Introduction
Empirical evidence indicates that host-parasite checklists are incomplete records of the ‘true’ geographic diversity of parasites (Poulin et al. Reference Poulin, Besson, Bélanger Morin and Randhawa2016) and that climate change and/or fisheries are responsible for distribution shifts in fishes (Perry et al. Reference Perry, Low, Ellis and Reynolds2005; Pörtner and Peck Reference Pörtner and Peck2010; Campana et al. Reference Campana, Stefánsdóttir, Jakobsdóttir and Sólmundsson2020; Fredston-Hermann et al. Reference Fredston-Hermann, Selden, Pinsky, Gaines and Halpern2020). Generally, it is assumed that the biology and ecology of commercial and recreational fish species have been relatively well-studied and that their parasite faunas are well-described. However, there are numerous recent examples from all continents where new parasite species (e.g. Pérez-Ponce de León et al. Reference Pérez-Ponce de Léon, Anglade and Randhawa2018; Palacios-Abella et al. Reference Palacios-Abella, Montero, Merella, Mele, Raga and Repullés-Albelda2021; Garrido-Olvera et al. Reference Garrido-Olvera, García-Prieto, Osorio-Sarabia, Sánchez-Martínez, Rábago-Castro, Hernández-Mena and Pérez-Ponce de León2022; Martin and Cutmore Reference Martin and Cutmore2022; Yong et al. Reference Yong, Martin and Smit2023; Nguyen et al. Reference Nguyen, Van Nguyen, Van Hien, Chinh, Truong, Van Kim, Hoai, Duc, Greiman and Nguyen2024) or new host or geographical records (Anglade and Randhawa Reference Anglade and Randhawa2018; Ramdani et al. Reference Ramdani, Trilles and Ramdane2021; Fajer-Ávila et al. Reference Fajer-Ávila, García-Prieto, Soler-Jiménez, Medina-Guerrero and Morales-Serna2022; Fonseca et al. Reference Fonseca, Felizardo, Torres, Gomes and Knoff2022; Moravec and Nie Reference Moravec and Nie2025; Pubert and Randhawa Reference Pubert and Randhawa2025) have been described from fish of commercial or recreational importance. Many of these fish species are known hosts for parasites of zoonotic importance, such as anisakid nematodes (e.g. Levsen et al. Reference Levsen, Svanevik, Cipriani, Mattiucci, Gay, Hastie, Bušelić, Mladineo, Karl, Ostermeyer and Buchmann2018). Furthermore, several parasite species of commercial and recreational fishes are considered biological tags in stock identification (e.g. MacKenzie and Hemmingsen Reference MacKenzie and Hemmingsen2015). Therefore, it is imperative to continue monitoring the parasite faunas of commercial and recreational fishes to record temporal and spatial shifts in relation to environmental and ecological variables.
In this study, we sampled the parasite fauna of haddock (Melanogrammus aeglefinus) in Icelandic waters. Haddock is a demersal species of the family Gadidae (Order Gadiformes) inhabiting waters throughout the North Atlantic with adults commonly found at depths of 50–200 m (Tam et al. Reference Tam, Link, Large, Bogstad, Bundy, Cook, Dingsør, Dolgov, Howell, Kempf and Pinnegar2016). It is an important commercial fish species in the Icelandic fishery, with variable landings since 2000 ranging from 33,141 tonnes (2014) to 108,181 tonnes (2007) (Marine and Freshwater Research Institute [MFRI] 2024). Haddock undergoes an ontogenetic shift in diet, with young fish feeding primarily on small benthic invertebrates (Klimpel and Rückert Reference Klimpel and Rückert2005) before incorporating a greater proportion of teleosts, such as capelin (Mallotus villosus), as adults (Antipova et al. Reference Antipova, Ponomarenko and Yaragina1980; Pálsson Reference Pálsson1983; Link et al. Reference Link, Almeida, Valentine, Auster, Reid and Vitaliano2005). In Icelandic waters, the size of landed haddock generally ranges between 40 and 70 cm total length (TL), but larger in the long line fishery, and primarily seven to 10 years of age (MFRI 2024). The proportion of mature haddock to the north of Iceland is lower than in the south (MFRI 2024) due to the haddock abundance and distribution being positively correlated with water temperatures (Astthorsson et al. Reference Astthorsson, Gislason and Jonsson2007). Icelandic waters are influenced by two major currents, the warmer North Atlantic Current to the south and the cold East Greenland Current to the north (Jónsson and Valdimarsson Reference Jónsson and Valdimarsson2005; Astthorsson et al. Reference Astthorsson, Gislason and Jonsson2007; Logemann et al. Reference Logemann, Ólafsson, Snorrason, Valdimarsson and Marteinsdóttir2013), thus forming distinct ecoregions along the Northwest-Southeast axis (Spalding et al. Reference Spalding, Fox, Allen, Davidson, Ferdaña, Finlayson, Halpern, Jorge, Lombana, Lourie and Martin2007). Given these environmental and oceanographic differences and differing food abundance, e.g. capelin (Vilhjálmsson Reference Vilhjálmsson2002), leading to differences in haddock body condition (Jónsdóttir et al. Reference Jónsdóttir, Björnsson, Ragnarsson, Elvarsson and Sólmundsson2024) in waters to the north and south of Iceland, it is hypothesised that these factors might contribute to haddock caught in different areas harbouring different parasite faunas.
