Introduction
Scombrid fish of the genus Thunnus are top predators in pelagic marine ecosystems and support extensive fisheries in tropical and subtropical oceans across the world (Moore et al. Reference Moore, Lestari, Cutmore, Proctor and Lester2019). Stocks have been reduced due to unregulated harvesting, which has led to governmental agencies to impose stringent quotas for particular species. Meanwhile, aquaculture practices for some species have been developed in certain countries (Lee Reference Lee1998). The information regarding the diseases of these commercially important fish has been reviewed by Munday et al. (Reference Munday, Sawada, Cribb and Hayward2003) and Aiken et al. (Reference Aiken, Bott, Mladineo, Montero, Nowak and Hayward2007). Populations of Thunnus spp. kept in captivity have also been surveyed for parasites in the Mediterranean Sea (Mladineo and Tudor Reference Mladineo and Tudor2004; Nowak et al. Reference Nowak, Mladineo, Aiken, Bott and Hayward2006) and in Australia (Munday et al. Reference Munday, Sawada, Cribb and Hayward2003; Deveney et al. Reference Deveney, Bayly, Johnston and Nowak2005; Nowak et al. Reference Nowak, Mladineo, Aiken, Bott and Hayward2006). Information about the parasite fauna of Thunnus spp. in Mexico is very scarce; only three studies have reported the presence of some parasite species in these hosts. For instance, Sánchez-Serrano and Cáceres-Martínez (Reference Sánchez-Serrano and Cáceres-Martínez2011) analysed 30 specimens of the Pacific bluefin tuna, T. orientalis Temminck and Schlegel, from off the coast of Ensenada, Baja California. Four helminth taxa were reported in that study, including two didymozoid trematodes, one acanthocephalan, and one nematode; however, they were not identified to species level. Aiken et al. (Reference Aiken, Bott, Mladineo, Montero, Nowak and Hayward2007) reported molecular data for two species of platyhelminths, Cardicola sp. and Capsala sp., as parasites of T. orientalis from Isla Coronado on the northern Mexican Pacific coast. More recently, Román-Reyes et al. (Reference Román-Reyes, Ortega García, Galvan Magaña and Grano-Maldonado2019) reported the presence of the copepod Pennella filosa (Linnaeus, 1758) on the skin of T. albacares (Bonaterre) in the northwestern coast of Mexico.
The yellowfin tuna, T. albacares, is a commercially important fish species that represents a significant source of government revenue in many countries, as it is one of the most heavily harvested fish by weight in the tropical waters of the Pacific and Indian oceans (see Moore et al. Reference Moore, Lestari, Cutmore, Proctor and Lester2019, and references therein). Tunnus albacares populations are widely distributed in Mexican waters of both the Atlantic and Pacific coasts. In the coastal waters of Sinaloa, on the Pacific coast, yellowfin tuna is a high-value fish for commercial and recreational fishing. The tuna fleet of Sinaloa lands 77,761 tons of yellowfin tuna annually (SIPESCA 2023). Although several studies have documented the metazoan parasite biodiversity of marine fishes off the coast of Sinaloa (see Grano-Maldonado and Pérez-Ponce de León Reference Grano-Maldonado and Pérez-Ponce de León2023), information about the parasite fauna of the yellowfin tuna is very scarce. The main objective of this study was to identify the metazoan parasite fauna of T. albacares captured off the coast of Sinaloa and landed in Mazatlán Bay for processing, using a combination of morphological and molecular data.
Materials and methods
Sample collection
During the 2023 and 2024 fishing seasons of the tuna fleet along the coast of Sinaloa, a subsample of 17 individuals was analysed for parasites. The coast of Sinaloa is included in the Cortezian Marine Ecoregion (CME) (Spalding et al. Reference Spalding, Fox, Allen, Davidson, Ferdana, Finlayson, Halpern, Jorge, Lombana and Lourie2007). Fish were frozen after capture and subsequently landed for processing in Mazatlán. Once in the food processing plant, fish were thawed, the surface was screened for ectoparasites, and the viscera and the gills were separated before filleting. The viscera and the gills were kept in plastic bags and placed on ice. The screening for parasites was conducted at the Facultad de Ciencias del Mar, Autonomous University of Sinaloa. The gills were screened for ectoparasites by a gill wash with 0.85% saline solution prepared with NaCl; the supernatant was removed, and the sediment was poured into a Petri dish for observation under a stereomicroscope (Olympus SZ40). The internal organs (stomach, intestine, liver, spleen, and gonads) were dissected, placed in Petri dishes with 0.85% saline, and observed under the stereomicroscope. As expected, all sampled parasites were dead; the specimens were washed in 0.85% saline and preserved in 96% ethanol for morphological and molecular analyses. For morphological studies, platyhelminths and acanthocephalans were stained with Mayer’s paracarmine and mounted on permanent slides with Canada balsam, while nematodes and crustaceans were cleared with 50% glycerol. Voucher specimens of some helminths and crustaceans were deposited in the Colección Nacional de Helmintos (CNHE), or in the Colección Nacional de Crustáceos (CNCR), Instituto de Biología, Universidad Nacional Autónoma de México (Mexico), with the accession numbers: CNHE 12101-12104, and the CNCR: 37725-37726. The prevalence and mean intensity of infection were estimated following Bush et al. (Reference Bush, Lafferty, Lotz and Shostak1997).
