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This review provides an inventory of the biodiversity of the anisakid species identified so far from fish and marine mammals of the NE Atlantic Ocean. The paper reviews and discusses various taxonomical and epidemiological aspects related to biodiversity assessment, with emphasis on: (1) taxa recognized as 'biological species' based on molecular/genetic markers; (2) current molecular/genetic approaches to identify the species at different developmental stages; (3) ecological data related to the actual geographical distribution and definitive host preferences of the species; (4) their distribution in various, commercially important fish species in northern European waters; (5) their possible occurrence in farmed fish; and, finally, (6) an update of their zoonotic potential as causative agents of anisakidosis in humans. [ABSTRACT FROM AUTHOR]

Anisakid nematodes are a globally distributed group of marine mammal parasites. Kogiid whales, including the pygmy sperm whale Kogia breviceps, host an assemblage of specific anisakid species. Currently, three species are known to be specific to kogiid hosts, i.e., Skrjabinisakis paggiae, S. brevispiculata, and the less studied Pseudoterranova ceticola. The aim of this study was to investigate the species diversity of anisakid nematodes sampled from a pygmy sperm whale stranded in 2013 at the edge of its distribution range in the Northeast Atlantic, specifically in the North of Scotland. Nematodes were assigned to genus level based on morphology and identified by sequence analysis of the mtDNA cox2 gene and the rDNA ITS region. The present finding represents the first observation of syntopic occurrence of adult stages of S. brevispiculata, S. paggiae, and P. ceticola in a pygmy sperm whale in the Northeast Atlantic, and represent the northernmost record of these species in this area. Skrjabinisakis brevispiculata was the most abundant species, accounting for 55% of the identified nematodes, predominantly in the adult stage. Anisakis simplex (s.s.) was also abundant, with most specimens in the preadult stage, followed by S. paggiae and P. ceticola. The pygmy sperm whale is rarely documented in Scottish waters, and its occurrence in the area could suggest expansion of its geographic range. The presence of S. brevispiculata, S. paggiae, and P. ceticola in this whale species in this region may indicate a shift in the whole host community involved in the life cycle of these parasites in northern waters. However, it is also plausible that these parasites were acquired while the whale was feeding in more southern regions, before migrating northbound., (© P. Cipriani et al., published by EDP Sciences, 2024.)

Parasite biodiversity in cetaceans represents a neglected component of the marine ecosystem. This study aimed to investigate the distribution and genetic diversity of anisakid nematodes of the genus Anisakis sampled in cetaceans from the Northeast Atlantic Ocean and the Mediterranean Sea. A total of 478 adults and pre-adults of Anisakis spp. was identified by a multilocus genetic approach (mtDNA cox2, EF1 α - 1 nDNA and nas 10 nDNA gene loci) from 11 cetacean species. A clear pattern of host preference was observed for Anisakis spp. at cetacean family level: A. simplex (s.s.) and A. pegreffii infected mainly delphinids; A. physeteris and A. brevispiculata were present only in physeterids, and A. ziphidarum occurred in ziphiids. The role of cetacean host populations from different waters in shaping the population genetic structure of A. simplex (s.s.), A. pegreffii and A. physeteris was investigated for the first time. Significant genetic sub-structuring was found in A. simplex (s.s.) populations of the Norwegian Sea and the North Sea compared to those of the Iberian Atlantic, as well as in A. pegreffii populations of the Adriatic and the Tyrrhenian Seas compared to those of the Iberian Atlantic waters. Substantial genetic homogeneity was detected in the Mediterranean Sea population of A. physeteris. This study highlights a strong preference by some Anisakis spp. for certain cetacean species or families. Information about anisakid biodiversity in their cetacean definitive hosts, which are apex predators of marine ecosystems, acquires particular importance for conservation measures in the context of global climate change phenomena., (© 2022. The Author(s).)