A synopsis of haddock parasites in Canadian waters lists > 50 species infecting haddock (Margolis and Arthur Reference Margolis and Arthur1979; McDonald and Margolis Reference McDonald and Margolis1995), many of which have been recovered from European waters. However, only 11 parasite species infecting haddock have been reported from Icelandic waters: (1) the myxozoan Myxobolus aeglefini Auerback, 1906 (Karlsbakk et al. Reference Karlsbakk, Kristmundsson, Albano, Brown and Freeman2017); two copepods (2) Clavella dubia Scott and Scott, 1913 (Leigh-Sharpe and Perkins Reference Leigh-Sharpe and Perkins1924) and (3) Lepeophtheirus pectoralis (Müller 1776) (Stephensen Reference Stephensen1940); (4) one adult cestode Abothrium gadi Van Beneden, 1870 (Baer Reference Baer1962); (5) one larval cestode Hepatoxylon trichiuri (Holten 1802) (Baer Reference Baer1962); one larval trematode (6) Prosorhynchoides gracilescens (Rudolphi 1819) metacercariae (Eydal et al. Reference Eydal, Helgason, Kristmundsson and Bambir1999) and one adult trematode (7) Lepidapedon rachion (Cobbold 1858) (Brinkmann Jr Reference Brinkmann1956); and four larval nematodes (8) Anisakis sp., (9) Contracaecum sp., (10) Hysterothylacium aduncum (Rudolphi 1802) as Contracaecum aduncum (Kreiss Reference Kreis1958), and (11) Phocanema decipiens (Krabbe 1878) as Pseudoterranova decipiens (Ólafsdóttir Reference Ólafsdóttir2001).
The objective of this study is to describe and quantify the parasite fauna (primarily macroparasites; large, usually visible to the eye) from haddock caught in the north and south of Iceland. Recently, questions have been raised about the stock structure of haddock in Icelandic waters (Jónsdóttir et al. Reference Jónsdóttir, Björnsson, Ragnarsson, Elvarsson and Sólmundsson2024). Given the limited spatial information available on the parasite fauna of haddock in Icelandic waters, this study provides an initial baseline for assessing the potential of certain parasites as biological tags for stock identification in the area. Our study demonstrates the importance of reporting parasite information separately from different: (1) organs given that the relative abundance of a species varies greatly between organs; and (2) localities that vary on the basis of ecology or oceanography, such as from waters to the north and south of Iceland. This is of particular relevance in the context of using parasites as biological tags since reporting parasite information by organ will inform fisheries scientists as to which organ(s) to focus on, and on the spatial variability of parasites, since this is a key tenet for this method.
Materials and methods
Sampling
A total of 26 haddock were sampled from fishermen’s catch (fresh, unbled, ungutted) between February and April 2023. Sixteen haddock were sampled from catches landed in Dalvík (North of Iceland; 53 to 125 m depths) and 10 from fish landed in Sandgerði (South of Iceland; 68 or 77 m depths) (Table 1 and Fig. 1). Fish from Sandgerði were collected in a cooler and on ice and transported to the lab (45-minute drive). Fish from Dalvík were transported on ice overnight to the Reykjavík fish market and picked up for transport to the lab (5-minute drive). Each fish was measured (TL; nearest cm) and sexed when possible (male, female). Organs were separated and examined in saline or bagged individually and frozen (organs highlighted in bold below). Parasites were collected from each organ/site (brain, body cavity, eyes, fins, gills, gonads, heart, intestine, liver, mesenteries, mouth, nares, pyloric caecae, skin, spleen, stomach), placed in saline (8 g of sea salt per L of distilled water), counted (on surface of an organ or within), and preserved in 96% ethanol for molecular analyses. It should be noted that the pyloric caecae were examined in 20 of 26 haddock. The external and nasal cavity exams were performed immediately upon returning to the lab. For the nasal cavity, saline was flushed with a plastic pipette into one nare and collected in a Petri dish as it exited the second nare (repeated for the second pair). During the extraction of the brain, otoliths were collected and stored dry in envelopes and a genetic sample (gill filaments, stored in 96% ethanol) was retained from each haddock during examination of gills (for future studies).
Table 1. Summary sampling locations for haddock (Melanogrammus aeglefinus; N = 26) examined for parasites
Figure 1. Map of Iceland depicting locations where haddock were caught (N = 26; 16 in the north, 10 in the south).
