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Population genetics and infection dynamics of Phocanema krabbei and P. decipiens (Nematoda: Anisakidae) in commercially exploited fish from the Northeast Atlantic

This study provides the first comprehensive assessment of *Phocanema krabbei* and *P. decipiens* infections in Northeast Atlantic fish, revealing that *P. krabbei* dominates the region with a single panmictic population, primarily infecting *Lophius* species and preferring muscle tissues, while highlighting significant public health and economic implications for the seafood industry.

Original authors: Andrea Ramilo, Helena Rodríguez, Santiago Pascual, Elvira Abollo

Published 2026-06-25

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: Andrea Ramilo, Helena Rodríguez, Santiago Pascual, Elvira Abollo

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ⚕️ This is an AI-generated explanation of a preprint that has not been peer-reviewed. It is not medical advice. Do not make health decisions based on this content. Read full disclaimer

The Big Picture: A Hidden Guest in the Seafood Buffet

Imagine the ocean as a massive, interconnected neighborhood. In this neighborhood, there are tiny, invisible tenants called nematodes (a type of roundworm). Specifically, this study looks at two cousins in the Phocanema family: P. krabbei and P. decipiens.

These worms are like uninvited guests that hitchhike on fish. If you buy a fish at the market and it has these worms in the meat you plan to eat, it's a problem. It's not just a health risk; it's an "ugly" problem that makes people throw the fish away, costing the fishing industry a lot of money.

This study is like a massive, continent-wide census. The researchers looked at 16,156 fish from nine different species, caught all along the Northeast Atlantic (from Scotland down to Portugal), to answer three main questions:

Who has these worms?
Which species of worm is it?
Are the worms in different parts of the ocean related to each other, or are they distinct families?

1. The "Who" and "Where": The Fish and the Worms

The Findings:
The researchers found that not all fish are equally popular with these worms.

The VIPs: The worms loved Monkfish (both Lophius piscatorius and L. budegassa). In fact, Monkfish were the "hotspots," hosting over 70% of all the worms found.
The Casuals: Hake and other fish had some worms, but far fewer.
The Ghosts: Some fish, like mackerel and sardines, had zero worms.

The Geographic Map:
Think of the ocean as a temperature gradient. The worms were most common in the north (near Scotland and Norway) and became much rarer as you moved south toward Portugal. This matches the movement of their "bosses"—seals. The worms need seals to finish their life cycle, so where the seals hang out, the worms hang out.

The Twist:
The study found these worms in fish species that had never been confirmed to carry them before (like the black monkfish and the whiting). It's like discovering that a new type of car is secretly driving on a road where no one thought they existed.

2. The "What": A Genetic Identity Check

The researchers took the worms they found and ran a DNA test (like a fingerprint scan) to see exactly which species they were.

The Result: It was a landslide victory for P. krabbei. About 96.4% of the worms were this species.
The Underdog: P. decipiens was the minority, making up only 3.6%.

Where they lived inside the fish:
The worms weren't hiding randomly. They had a strong preference for the muscle meat (the "loins" and "belly flaps") rather than the guts or the tail.

Analogy: Imagine the fish is a house. The worms prefer to sleep in the living room (the belly and loin meat) where people eat, rather than the basement (the guts) which gets thrown away. This is bad news for food safety because standard cleaning (gutting) doesn't remove them.

3. The "Family Tree": Are They One Big Crowd or Separate Tribes?

This is the most complex part, but here is the simple version. The researchers asked: Are the worms in Scotland genetically different from the worms in Portugal? Or are they all part of one big, mixed-up family?

The Analogy: The "Global Village" vs. "Isolated Villages"

Isolated Villages: If the worms in Scotland were a distinct tribe and the worms in Portugal were a different tribe, they would have very different DNA fingerprints.
The Global Village: If they are all one big, mixed-up crowd, their DNA would look very similar across the whole region.

The Result: The Global Village.
The study found that the worms are one single, mixed-up population.