Molecular analyses
Some individual helminths were processed for molecular analyses. Genomic DNA was isolated using DNAzol Reagent (Invitrogen) according to the manufacturer’s protocol. The D1-D3 domains of the large subunit of the ribosomal DNA (28S) were amplified using the primers 391 (5-AGCGGAGGAAAAGAAACTAA-3) and 536 (5-CAGCTATCCTGAGGGAAAC-3) (García-Varela and Nadler Reference García-Varela and Nadler2005). The Cytochrome c oxidase subunit 1 (COI) was amplified using the primers LCO1490 (5′-GGT CAA CAA ATC ATA AAG ATA TTG G-3′) and HCO2198 (5′-TAA ACT TCA GGG TGA CCA AAA AAT CA-3′) (Folmer et al. Reference Folmer, Black, Hoeh, Lutz and Vrijenhoek1994). The amplification and sequencing protocols followed those previously described in Grano-Maldonado et al. (Reference Grano-Maldonado, Andrade-Gómez, Mendoza-Garfias, Solórzano-García, García-Pantoja, Nieves-Soto and Pérez-Ponce de León2024a, Reference Grano-Maldonado, Sereno-Uribe, Payán, Pérez-Ponce de León and García-Varelab). Sequences were assembled and edited using Geneious v7 (Kearse et al. Reference Kearse, Moir, Wilson, Stones-Hava, Cheung, Sturrock, Buxton, Cooper, Markowitz, Duran, Thierer, Ashto-Meintjes and Drummond2012). Sequences of individual parasite taxa were assessed by their percentage identity, conducted through a BLAST search in the NCBI database. Arbitrarily, a sequence identity value ≥99% for the 28S rRNA gene and ≥95% for the COI mitochondrial gene was considered valid in this study to achieve a species-level designation. To further corroborate the species identified molecularly, phylogenetic trees were built to test the position of the newly sequenced individuals in relation to those deposited in the GenBank dataset. Four datasets were constructed separately to assess the family level relationships, three of them for 28S rDNA, i.e., Anisakidae Railliet and Henry, 1912; Sphyriocephalidae Pintner, 1913 and Hirudinellidae Dollfus, 1932; and one for COI mtDNA to asses relationships of Rhadinorhynchidae Lühe, 1912. The phylogenetic analyses were performed using maximum likelihood (ML) on CIPRES Science Gateway v3.3 (Miller et al. Reference Miller, Pfeiffer and Schwartz2010). The nucleotide substitution models (AIC criterion in JmodelTest2) were the following: Rhadinorhynchus GTR+I+G; Hirudinella and Heterosphyriocephalus GTR+G; Anisakis HKY+G. The ML was carried out with the RAxML-HPC2 on ACCESS (8.2.12) (Stamatakis Reference Stamatakis2014), using 1000 bootstrap replicates. Trees were drawn using FigTree v.1.3.1 (Rambaut Reference Rambaut2012).
Results
A total of 17 yellowfin tuna were sampled with an overall range size between 63 and 185 cm; 10 males (Mean TL: 117.3.6 ± 45.1 cm; weight: 44.7 ± 27.5 kg), and 7 females (Mean TL: 120.9 ± 45.1 cm; weight: 46.7 ± 26.0 kg) were sampled. One hundred twenty-five parasites were collected. All hosts were infected with at least one parasite species (1–6 species). Ten parasite taxa were identified, comprising three monogeneans (Capsala sp., Hexostoma thynni, and Neobenedenia girellae), one trematode (Hirudinella ahi), one cestode (Heterosphyriocephalus tergestinus), one acanthocephalan (Rhadinorhynchus laterospinosus), one nematode (Anisakis typica), and three copepods (Euryphorus brachypterus, Pseudocycnus appendiculatus, and Brachiella thynni) (Table 1). Parasite identification was accomplished either by using morphological characters solely, as in the case of the three parasitic copepods, Capsala sp. (identified up to genus level, see discussion), and H. thynni, or by using molecular data. In all cases, BLAST search allowed us to identify the taxa to species level. Five parasite taxa were successfully sequenced. The 28S rDNA gene was targeted for N. girellae, H. ahi, H. tergestinus, and A. typica reaching sequence identity values of 99%, 99.6%, 99.6%, and 99.8%, respectively; moreover, the mitochondrial cytochrome oxidase subunit 1 gene (COI) was targeted for R. laterospinosus, reaching a sequence identity of 98%. For additional validation, phylogenetic analyses were conducted to confirm interrelationships between the newly sequenced individuals and sequences deposited in GenBank (Figures 1 and 2).