Statistical analyses
Parasite prevalence (proportion of infected fish), mean intensity of infection (mean number of parasites per infected host), and mean abundance (mean number of parasites per host sampled, including uninfected hosts) were calculated according to Bush et al. (Reference Bush, Lafferty, Lotz and Shostak1997) for each parasite and reported separately per organ and region (north, south). A general linear model approach was used to examine the influence of fish length and region (as fixed effects) on larval nematode abundance (excluding data from pyloric caecae) and on species richness. Due to low sample size, sex (sex could not be assessed in > 30% of samples) and interactions were not included in the GLM due to low statistical power.
Estimates of the ‘true’ number of parasite species were computed using the software SuperDuplicates (Chao et al. Reference Chao, Colwell, Chiu and Townsend2017) to determine whether our sample sizes were sufficient to recover all parasite species comprising the parasite component communities, i.e. all parasite infrapopulations (all parasite individuals from a species infecting an individual host) from a subset of hosts (sensu Bush et al. Reference Bush, Lafferty, Lotz and Shostak1997) from haddock sampled from the North and from the South. These estimates were derived from using prevalence (including all parasites recovered) and abundance (excluding Loma branchialis [Nemeczek 1911] and Myxobolus aeglefini [Auerbach 1906]) data, respectively (random bootstrapping, 200 loops). For this exercise, since the pyloric caecae data were missing from three haddock per region, this organ was not excluded from our estimating procedure.
Molecular analysis
Genomic DNA was extracted from a maximum of three specimens per morphotype per organ per three infected hosts, i.e. maximum of nine specimens per morphotype per organ, using the protocol described in Devlin et al. (Reference Devlin, Diamond and Saunders2004). This was not applied to crustaceans or microparasites. A 1-mm-thick disc from large parasites (e.g. nematodes, acanthocephalans) or the entire parasite (e.g. metacercariae) was used for extractions. The remaining tissue from large parasites was preserved in 96% ethanol and deposited at the Natural Science Institute of Iceland (Náttúrufræðistofnun) (MRNI voucher numbers P1991, P1993–P2006, P2008–P2009, P2011–P2047, P2049–P2050, P2054–P2066) as hologenophores (sensu Pleijel et al. Reference Pleijel, Jondelius, Norlinder, Nygren, Oxelman, Schander, Sundberg and Thollesson2008). Different gene regions were targeted for amplification for different taxa. For nematodes, the ITS region was amplified using primers 93 and 94 (Nadler et al. Reference Nadler, D’Amelio, Dailey, Paggi, Siu and Sakanari2005) according to amplification protocols described in Nadler et al. (Reference Nadler, D’Amelio, Dailey, Paggi, Siu and Sakanari2005). For acanthocephalans, the mitochondrial cytochrome oxidase c subunit I (COI) region was amplified using primers LCO1490a (García-Varela and Pérez-Ponce de León Reference García-Varela and de León G2008) and HCO2198 (Folmer et al. Reference Folmer, Black, Hoeh, Lutz and Virjenhoek1994) according to the amplification protocol described in García-Varela and Pérez-Ponce de León (Reference García-Varela and de León G2008). The 28S rDNA region was amplified for trematodes using primers BD3 (Hernández-Mena et al. Reference Hernández-Mena, García-Prieto and García-Varela2014) and 536 (Stock et al. Reference Stock, Campbell and Nadler2001) according to the amplification protocol described in García-Varela and Nadler (Reference García-Varela and Nadler2005). The same region was amplified for cestodes and unknowns (Cysts of Unknown Etiology, CUE) using primers T01N and T13N (Harper and Saunders Reference Harper and Saunders2001) according to the amplification protocol described in Harper and Saunders (Reference Harper and Saunders2001). PCR reactions (25 μL) contained 12.50 μL of MyTaq 2x Master Mix (New England Biolabs), 0.35 μL of both forward and reverse primers (50 nM), 0.5 μL of DNA template, and 11.30 μL of ddH2O. PCR products were visualised under UV light (1.5% agarose gel and ethidium bromide). Ten μL of each successful PCR product was purified using 2 μL of ExoSAP-IT (Applied Biosystems). Purified PCR products were bi-directionally sequenced (Sanger sequencing) at Microsynth (Germany) using PCR primers (20 nM) and internal primers BD2 (Luton et al. Reference Luton, Walker and Blair1992) and 504 (García-Varela and Nadler Reference García-Varela and Nadler2005) for trematodes. No tapeworms, metacercariae, and CUEs were amplified. Sequences were manually edited in Sequencher 5.4.6 (Gene Codes Corporation) and all sequences were screened for orthology with sequences from respective parasite taxa using BLASTn (McGinnis and Madden Reference McGinnis and Madden2004).