Adult Anisakis Dujardin, 1845 were found in two specimens of killer whale Orcinus orca and one specimen of franciscana Pontoporia blainvillei stranded from off the coast of Buenos Aires Province, Argentina. Genetic identification of the nematodes (N = 144) was performed by sequence analysis of the mitochondrial (mtDNA cox2) and the nuclear (nas 10 nDNA) gene loci. Anisakis pegreffii and Anisakis berlandi were detected in the two individuals of O. orca, while Anisakis typica and A. pegreffii were identified in P. blainvillei. Morphological and morphometric analysis also carried out on adult specimens of A. pegreffii and A. berlandi has allowed to underlining the usefulness of genetic/molecular markers in their recognition. This represents the first record of A. pegreffii in O. orca and P. blainvillei and of A. berlandi in O. orca. This is also the first sympatric and syntopic occurrence, as adults, of A. pegreffii and A. berlandi from the Austral Region of the Atlantic Ocean waters. These results provide insights into the knowledge of the host ranges and geographical distribution of these parasites in the basin waters of the region. Pontoporia blainvillei showed low abundance values of infection with Anisakis spp., which is the general pattern for coastal dolphins in the area, whereas O. orca harboured higher abundance of Anisakis spp. than those previously recorded among cetacean species in the Argentine Sea. Differences in the Anisakis spp. distribution and their parasitic loads, observed among the three host specimens, are discussed in relation to the oceanographic parameters, as well as to the host ecology. The usefulness of genetic/molecular markers in the recognition of adults of the sibling species A. pegreffii and A. berlandi with considerable overlapping in morphometric and morphological characters was underlined. The distribution of Anisakis species from Southwestern Atlantic waters is discussed in relation to their value as indicators for studies on the zoogeography of their hosts at a regional-scale level.

A survey on Anisakis simplex (sensu stricto (s.s.)) from blue whiting, Micromesistius poutassou, in the north-eastern Atlantic Ocean revealed the occurrence of high infection levels of third larval stages in visceral organs and flesh. Larvae were genetically identified with a multilocus approach as A. simplex (s.s.). Histochemical, immunohistochemical and ultrastructural observations were conducted on 30 M. poutassou specimens. Gonads, pyloric caeca and flesh harboured encapsulated larvae of A. simplex (s.s.) but no intense host reaction was encountered around the parasite in the above organs. In the liver, the most infected organ, the larvae co-occurred with the coccidian Goussia sp. Within the granuloma around the A. simplex (s.s.) larvae, two concentric layers were recognized, an inner mostly comprising electron-dense epithelioid cells and an outer layer made of less electron-dense epithelioid cells. Macrophages and macrophage aggregates (MAs) were abundant out of the granulomas, scattered in parenchyma, and inside the MAs, the presence of engulfed Goussia sp. was frequent. In liver tissue co-infected with Goussia sp. and A. simplex (s.s.), hepatocytes showed cytoplasmic rarefaction and acute cell swelling. Results suggest that the host-induced encapsulation of A. simplex (s.s.) larvae is a strategic compromise to minimize collateral tissue damage around the larval infection sites, to facilitate the survival of both parasite and host.

Few reports exist upon the occurrence and localization of zoonotic anisakid nematodes in T. sagittatus, especially in the mantle of the squid. The occurrence and site of infection of larval anisakids in 98 T. sagittatus caught West off St. Kilda, NE Atlantic Ocean, were investigated. Squids were examined for anisakids using the UV-Press method. In total, 689 nematodes were detected in the viscera and mantle. According to morphology, all the larvae (L 3 ) were assigned to genus Anisakis. Diagnostic allozymes and mtDNA cox2 sequence analysis permitted to genetically identify all larvae as Anisakis simplex (s.s.) (N = 100). Overall prevalence (P = 81%) and mean intensity (mI = 8.6) of infection with A. simplex are provided. Most of the larvae present in the mantle cavity were embedded in the stomach wall or attached in the outer layer of the stomach and caecum (49%). Over a third of squids (37%) hosted A. simplex (s.s.) larvae in the mantle. A novel schematized representation of larvae distribution in the mantle is provided, showing where they were mostly located. According to the results obtained, the risk of anisakiasis associated with consumption of raw or undercooked T. sagittatus should be considered., (Copyright © 2020 The Author(s). Published by Elsevier B.V. All rights reserved.)