Table 1. Metazoan parasites of the yellowfin tuna, Thunnus albacares, off the coast of Sinaloa, northwestern Mexico
Figure 1. Maximum Likelihood trees inferred with 28S rDNA showing the phylogenetic position of parasites of T. albacares from off the coast of Sinaloa. A) Anisakis typica; B) Heterosphyriocephalus tergestinus; and C) Hirudinella ahi. Asterisks in nodes indicate bootstrap support values higher than 80
Figure 2. Maximum Likelihood trees inferred with COI mtDNA showing the phylogenetic position of Rhadinorhynchus laterospinosus from T. albacares from off the coast of Sinaloa. Asterisks in nodes indicate bootstrap support values higher than 80.
Six of the 10 parasite taxa were recovered as ectoparasites (three monogeneans and three copepods). The gills were the infection site with the highest parasite species richness since one species of monogenean and two species of copepods were collected. Only two of the parasite taxa were found as larval stages, including the third-stage larvae of the nematode A. typica and the plerocercoid of H. tergestinus; all the other parasites were adult forms. The capsalid monogenean was the only taxa not identified to species level (Table 1). The larval forms of A. typica reached the highest prevalence of infection (76.5%); these larval forms and the acanthocephalan R. laterospinosus reached the highest mean intensity values with 3.4 and 3.8 parasites per infected host, respectively.
Discussion
This study contributes novel information about the parasite fauna of marine fish on the northwestern Pacific coast of Mexico; only one parasite taxon had been previously reported as a parasite of the yellowfin tuna, the copepod Penella filosa L. (Román-Reyes et al. Reference Román-Reyes, Ortega García, Galvan Magaña and Grano-Maldonado2019). All the metazoan parasite species recovered from the yellowfin tuna in this survey represent new geographical records, since they are reported for the first time as parasites of this commercially important fish in waters off the coast of Sinaloa in the Cortezian Marine Ecoregion. In addition, four species of metazoan parasites represent new host records, as they are reported for the first time as parasites of T. albacares, namely the monogenean N. girellae, the cestode H. tergestinus, the acanthocephalan R. laterospinosus, and the copepod B. thynni. Supplementary Table S1 lists the species of metazoan parasites of T. albacares across its global distributional range. The list, compiled from at least 35 bibliographical sources, includes 57 species (15 monogeneans, 28 digeneans, 2 cestodes, 2 acanthocephalans, 2 nematodes, and 8 copepods). Interestingly, most papers are isolated reports about the presence of a particular species or group of species of T. albacares in a specific area (e.g., Calhoun et al. Reference Calhoun, Curran, Pulis, Provaznik and Franks2013; Kohn et al. Reference Kohn, Baptista-Farias, Dos Santos and Gibson2004; Purivirojkul et al. Reference Purivirojkul, Chaidee and Thapanand-Chaidee2011). Few studies have reported the entire parasite fauna of T. albacares in a particular area (see Bane Reference Bane1969; Fernandes et al. Reference Fernandes, Kohn and Santos2002; Aiken et al. Reference Aiken, Bott, Mladineo, Montero, Nowak and Hayward2007), and some records are presented while describing the lesions caused by particular parasite species on the yellowfin tuna (e.g., Justo et al. Reference Justo, Tortelly, Menezes and Kohn2008, Reference Justo, Tortelly, Menezes and Kohn2009; Bullard et al. Reference Bullard, Womble, Maynard, Orélis-Ribeiro and Arias2015). Although the sample size in our study is relatively small, 10 parasite species were identified, and four of them were added as new host records to the host-parasite list of this commercially important fish species, which indicates that more studies are necessary to gather a complete list of the parasites that may infect this host species across its distributional range.