For Anisakis, a phylogenetic approach using sequences from GenBank (Table 2) was used to determine the specific affinity of our sequences. Sequences were aligned using MacClade 4.07 (Maddison and Maddison Reference Maddison and Maddison2005). The automatic model selection – Smart Model Selection (Lefort et al. Reference Lefort, Longueville and Gascuel2017) – function in PhyML 3.0 (Guindon et al. Reference Guindon, Dufayard, Lefort, Anisimova, Hordijk and Gascuel2010) determined the best nucleotide-substitution model for the data. The Kimura 2-parameter (K80) with gamma distribution (G; estimated at 0.249) provided the best fit to the data according to the Bayesian Information Criterion (BIC). The alignment was analysed by maximum likelihood (ML) in PhyML 3.0 with 1000 bootstrap replicates.
Table 2. Taxa used in the phylogenetic analysis, including GenBank accession numbers and references
Results
A total of 26 haddock were sampled (mean TL: 58.81 cm ± [SD] 14.09 cm; range 30–78 cm): five males (66.00 cm ± 14.90 cm; 40–78 cm), 13 females (64.54 cm ± 8.78 cm; 52–76 cm), and eight haddock for which sex could not be determined (45.00 cm ± 11.58 cm; 30–63 cm). Total length data by sex for each locality are summarised in Table 3.
Table 3. Summary of mean total length (TL) and mean parasite abundance (with standard deviation; SD) and range per haddock (Melanogrammus aeglefinus) (N = 26) sampled from the north and south of Iceland
Parasite species richness
All hosts were infected with at least three different parasite species (6.50 ± 2.23; 3 to 11 different species) with haddock in the South (7.80 ± 1.14; 6 to 9 different species) harbouring a greater richness than those from the North (5.69 ± 2.39; 3 to 11 different species) (albeit with weak statistical evidence: GLM; South, t = 2.024, p = 0.0548, partial r2 = 0.151). Overall, 19 different parasite species were observed, 18 species in the North and 18 species in the South, with 17 parasite species shared between haddock sampled from both areas. Prevalences, mean abundances, and mean intensities of infection for each of the 18 parasite species per locality are summarised in Table 4. Based on prevalence (including the microparasites Lo. branchialis and M. aeglefini), the ‘true’ species richness for the North was estimated at 21.51 parasite species (18.73 to 34.88), compared to an observed richness of 18 species, with an estimated 3.51 (16.33%) undetected parasite species. For the South, the ‘true’ species richness was estimated at 22.17 parasite species (19.09 to 33.96), compared to an observed richness of 18 species, thus an estimated 4.17 (18.81%) undetected parasite species. Based on abundance (excluding Lo. branchialis and M. aeglefini), the ‘true’ species richness for the North was estimated at 17.36 parasite species (16.18 to 26.17), compared with the 16 species observed, with an estimated 1.36 (7.85%) undetected parasite species. For the South, the ‘true’ species richness was estimated at 19.23 parasite species (16.75 to 29.93), compared to an observed richness of 16 species, with an estimated 3.23 (16.78%) undetected parasite species.
Table 4. Summary of the parasite component community from haddock (Melanogrammus aeglefinus) (N = 26) collected from waters to the North (N = 16) and South (N = 10) of Iceland. Note that only 20 pyloric caecae were examined (13 from North and 7 from South). Values correspond to Prevalence (P), Mean Intensity (MI), and Mean Abundance (MA)
A, Adult; C, cystacanth; CUE, cyst of unknown etiology; Ext, external; Int, internal; L, larvae; M, metacercariae; *, only prevalence recorded
Parasite abundance
Parasite mean abundance (excluding data from the pyloric caecae) from the 26 haddock sampled was 54.54 ± 39.13 (5 to 136). Parasite mean abundance by sex: five males (53.00 ± 48.25; 13 to 136), 13 females (70.92 ± 37.04; 5 to 132), and eight for which sex could not be determined (28.88 ± 23.15; 6 to 72). Parasite abundance data by sex for each locality are summarised in Table 3. Parasite abundance data from the different organs are summarised in Tables 5 (North) and 6 (South).
Table 5. Parasite component community from haddock (Melanogrammus aeglefinus) (N = 16) collected from waters North of Iceland (per organ). Values correspond to mean Abundance (Prevalence %) [mean Intensity of infection]. The parasite(s) may be located outside (Ext) or inside (Int) of the organ. ‘-’ corresponds to 0.00 (0.00) [0.00]. Organs not listed were uninfected
A, Adult; C, cystacanth; CUE, cyst of unknown etiology; L, larvae; M, metacercariae; *, only prevalence recorded; †, in cartilage surrounding the brain; ‡, in sclera; ß, N = 13 examined for parasites
Table 6. Parasite component community from haddock (Melanogrammus aeglefinus) (N = 10) collected from waters South of Iceland (per organ). Values correspond to mean Abundance (Prevalence %) [mean Intensity of infection]. The parasite(s) may be located outside (Ext) or inside (Int) of the organ. ‘-’ corresponds to 0.00 (0.00) [0.00]. Organs not listed were uninfected
A, Adult; C, cystacanth; CUE, cyst of unknown etiology; L, larvae; M, metacercariae; *, only prevalence recorded; †, in cartilage surrounding the brain; ‡, in sclera; ß, N = 7 examined for parasites.