The Argentinean short-finned squid (Illex argentinus) is an oceanic, neritic species widely distributed off the east coast of South America, representing the most abundant commercially exploited squid species in these waters. Despite the great commercial importance of Argentinean short-finned squid as a food resource, and as frozen product exported to Europe, the presence of zoonotic anisakid nematodes, especially in the mantle of the squid, is poorly known. The occurrence and site of infection of larval ascaridoid nematodes in 70 I. argentinus caught off the Falkland Islands were investigated. Squids were examined using the UV-Press method. In total, 30 nematodes were detected in the viscera and mantle. According to morphology, 27 were third-stage larvae (L 3 ) belonging to genus Anisakis, while three were L 3 assigned to Hysterothylacium. Anisakis pegreffii (n = 27) were identified by sequence analysis of the mtDNA cox2 and the partial EF1 α-1 region of nDNA genes; Hysterothylacium aduncum (N = 3) were identified by sequence analysis of the ITS rDNA region. These findings represent the first molecular identification of A. pegreffii and H. aduncum in I. argentinus. Both prevalence (P = 15.7%) and abundance (A = 0.39) of infection with A. pegreffii were low, and even lower values of infection were recorded for H. aduncum (P = 2.1%, A = 0.04). Only 3 out of 70 (4.3%) squids hosted A. pegreffii larvae in the mantle. Larvae infecting viscera were coiled and mainly attached to outer surface of visceral organs. Mantle-infecting larvae were situated in the posterior half. Thus, these results suggest that - although low - the risk of acquiring anisakiasis from consumption of raw, marinated and/or undercooked short-finned squid products still exists., (Copyright © 2019. Published by Elsevier B.V.)

Numerous specimens of the 3 sibling species of the Anisakis simplex species complex (A. pegreffii, A. simplex (senso stricto)), and A. simplex sp. C) recovered from cetacean species stranded within the known geographical ranges of these nematodes were studied morphologically and genetically. The genetic characterization was performed on diagnostic allozymes and sequences analysis of nuclear (internal transcribed spacer [ITS] of ribosomal [r]DNA) and mitochondrial (mitochondrial [mt]DNA cox2 and rrnS) genes. These markers showed (1) the occurrence of sympatry of the 2 sibling species A. pegreffii and A. simplex sp. C in the same individual host, the pilot whale, Globicephala melas Traill, from New Zealand waters; (2) the identification of specimens of A. pegreffii in the striped dolphin, Stenella coeruleoalba (Meyen), from the Mediterranean Sea; and (3) the presence of A. simplex (s.s.) in the pilot whale and the minke whale, Balaenoptera acutorostrata Lacépède, from the northeastern Atlantic waters. No F1 hybrids were detected among the 3 species using the nuclear markers. The phylogenetic inference, obtained by maximum parsimony (MP) analysis of separate nuclear (ITS rDNA region), combined mitochondrial (mtDNA cox2 and rrnS) sequences datasets, and by concatenated analysis obtained at both MP and Bayesian inference (BI) of the sequences datasets at the 3 studied genes, resulted in a similar topology. They were congruent in depicting the existence of the 3 species as distinct phylogenetic lineages, and the tree topologies support the finding that A. simplex (s.s.), A. pegreffii, and A. berlandi n. sp. (= A. simplex sp. C) represent a monophyletic group. The morphological and morphometric analyses revealed the presence of morphological features that differed among the 3 biological species. Morphological analysis using principal component analysis, and Procrustes analysis, combining morphological and genetic datasets, showed the specimens clustering into 3 well-defined groups. Nomenclatural designation and formal description are given for A. simplex species C: the name Anisakis berlandi n. sp. is proposed. Key morphological diagnostic traits are as follows between A. berlandi n. sp. and A. simplex (s.s.): ventriculus length, tail shape, tail length/total body length ratio, and left spicule length/total body length ratio; between A. berlandi n. sp. and A. pegreffii: ventriculus length and plectane 1 width/plectane 3 width ratio; and between A. simplex (s.s.) and A. pegreffii: ventriculus length, left and right spicule length/total body length ratios, and tail length/total body length ratio. Ecological data pertaining to the geographical ranges and host distribution of the 3 species are updated.