Species identity using morphology and DNA
The identification of five of the parasite taxa was accomplished using DNA sequences (N. girellae, H. ahi, H. tergestinus, A. typica, and R. laterospinosus). The first four species reached a percentage identity through the BLAST search higher than 99% for the 28S rRNA gene, whereas for the acanthocephalan, the percentage identity for COI was 98%. The remaining five species were identified solely on morphological grounds; only one of them was not identified to species level, the monogenean Capsala sp. Species of Capsala are characteristically large monogeneans and common ectoparasites of marine fish (Bullard et al. Reference Bullard, Womble, Maynard, Orélis-Ribeiro and Arias2015); they are clearly distinguished from the other capsalid reported in this study, N. girellae, by having a septate haptor. The genus Capsala currently comprises 25 species, according to WoRMS (2025) (accessed at https://www.marinespecies.org/aphia.php?p=taxdetails&id=119263 on 5 June 2025). Of these species, three have been reported parasitizing marine fishes in the Mexican Pacific coast, i.e., Capsala laevis (Verril, 1857), and C. pricei as parasites of the striped marlin, Kajikia audax, and Capsala caballeroi Winter, 1955 parasitising the scombrid Sarda orientalis (see Mendoza-Garfias et al. Reference Mendoza-Garfias, García-Prieto and Pérez-Ponce de León2017 and references therein). Particularly in tuna fish, Aiken et al. (Reference Aiken, Bott, Mladineo, Montero, Nowak and Hayward2007) reported Capsala sp. as a parasite of T. orientalis in Isla Coronado, off the coast of Baja California, in the northeastern Pacific coast of Mexico. Bullard et al. (Reference Bullard, Womble, Maynard, Orélis-Ribeiro and Arias2015) reported Capsala biparasiticum (Goto, 1894) from the buccal cavity of the yellowfin tuna, T. albacares; however, the record was made in tuna captured in the Gulf of Mexico, on the Atlantic coast. Unfortunately, very few individuals of Capsala were sampled, and they were in poor condition, making it difficult to identify them and confirm whether or not the specimens corresponded to any of the previously reported Capsala species parasitising marine fishes from the Mexican Pacific.
Most of the parasite taxa found in T. albacares in this study were adults. Only two of them were larval forms. One of them was the third-stage larvae of A. typica. This nematode, like many other anisakids, completes its life cycle in marine mammals (Mattiucci et al. Reference Mattiucci, Paoletti, Cipriani, Webb, Timi, Nascetti, Klimpel, Kuhn and Mehlhorn2017; Shamsi et al. Reference Shamsi, Briand and Justine2017; Mostafa et al. Reference Mostafa, Abdel-Ghaffar, Fayed and Hassan2023). An important aspect of A. typica, which is found in the yellowfin tuna, is its zoonotic potential. Considering that fresh yellowfin tuna is preferably consumed raw in the form of sushi or sashimi, the presence of anisakids may raise food safety concerns. The presence of anisakids such as Anisakis simplex and A. pegreffi has been reported in bluefin tuna (Thunnus thynnus) in the Mediterranean and in the North East Atlantic (Mladineo and Poljak Reference Mladineo and Poljak2014; Bao et al. Reference Bao, Levsen, Giulietti, Wiech, Ferter, Karlsbakk and Cipriani2025), as well as A. pegreffi and A. typica in bluefin tuna caught off Brazil (Mattiucci et al. Reference Mattiucci, Paggi, Nascetti, Portes Santos, Costa, Di Beneditto, Ramos, Argyrou, Cianchi and Bullini2002). Bao et al. (Reference Bao, Levsen, Giulietti, Wiech, Ferter, Karlsbakk and Cipriani2025) suggested that the detection of larvae in the caeca and intestines of the bluefin tuna populations they studied may indicate that there is no need to continue investigating the potential food safety concerns associated with raw tuna consumption because anisakis larvae could not be found in the muscle. However, in this study, two of the 44 larvae of A. typica were collected from the muscle, although it is acknowledged that the mean intensity of infection of anisakids larvae in the yellowfin tuna is very low, and the possibility that the larvae migrated to the flesh post-mortem cannot be ruled out.