When excluding haddock for which the pyloric caecae were not examined, the mean parasite abundance (including pyloric caecae) from the 20 haddock sampled was 71.70 ± 56.77 (6 to 249). Parasite mean abundance by sex: four males (93.00 ± 104.87; 22 to 249), 10 females (84.10 ± 39.26; 12 to 148), and six for which sex could not be determined (36.83 ± 27.29; 6 to 80). In the North (N = 13), mean parasite abundance was 71.08 ± 68.32; 6 to 249. Parasite mean abundance by sex: two males (13 and 136, respectively), six females (76.83 ± 50.51; 12 to 148), and five haddock for which sex could not be determined (38.40 ± 30.20; 6 to 80). In the South (N = 7), the mean parasite abundance was 72.86 ± 29.47; 29 to 106. Parasite mean abundance by sex: two males (50 and 51), four females (95.00 ± 10.42; 81 to 106), and one haddock for which sex could not be determined (29).
A Shapiro-Wilk test determined that the ln of larval nematode abundance (excluding data from pyloric caecae) did not deviate significantly from a normal distribution (W = 0.961, p = 0.410), and Levene’s test supported homogeneity of variance (F = 3.032, p = 0.095). The general linear model revealed no statistically significant influence of TL (t = -0.130, p = 0.898, partial r2 < 0.001) and region (South, t = -0.925, p = 0.3644, partial r2 = 0.036) on larval nematode abundance (Fig. 2).
Figure 2. Linear regression analysis showing the relationship between ln of larval nematode abundance and haddock total length for haddock caught in the north (blue; N = 16) and south (red; N = 10). Please note that despite trend lines being drawn on the plot, neither relationship was statistically significant.
Molecular identifications
Specific information regarding GenBank accession numbers and sequences used to confirm morphological identifications, including organ of infection, sequence length (bp), and percentage similarity with previously published GenBank sequences, are summarised in Supplementary Table S1.
Larval nematodes
Sequences from 30 larval nematodes infecting haddock from the North and 11 infecting haddock in southern Icelandic waters were assignable to A. simplex (Supplementary Table S1; Fig. 3). The phylogeny, based on 951 bp, yielded strong bootstrap support for a clade which included all Anisakis species (100%) and an Anisakis simplex sensu lato clade (91%) (Fig. 3). In light of the poor bootstrap support within this latter clade (65%–93%), the specific identification of our Anisakis samples is hereby referred to as Anisakis simplex s. l.
Figure 3. Consensus tree based on Maximum Likelihood majority-rule inference for the Anisakinae Railliet and Henry, 1912 showing our samples from haddock (Melanogrammus aeglefinus) in bold grouping within the Anisakis simplex sensu lato clade. Nodal support is based on 1000 bootstrap replicates with values < 0.65 not shown.
Two larval nematodes infecting haddock from the North and three infecting haddock in southern Icelandic waters were assignable to Contracaecum osculatum (Rudolphi 1802) (Supplementary Table S1). A single larval nematode infecting haddock from the north was assignable to P. decipiens (Supplementary Table S1; Fig. 3). Furthermore, the phylogeny yielded strong bootstrap support (100%) for this conspecificity (Fig. 3).
Adult nematodes
Four adult nematodes infecting haddock from the North (N = 2) and South (N = 2) located in the stomach were assignable to the Cystidicolidae Skrjabin, 1946 (Supplementary Table S1). Two adult nematodes infecting haddock from the North (N = 1) and South (N = 1) located in the intestine were assignable to the Hysterothylacium aduncum (Rudolphi 1802) (Supplementary Table S1). One adult nematode infecting a haddock from the North located in the intestine was assignable to the cucullanid genus Dichelyne Jägerskiöld, 1902 (Supplementary Table S1).
Adult trematodes
Sequences from four adult trematodes infecting haddock from the North and three infecting haddock in southern Icelandic waters were assignable to Derogenes varicus (Müller 1774) complex sp. DV1 (D. varicus sensu stricto) (Supplementary Table S1). Two adult trematodes from the intestine of haddock in the South were assignable to Lepidapedon rachion (Cobbold 1858) (Supplementary Table S1).
Adult acanthocephalans
Six adult acanthocephalans infecting haddock from the North (N = 3) and South (N = 3) located in the intestine were assignable to Echinorhynchus gadi Zoega in Müller (1776) (Supplementary Table S1).