Larvae of the genus Pseudoterranova constitute a risk for human health when ingested through raw or undercooked fish. They can provoke pseudoterranovosis in humans, a fish-borne zoonotic disease whose pathogenicity varies with the species involved, making their correct specific identification a necessary step in the knowledge of this zoonosis. Larvae of Pseudoterranova decipiens s.l. have been reported in several fish species from off the Argentine coasts; however, there are no studies dealing with their specific identification in this region. Here, a genetic identification and morphological characterization of larval Pseudoterranova spp. from three fish species sampled from Argentine waters and from Notothenia coriiceps from Antarctic waters was carried out. Larvae were sequenced for their genetic/molecular identification, including the mitochondrial cytochrome c oxidase subunit II (mtDNA cox2), the first (ITS-1) and the second (ITS-2) internal transcribed spacers of the nuclear ribosomal DNA, and compared with all species of the P. decipiens (sensu lato) species complex (sequences available in GenBank). Further, adults of Pseudoterranova spp. from the definitive host, the southern sea lion, Otaria flavescens, from Argentine and Chilean coasts were sequenced at the same genes. The sequences obtained at the ITS-1 and ITS-2 genes from all the larvae examined from fish of Argentine waters, as well as the adult worms, matched 100% the sequences for the species P. cattani. The sequences obtained at mtDNA cox2 gene for Antarctic larvae matched 99% those available in GenBank for the sibling P. decipiens sp. E. Both MP and BI phylogenetic trees strongly supported P. cattani and P. decipiens sp. E as two distinct phylogenetic lineages and depicted the species P. decipiens sp. E as sister taxon to the remaining taxa of the P. decipiens complex. Larval morphometry was similar between specimens of P. cattani from Argentina, but significantly different from those of P. decipiens sp. E, indicating that larval forms can be distinguished based on their morphology. Pseudoterranova cattani is common and abundant in a variety of fish species from Chile, whereas few host species harbour these larvae in Argentina where they show low levels of parasitism. This pattern could arise from a combination of factors, including environmental conditions, density and dietary preferences of definitive hosts and life-cycle pathways of the parasite. Finally, this study revealed that the life-cycle of P. cattani involves mainly demersal and benthic organisms, with a marked preference by large-sized benthophagous fish., (Copyright © 2013 Elsevier B.V. All rights reserved.)

Thirteen parasite taxa were identified in the Mediterranean swordfish by morphological and genetic/molecular methods. The comparison of the identified parasite taxa and parasitic infection values observed in the Mediterranean swordfish showed statistically significant differences with respect to those reported for its Atlantic populations. A stepwise Linear Discriminant Analysis of the individual fish examined showed a separation among three groups: one including fish from the Mediterranean Sea (CTS, STS, and IOS); one consisting of fish from the Central South (CS), Eastern Tropical (ET), and Equatorial (TEQ) Atlantic; and a third comprising the fish sampled from the North-West Atlantic (NW); the CN Atlantic sample was more similar to the first group rather than to the other Atlantic ones. The nematodes Hysterothylacium petteri and Anisakis pegreffii were the species that contributed most to the characterization of the Mediterranean swordfish samples with respect to these Atlantic ones. Anisakis brevispiculata, A. physeteris, A. paggiae, Anisakis sp. 2, Hysterothylacium incurvum, Hepatoxylon trichiuri, Sphyriocephalus viridis, and their high infection levels were associated with the swordfish from the Central and the Southern Atlantic areas. Finally, H. corrugatum, A. simplex (s.s.), Rhadinorhynchus pristis, and Bolbosoma vasculosum were related to the fish from the North-West (NW) Atlantic area. These results indicate that some parasites, particularly Anisakis spp. larvae identified by genetic markers, could be used as "biological tags" and support the existence of a Mediterranean swordfish stock., (© S. Mattiucci et al., published by EDP Sciences, 2014.)

