Chicago/Turabian Style

Cobo-Arroyo, Sara, Sofía Blanco, Angel P. Diz, and Paloma Morán. 2025. "High Prevalence of Pea Crab in a Mussel Population (Mytilus edulis) of the Atlantic French Coast" Ecologies 6, no. 3: 60. https://doi.org/10.3390/ecologies6030060

APA Style

Cobo-Arroyo, S., Blanco, S., Diz, A. P., & Morán, P. (2025). High Prevalence of Pea Crab in a Mussel Population (Mytilus edulis) of the Atlantic French Coast. Ecologies, 6(3), 60. https://doi.org/10.3390/ecologies6030060

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