Discussion
Of the 19 parasite species recovered from this survey, eight represent new parasite geographical records from haddock, being recovered for the first time from this commercially important fish species from waters both to the north and south of Iceland: (1) the microsporidian Lo. branchialis; the copepods (2) Cresseyus (formerly Holobomolochus) confusus (Stock 1953) recovered from the nasal cavity and (3) Lernaeocera branchialis (Linnaeus 1767) from the gills; (4) cystacanths of the acanthocephalan Corynosoma sp. from the body cavity, gonads, and liver; (5) the adult acanthocephalan E. gadi recovered from the intestine and pyloric caecae; (6) the adult trematode D. varicus s.s. from the gills, heart, nasal cavity, and stomach; and the adult nematodes (7) Dichelyne sp. from the intestine and (8) adult cystidicolid nematodes from the stomach. Additionally, using molecular tools, we were able to provide specific identifications to the nematodes A. simplex s. l. and C. osculatum, thus providing two new species records from haddock in Icelandic waters. The genera Anisakis and Contracaecum had been recorded from haddock previously in Icelandic waters (Kreis Reference Kreis1958).
All parasite species recorded for the first time from haddock in Iceland from this study have been collected from other marine fish species in Icelandic waters and from haddock elsewhere in the North Atlantic (see Supplementary Table S2). Adult acanthocephalans identified as E. gadi have been recovered from Icelandic waters from various fish species and in haddock from North Atlantic waters (see Supplementary Table S2). However, due to E. gadi being considered a species complex (see Väinölä et al. Reference Väinölä, Valtonen and Gibson1994; Wayland et al. Reference Wayland, Gibson and Sommerville2005), we err on the side of caution and provide a specific identification of our specimens as E. gadi sensu lato. Although our sequencing results confirm the presence of D. varicus sensu stricto, it is not possible to determine whether previous records of D. varicus from Icelandic waters and from haddock in North Atlantic waters correspond to D. varicus s. l. or D. varicus s.s. (see Bouguerche et al. Reference Bouguerche, Huston, Karlsbakk, Ahmed and Holovachov2024). The nematode genus Dichelyne has been observed in marine fishes from Icelandic waters and in haddock from North Atlantic waters (see Supplementary Table S2). The species Dichelyne minutus (Rudolphi 1819) (as Cucullanus cirratus) has been recovered from Faroese waters (Køie Reference Køie1993), Atlantic Canadian waters (Kuitunen-Ekbaum Reference Kuitunen-Ekbaum1937), and the Scotian Shelf (Scott Reference Scott1981) and seems to be the only species of the genus recorded from haddock; it is likely that our specimens are assignable to D. minutus. However, we err on the cautious side and report it herein as Dichelyne sp.; distinct from the Dichelyne sp. of Pubert and Randhawa (Reference Pubert and Randhawa2025) recovered from plaice in Icelandic waters. Cystidicolid nematodes have been recorded from Icelandic waters as Ascarophis sp. (Ólafsdóttir Reference Ólafsdóttir1999), Ascarophis crassicollis Dollfus and Campana-Rouget, 1956 (Perdiguero-Alonso et al. Reference Perdiguero-Alonso, Montero, Raga and Kostadinova2008), Spinitectus oviflagellus Fourment, 1883 (Ólafsdóttir Reference Ólafsdóttir1999), and Spinitectus sp. (Eydal and Ólafsdóttir Reference Eydal and Ólafsdóttir2003) (see Supplementary Table S1). Additionally, they have been recorded from haddock in North Atlantic waters as Ascarophis arctica Polyansky, 1952 (Appy Reference Appy1981; Scott Reference Scott1981), A. crassicollis (Køie Reference Køie1993), Ascarophis filiformis Polyansky, Reference Polyansky1952 (Scott Reference Scott1981), and Ascarophis morrhuae Van Beneden, 1871 (Polyansky Reference Polyansky1952) (see Supplementary Table S2). To our knowledge, no other cystidicolid has been reported from haddock, therefore, we make the generic assignation of our samples to the genus Ascarophis without providing a specific identification, i.e. Ascarophis sp.
Anisakis simplex is a nematode that uses marine mammals such as balaenopterids, delphinids, and phocids as definitive hosts, and teleosts and cephalopods as paratenic or intermediate hosts (Mattiucci et al. Reference Mattiucci, Cipriani, Paoletti, Levsen and Nascetti2017). In Icelandic waters, the genus Anisakis has been recorded previously from nine teleost species, including haddock, as Anisakis sp. and from 10 teleost species as A. simplex (see Supplementary Table S2). Based on molecular results, A. simplex s. l. recovered from haddock in this study are conspecific with those from plaice sampled in Icelandic waters (see Pubert and Randhawa Reference Pubert and Randhawa2025).