To assess fish stocks boundaries and state, the tools of population genetics have been widely used, contributing to the evaluation of relevant parameters such as the identification of stock boundaries, the assessment of gene flow and the estimation of effective population size. Also, increasing evidences show that the monitoring of the genetic diversity level is a reliable method to check the status of fish stocks. However, genetics cannot answer all the questions. For example, in high gene flow species the genetic approach could have not enough resolution to identify stock limits, while the use of parasites as biological tags could provide insights into stock structure. Even better, the so-called holistic approach, applying simultaneously a wide range of complementary techniques, is the only one considered able to provide a reliable and complete picture of fish stocks and to address a sustainable exploitation of marine resources. The work will present some examples from multidisciplinary studies concerning commercially relevant species with different biological features: the demersal European hake (Merluccius merluccius), the small pelagic horse mackerel (Trachurus trachurus) and the large pelagic swordfish (Xiphias gladius). In all these case studies merging genetic, parasitological and environmental data helped to reveal the real patterns of stocks structure.

In the present paper, recent results obtained on the use of different distributions observed in larval species of Anisakis, genetically identified by means of allozyme markers, for stock characterization of demersal (Merluccius merluccius), small (Trachurus trachurus) and large pelagic (Xiphias gladius) finfish species in European waters, are reviewed and discussed. Several species of Anisakis were identified in the three fish hosts: A. simplex (s.s.), A. physeteris, A. typica, A. ziphidarum, A. pegreffii, A. brevispiculata and A. paggiae. Canonical discriminant analysis performed on all the samples of the three fish species collected in areas comprising their geographical range, according to the different species of Anisakis identified, showed distinct fish populations in European waters. In all the three fish hosts, the pattern of distribution of Anisakis larvae allowed discrimination of Mediterranean stocks from Atlantic stocks. In the case of swordfish, the possible existence of a southern Atlantic stock separated from a northern one is also suggested. Congruence and discordance with the population genetic data inferred from allozyme markers on the same samples of the three fish species are also discussed.

Genetic variation of Contracaecum ogmorhini (sensu lato) populations from different otariid seals of the northern and southern hemisphere was studied on the basis of 18 enzyme loci as well as preliminary sequence analysis of the mitochondrial cyt b gene (260 bp). Samples were collected from Zalophus californianus in the boreal region and from Arctocephalus pusillus pusillus, A. pusillus doriferus and A. australis from the austral region. Marked genetic heterogeneity was found between C. ogmorhini (sensu lato) samples from the boreal and austral region, respectively. Two loci (Mdh-2 and NADHdh) showed fixed differences and a further three loci (Iddh, Mdh-1 and 6Pgdh) were highly differentiated between boreal and austral samples. Their average genetic distance was D(Nei) = 0.36 at isozyme level. At mitochondrial DNA level, an average proportion of nucleotide substitution of 3.7% was observed. These findings support the existence of two distinct sibling species, for which the names C. ogmorhini (sensu stricto) and C. margolisi n. sp., respectively, for the austral and boreal taxon, are proposed. A description for C. margolisi n. sp. is provided. No diagnostic morphological characters have so far been detected; on the other hand, two enzyme loci, Mdh-2 and NADHdh, fully diagnostic between the two species, can be used for the routine identification of males, females and larval stages. Mirounga leonina was found to host C. ogmorhini (s.s.) in mixed infections with C. osculatum (s.l.) (of which C. ogmorhini (s.l.) was in the past considered to be a synonym) and C. miroungae; no hybrid genotypes were found, confirming the reproductive isolation of these three anisakid species. The hosts and geographical range so far recorded for C. margolisi n. sp. and C. ogmorhini (s.s.) are given.