The second larval form obtained in this study was the plerocercoid of the cestode H. tergestinus, which completes its life cycle in elasmobranchs (Dallarés et al. Reference Dallarés, Carrassón and Schaeffner2017). This indicates that a shark should feed upon the yellowfin tuna for the cestode to complete its life cycle. It is not clear if yellowfin tuna represent a dead-end host for the cestode, since the trophic spectrum of thresher sharks such as A. pelagicus is narrow and includes mostly cephalopods and teleosts as hakes and they are considered specialist predators (Calle-Morán and Galván-Magaña Reference Calle-Morán and Galván-Magaña2020). The tapeworm was originally described (as Sphyriocephalus tergestinus) from the Thresher shark, Alopias vulpinus (Bonnaterre), in the Mediterranean Sea (Dallarés et al. Reference Dallarés, Carrassón and Schaeffner2017). In the same paper, the authors provided evidence to re-allocate the species in the genus Heterosphyriocephalus Palm, 2004; additionally, they described a new species of the genus, H. encarnae Dallarés, Carrassón and Schaeffner, Reference Dallarés, Carrassón and Schaeffner2017, from the stomach of Alopias pelagicus Nakamura, collected in the Gulf of California, off Boca del Alamo, on the Pacific coast of Mexico. The new species was easily separated from other congeners mainly by its small size, small number of proglottids, a long velum with an irregular and folded margin, and the absence of a pars post-bulbosa (Dallarés et al. Reference Dallarés, Carrassón and Schaeffner2017). Unfortunately, DNA sequences for the new species were not provided, although the wide geographical distribution range of the plerocercoid of H. tergestinus was reported; the report included their presence in teleost fishes from localities of the Mediterranean Sea, the North Atlantic, and the Indian Pacific Oceans, and also that from the western coastline of Mexico by Dollfus (Reference Dollfus1967). Based on geographical distribution, it is expected that the specimens sampled in this study correspond to H. encarnae since the yellowfin tuna were captured off the coast of Sinaloa, which is very close to the type locality of H. encarnae, within the CME. The lack of sequence data for H. encarnae precludes at the moment, testing the hypothesis that the specimens from this study may correspond to that species. Still, the sequence identity value obtained for these samples was 99.6% similar to that of H. tergestinus, clearly indicating that they belong to the same species.
This study further corroborates the usefulness of incorporating DNA sequence data into the taxonomic identification of fish parasites, as succinctly demonstrated by Aiken et al. (Reference Aiken, Bott, Mladineo, Montero, Nowak and Hayward2007), while providing molecular evidence for the cosmopolitan distribution of platyhelminth parasites of tunas (Thunnus spp.) worldwide. The genetic library of parasite species affecting economically important fish is increasing steadily, generating baseline genetic data for studies like the one presented herein. This is the first study aimed at describing the parasite fauna of yellowfin tuna in the CME. The results of this study provide a key basis for identifying parasites in the northwest Mexican Pacific. They are complementary to the list of parasite species that infect yellowfin tuna worldwide (see Supplementary Table S1). This information is also essential for the development of aquacultural practices on this fish species. Still, more studies are necessary to fully understand the parasite fauna of fish populations of commercially important fish in the wild, especially those of pelagic species such as T. albacares. Generally, it is assumed that the parasite fauna of these fish is well understood. However, new parasite species, host records, and geographical records are published regularly across all continents. Moreover, this type of study may also provide data on species of parasites with potential to be used as biological tags in stock identification (MacKenzie and Hemmingsen Reference Mackenzie and Hemmingsen2015; Irigoitia et al. Reference Irigoitia, Incorvaia and Timi2017), and studies on host migrations (Binning et al. Reference Binning, Craft, Zuk and Shaw2022). Precise taxonomic identification of parasites is fundamental to address questions in these areas.
Supplementary material
The supplementary material for this article can be found at http://doi.org/10.1017/S0022149X25100795.
Acknowledgments
We thank Sandra Pérez, Elizabeth Hernández, Eden Rodríguez, Jacqueline Muñoz, and Daniela Maciel, students of the Facultad de Ciencias del Mar, for their help during the necropsy work. LAG thanks SECIHTI for the postdoctoral fellowship ‘Estancias Posdoctorales por México Modalidad Académica’ (CVU 640068). We are grateful to Laura Marquez and Nelly López, LaNaBio, for their help with sequencing DNA. We thank Norberto Colín for allowing us to use the Molecular Biology lab of ENES-Mérida. We also thank Laura Elena López León for providing English editing services for the manuscript, and the two anonymous reviewers for their valuable comments and suggestions which helped to improve it.
Financial support
This study was partially funded by a grant from the Programa de Apoyo a Proyectos de Investigación e Innovación Tecnológica PAPIIT-UNAM IN 200824 to GPPL.
Competing interests
The authors declare no competing interests.
Data availability
All study data are included in the article and in the Supplementary material. DNA sequence data was deposited in GenBank, and voucher specimens were deposited at the National Collections of Helminths and Crustaceans.
Study permits
Not applicable.
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