Certain parasites were recovered from odd sites of infection. For instance, the trematode D. varicus s.s., a parasite typically found in the stomach of fishes (Blair et al. Reference Blair, Bray and Barker1998; Gibson Reference Gibson, Gibson, Jones and Bray2002), was recovered from the gills, heart, nasal cavity, and stomach. Post-mortem migration from the stomach has been observed in this species (e.g. Kostadinova et al. Reference Kostadinova, Power, Fernández, Balbuena, Raga and Gibson2003; Hermida et al. Reference Hermida, Cruz and Saraiva2014) and could explain the recovery of worms from the gill chamber. The presence of D. varicus s.s. from the heart might have resulted from contamination during the dissection while separating the oesophagus from the branchial chamber. No reasonable explanation can be provided for the presence of this trematode in the nasal cavity. Similarly, Co. osculatum larvae are generally confined to the liver, viscera, and body cavity (e.g. Valtonen et al. Reference Valtonen, Fagerholm and Helle1988; Levsen et al. Reference Levsen, Svanevik, Cipriani, Mattiucci, Gay, Hastie, Bušelić, Mladineo, Karl, Ostermeyer and Buchmann2018) with the potential to migrate to the flesh post-mortem (EFSA Panel on BIOHAZ 2011). Recovery of this nematode from the branchial chamber most likely stems from contamination during the dissection.
Humans sometimes act as accidental hosts for A. simplex when infected fish product is consumed and live larvae are ingested, leading to anisakiasis (Mattiucci et al. Reference Mattiucci, Fazii, De Rosa, Paoletti, Megna, Glielmo, De Angelis, Costa, Meucci, Calvaruso and Sorrentini2013; Shibata et al. Reference Shibata, Ueda, Akaike and Saida2014). Anisakiasis is more common in countries where raw or lightly cooked fish is consumed routinely (Oshima Reference Oshima1987; Adams et al. Reference Adams, Leja, Jinneman, Beeh, Yuen and Wekell1994; Baird et al. Reference Baird, Gasser, Jabbar and Lopata2014; Madrid et al. Reference Madrid, Gil, García, Debenedetti, Trelis and Fuentes2016). Incidences of anisakiasis have risen throughout the world over the past 30 years, attributable to the development of more sensitive diagnostic techniques, a global increase in demand for seafood, and the consumers’ preference for raw fish products, such as ceviche and sashimi (Cipriani et al. Reference Cipriani, Smaldone, Acerra, D’Angelo, Anastasio, Bellisario, Palma, Nascetti and Mattiucci2015). Anisakid larvae were not recovered from > 400 haddock sampled from the Baltic Sea (Levsen et al. Reference Levsen, Svanevik, Cipriani, Mattiucci, Gay, Hastie, Bušelić, Mladineo, Karl, Ostermeyer and Buchmann2018) but were from > 70% of the 60 haddock sampled from the Barents Sea (Levsen et al. Reference Levsen, Cipriani, Palomba, Giulietti, Storesund and Bao2022). Anisakis simplex s. l. was recovered from the flesh of 12.5% of haddock sampled from the North of Iceland, and none from the South (see Tables 5 and 6, respectively), suggesting that haddock fillets should be examined carefully for anisakids and that haddock could potentially act as a vector for this zoonotic parasite if its flesh is not prepared adequately.
Our survey is one of the few that takes a relatively holistic view of parasites of haddock and reports them in a single record. A total of 19 different parasite species (17 crustaceans and helminths, and two ‘microparasites’) were recovered from 26 haddock, with 17 of these parasites overlapping between haddock sampled from waters to the North and the South of Iceland. From the waters to the North of Iceland, 18 species were recovered from 16 haddock (21.51 were predicted). Similarly, 18 species were recovered from 10 haddock from waters to the South (22.17 were predicted). However, the number of parasite species recovered from each area seems to be an underestimation of the ‘true’ parasite biodiversity in haddock from Icelandic waters. This is likely attributable to four aspects: (1) the small sample size from each area; (2) the focus on metazoan parasites; (3) the way fish samples were handled and processed; and (4) cryptic diversity not captured by our limited barcoding. First, in host-parasite networks, it is expected that half of the links are only ever reported once (Poulin et al. Reference Poulin, Besson, Bélanger Morin and Randhawa2016), thus ‘rare’ parasites can be easily missed in surveys with inadequate sample sizes (Poulin Reference Poulin2004). The number of parasite species recovered from our survey is comparable with Scott (Reference Scott1981) who recovered 19 parasite species (17 helminths/copepods and two ‘microparasites’) from 214 haddock on the Scotian Shelf, but more than the eight metazoan parasites recovered from 24 haddock sampled from Faroese waters (Køie Reference Køie2000). There are a few parasite surveys from haddock that we can compare our data to. Other parasite surveys from haddock have generally focused on one organ, e.g. intestinal helminths (Kuitunen-Ekbaum Reference Kuitunen-Ekbaum1937) and nematodes from flesh/visceral cavity (Levsen et al. Reference Levsen, Svanevik, Cipriani, Mattiucci, Gay, Hastie, Bušelić, Mladineo, Karl, Ostermeyer and Buchmann2018), or a single group of parasites, e.g. E. gadi (Wayland et al. Reference Wayland, Gibson and Sommerville2005), larvae of the trypanorhynch cestode Grillotia sp. (Lubieniecki Reference Lubieniecki1977), and nematodes (e.g. Køie Reference Køie1993; Levsen et al. Reference Levsen, Svanevik, Cipriani, Mattiucci, Gay, Hastie, Bušelić, Mladineo, Karl, Ostermeyer and