Genetic variation at 21 gene-enzyme systems was studied in a sample of an adult population of Anisakis typica (Diesing, 1860) recovered in the dolphin Sotalia fluviatilis from the Atlantic coast of Brazil. The characteristic alleles, detected in this population, made it possible to identify as A. typica, Anisakis larvae with a Type I morphology (sensu Berland, 1961) from various fishes: Thunnus thynnus and Auxis thazard from Brazil waters, Trachurus picturatus and Scomber japonicus from Madeiran waters, Scomberomorus commerson, Euthynnus affinis, Sarda orientalis and Coryphaena hippurus from the Somali coast of the Indian Ocean, and Merluccius merluccius from the Eastern Mediterranean. Characteristic allozymes are given for the identification, at any life-stage and in both sexes, of A. typica and the other Anisakis species so far studied genetically. The distribution of A. typica in warmer temperate and tropical waters is confirmed; the definitive hosts so far identified for this species belong to delphinids, phocoenids and pontoporids. The present findings represent the first established records of intermediate/paratenic hosts of A. typica and extend its range to Somali waters of the Indian Ocean and to the Eastern Mediterranean Sea. A remarkable genetic homogeneity was observed in larval and adult samples of A. typica despite their different geographical origin; interpopulation genetic distances were low, ranging from D(Nei)=0.004 (Eastern Mediterranean versus Somali) to D(Nei)=0.010 (Brazilian versus Somali). Accordingly, indirect estimates of gene flow gave a rather high average value of Nm = 6.00. Genetic divergence of A. typica was, on average, D(Nei)=1.12 from the members of the A. simplex complex (A. simplex s.s, A. pegreffii, A. simplex C) and D(Nei)=1.41 from A. ziphidarum, which all share Type I larvae; higher values were found from both A. physeteris (D(Nei)=2.77)

In order to assess the taxonomic status of Anisakis brevispiculata Dollfus, 1966 population samples of this taxon from central and south-eastern Atlantic ocean were compared at 22 enzymatic loci with samples belonging to Anisakis physeteris Baylis, 1923 from the Mediterranean sea and central-eastern Atlantic ocean. Very low interpopulational genetic divergence was observed both within A. brevispiculata (average D(Nei) = 0.008) and within A. physeteris (D(Nei) = 0.009) despite the geographic distance among the samples, indicating high levels of gene flow in both taxa. On the other hand, the average genetic distance between A. brevispiculata and A. physeteris was found to be D(Nei) = 0.80, a value generally observed between well differentiated congeneric species. The reproductive isolation between A. brevispiculata and A. physeteris is indicated by the following observations: (1) no F(1) hybrids or recombinant genotypes were until now observed; and (2) the two Anisakis species do not seem to share their definitive hosts. The main definitive host of A. brevispiculata is the pygmy sperm whale (Kogia breviceps), while for A. physeteris it is the sperm whale (Physeter catodon). Only adult males differ slightly in spicule length, while females and larval stages are not differentiated morphologically. Both A. brevispiculata and A.physeteris show a type II larva. The correct recognition of A. brevispiculata from A. physeteris and from other Anisakis species studied, in either sexes and at any life stage, is made easy by allozyme markers (e.g. Icdh, Gapdh, Sod-1, Np, Aat-2, Adk-2, fEst-2, PepB, PepC-2, Mpi). Diagnostic keys, which can be used for routine identification in the field of these Anisakis worms, based on genetic markers, are given.

Genetic variation within and between population samples from 22 locations of the Atlantic Arctic-Boreal region, including 1657 specimens morphologically assigned to Contracaecum osculatum, was electrophoretically analysed at 17 loci. Highly significant deviations from the Hardy-Weinberg equilibrium were found at various loci in several samples, owing to the existence of three distinct gene pools within C. osculatum (sensu lato) from the study area. These gene pools correspond to three biological species (provisionally designated A, B and C), characterized by distinct genotypes at several diagnostic loci. Reproductive isolation between C.osculatum A, B and C is confirmed by the lack of F1, recombinant, or backcross genotypes in sympatric areas, despite the occurrence of multiple infections. Mean heterozygosity per locus is on average 0.11 in species A, 0.10 in B and 0.07 in C. High levels of gene flow were found within each of the three species, the values of Nm (number of migrant individuals) ranging from 3.41 (C. osculatum C) to 5.77 (C. osculatum A). Average Nei's genetic distance is 0.46 between A and B, 0.50 between A and C and 0.77 between B and C. From these values, times of evolutionary divergence from 2 to 4 million years can be estimated. Genetic relationships among populations and species of the C. osculatum complex are illustrated by principal component analysis. The role of both geographical isolation and host preferences in the speciation of C. osculatum (sensu lato) is discussed. A morphological distinction of the three species has not yet been possible (sibling species). However, there is evidence that the name C. osculatum (sensu stricto) should be used for species C, which shows a geographical distribution and definitive host corresponding to the neotype of C. osculatum (sensu stricto). Finally, a comparison is made between the members of the C. osculatum complex from the Atlantic Arctic-Boreal region and those of the Pseudoterranova decipiens complex from the same area, as to: (i) times of evolutionary divergence, (ii) geographical distribution, and (iii) host preferences.

Genetic variation of 1017 specimens of codworm, Pseudoterranova decipiens, collected from fish and seals at 23 sampling locations in the North Atlantic and Norwegian and Barents Seas, was analysed on the basis of 16 enzyme loci. Three reproductively isolated species, provisionally designated P. decipiens A, B and C, were detected, showing distinct alleles at the following loci: Mdh-1, 6Pgdh, Np, Pgm, Est-2 (between species A and B); Mdh-3, 6Pgdh, Np, Sod-1, Adk, Pgm, Est-2, Mpi (between A and C); Mdh-1, Mdh-3, Sod-1, Adk, Pgm, Est-2, Mpi (between B and C). One F1 hybrid was observed between P. decipiens A and B, but this apparently does not lead to any gene exchange between the two species, which do not show any evidence of introgression. No hybrids or introgressed individuals were observed between P. decipiens C and either A or B. Genetic distances among conspecific populations were low (average Nei's D 0.001-0.005), even though they were collected thousands of kilometres apart, indicating high levels of gene flow within each of the three species. The values of Nei's index D were 0.44 between P. decipiens A and B, 0.57 between B and C, and 0.79 between A and C. Estimated evolutionary divergence times, using Nei's formula, range from 2 to 4 million years. Differences between P. decipiens A, B and C were also found with respect to genetic variability, morphology, geographical distribution and hosts. Mean heterozygosity values of 0.08, 0.05 and 0.02 were obtained for P. decipiens A, B and C, respectively. Preliminary morphological examination of adult males, previously identified by multilocus electrophoresis, revealed differences in the relative size and pattern of caudal papillae. P. decipiens B is widespread in the study area, whereas P. decipiens A was found only in the North-East Atlantic and Norwegian Sea. In this area P. decipiens A is most common in the grey seal, Halichoerus grypus, while the common seal, Phoca vitulina, is the main host for P. decipiens B. In Canadian Atlantic waters, where P. decipiens A is apparently absent, P. decipiens B infects both grey and common seals; a few specimens were also found in the hooded seal, Cystophora cristata. The only definitive host so far identified for P. decipiens C is the bearded seal, Erignathus barbatus; P. decipiens C appears to be widespread, occurring in both the North-West Atlantic and Barents Sea.

The relationships between larvae and adults of Anisakis from the Mediterranean Sea and North-East Atlantic Ocean were analysed by multilocus electrophoresis. The correspondence of type I larvae with the A. simplex complex, including the sibling species A. simplex A and B, and of type II larvae with A. physeteris is confirmed. 19 of the 22 loci studied discriminated between the two larval types. Biochemical keys are given for the electrophoretic identification of A. simplex A, A. simplex B and A. physeteris, at both the larval and adult stages.