Buchmann2018). Second, a number of surveys have focused on ‘microparasites’ (e.g. Kabata Reference Kabata1963; Morrison and Sprague Reference Morrison and Sprague1981; Khan and Newman Reference Khan and Newman1982) without accounting for helminths and crustaceans. This survey did not include blood or mucus smears nor examination of the kidneys, all sites commonly infected with ‘microparasites’, therefore, the number of ‘microparasites’ reported from this survey is likely underestimated. Third, haddock were transported on ice in tubs, therefore, the skin-to-skin abrasion and contact with freshwater as the ice melts can cause several ectoparasite species, such as monogeneans, to detach or fall off, thus going undetected during the external examination. Therefore, similar to ‘microparasites’, the number of ectoparasite species is possibly underestimated in this survey. Furthermore, we did not apply incubation or digestion techniques to fish tissues (see McGladdery Reference McGladdery1983; Shamsi and Suthar Reference Shamsi and Suthar2016), therefore, it is possible that not all parasites were recovered and some species were underrepresented or missed from our samples. However, our methods were uniform, hence the bias was consistent across all fish sampled. Last, most parasite groups harbour cryptic diversity (Pérez-Ponce de León and Poulin Reference Pérez-Ponce de Léon and Poulin2016). Since not every single parasite was barcoded, it is possible that cryptic species might have been missed based on our approach of selecting nine individual parasites from each morphotype per organ. Together, these four plausible explanations provide some insights into the recovery of fewer parasite species than expected.
The temperature and salinity of waters to the South of Iceland are greater and less variable than those to the North (Astthorsson et al. Reference Astthorsson, Gislason and Jonsson2007; Logemann et al. Reference Logemann, Ólafsson, Snorrason, Valdimarsson and Marteinsdóttir2013). In this survey, a greater mean parasite species richness per host was observed and predicted in fish from the South relative to those from the North (observed, 7.8 vs. 5.7 parasite species per haddock, respectively), in addition to lower standard deviation (observed, 1.1 vs. 2.4, respectively) and range (observed, range of six to nine parasite species per haddock vs. three to 11, respectively). The greater stability of the marine ecosystem to the South of Iceland might provide a more stable environment and community of intermediate hosts, leading to the predictable proliferation of parasites with complex life cycles (Carney and Dick Reference Carney and Dick2000; Barrett et al. Reference Barrett, Thrall, Burdon and Linde2008; Warburton Reference Warburton2020). Conversely, the high variability in the North might lead to inter-annual variability in the availability of intermediate hosts and negatively impact the transmission dynamics of parasites with complex life cycles, while favouring the accumulation of long-lived parasites, such as larval nematodes. However, the latter was not supported by our data (Fig. 2).
In conclusion, several parasite species were observed infecting multiple organs and showing a strong preference for certain organs. For instance, A. simplex s. l. was recovered from the body cavity, flesh, gonads, heart, liver, mesenteries, pyloric caecae, spleen, and exterior of the stomach (Table 4), but showed a marked preference for the liver and pyloric caecae with abundances up to 100x greater than in other organs (Tables 5 and 6). Furthermore, the abundance of certain parasites differed between areas sampled, such as the copepod Cr. confusus (7 times more abundant in haddock from the South, Table 4), the nematode Co. osculatum (nearly 20 times more abundant in haddock from the North, Table 4), and the trematode D. varicus s.s. (more than 13 times more abundant in haddock from the South, Table 4). We recommend that surveys report parasites, and their respective descriptors, from each organ and from different sites, especially if the latter are influenced by different oceanographic or topographic features. Lastly, certain parasites of haddock in Iceland show potential as biological tags in stock identification, such as A. simplex s. l. from the liver and pyloric caecae, Cr. confusus from the nasal cavity, Co. osculatum from the liver, and D. varicus s.s. from the stomach. However, given the limited sampling in this study, the quest for potential biological tags should not be restricted to the aforementioned taxa.
Supplementary material
The supplementary material for this article can be found at http://doi.org/10.1017/S0022149X2510076X.
Acknowledgements
We are grateful to Hylnur Pétursson for helping us source haddock from commercial fishermen in the North of Iceland and to the Captain of the Benni Sæm GK26 for providing us with haddock from the South landed in Sandgerði. We thank Gerardo Pérez-Ponce de León for providing insightful feedback on a previous version of this manuscript and two anonymous reviewers for providing constructive feedback.
Financial support
This study was supported indirectly through a grant to HSR by the Eggerts Fund (Eggertssjóður), hosted at the University of Iceland, and the University of Iceland Contribution to Teachers’ Research Fund to HSR.
Competing interests
The authors declare none.
Ethical standard
Fish utilised in this study were sourced from commercial landings, hence no ethics protocols were required.
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