High Diversity, Low Structure: There are many different "names" (haplotypes) in the family tree, but they are all mixed together. A worm caught in Scotland might be a cousin to a worm caught in Portugal.
The "Super-Highways": The researchers found three specific DNA "types" (Haplotypes H4, H14, and H2) that were everywhere. They were found in every fish species and every geographic area.
The Conclusion: The ocean currents and the movement of fish act like a giant mixer. The worms travel from Scotland to Portugal on the backs of fish, mixing their genes so thoroughly that there are no distinct "local" populations. It is one giant, panmictic (randomly mating) population.

Summary of Key Takeaways

Monkfish are the main carriers: If you are worried about these worms, look at the Monkfish. They are the primary hosts.
The worms are in the meat: They prefer the belly and loin muscles, meaning they survive standard fish cleaning and can reach the consumer.
One big family: Despite being caught hundreds of miles apart, the worms in the Northeast Atlantic are all part of the same genetic family. They move freely through the food web.
A new range: This study proves these worms are found much further south (down to Portugal) than previously thought, expanding our map of where they live.

Why does this matter?
Because the worms are in the meat we eat, and they are everywhere from Scotland to Portugal, the fishing industry needs to be extra careful. You can't just check the guts; you have to check the muscle meat to ensure the fish is safe and looks good for sale.

Technical Summary: Population Genetics and Infection Dynamics of Phocanema krabbei and P. decipiens in the Northeast Atlantic

Problem Statement

Phocanema species (Nematoda: Anisakidae) are zoonotic parasites that pose significant public health risks to seafood consumers and cause substantial economic losses to the fishing industry due to the rejection of aesthetically compromised fish. While research in the Northeast Atlantic has historically focused on the genus Anisakis, studies on Phocanema remain limited, particularly regarding their southern distribution, host range, and population genetic structure. Existing data largely cover northern waters and a restricted set of host species. Furthermore, while Phocanema species are known to utilize pinnipeds as definitive hosts and various fish as intermediate or paratenic hosts, the genetic connectivity of these parasite populations across the marine trophic network—from Scotland to Portugal—remains poorly understood. This lack of comprehensive data hinders effective food safety monitoring and the assessment of infection dynamics in commercially exploited fish stocks.

Methodology

This study conducted a comprehensive epidemiological and genetic assessment of Phocanema infections in the Northeast Atlantic (FAO Major Fishing Area 27).

Sampling and Parasitological Analysis: A total of 16,156 fish specimens from nine commercially important species (Lophius piscatorius, L. budegassa, Merluccius merluccius, Lepidorhombus spp., Micromesistius poutassou, Engraulis encrasicolus, Sardina pilchardus, Scomber scombrus, and S. colias) were examined. Samples were sourced from commercial catches across multiple ICES divisions. Fish were processed using UV-Press and/or artificial peptic digestion to recover nematodes. Infection parameters (prevalence, mean intensity, and mean abundance) were calculated, and infection sites (viscera, belly flaps, loins, tail) were recorded.
Molecular Identification: Of the recovered larvae, 474 were selected for molecular identification. Genomic DNA was extracted, and a 469 bp fragment of the mitochondrial cytochrome oxidase 2 (cox2) gene was amplified and sequenced. Species identification was confirmed via BLAST analysis against GenBank sequences.
Population Genetic Analysis: A subset of 457 sequences identified as P. krabbei was analyzed to assess genetic diversity and population structure. Three distinct datasets were constructed:
1. Sequences grouped by host fish species.
2. Sequences grouped by ICES fishing divisions.
3. Sequences grouped by anatomical infection site (viscera, belly flaps, loins, tail).
  Analyses included calculation of haplotype diversity ( $H_d$ ), nucleotide diversity ( $\pi$ ), neutrality tests (Tajima's $D$ and Fu's $F_s$ ), median-joining haplotype networks, Analysis of Molecular Variance (AMOVA), and pairwise $F_{st}$ values.

Key Results

1. Infection Dynamics and Host Range

Prevalence: Phocanema spp. were detected in demersal fish but not in pelagic species. Prevalence varied significantly by host species and geographic area. Lophius piscatorius (monkfish) showed the highest prevalence (64.1%), followed by L. budegassa (31.8%). Merluccius merluccius (hake) showed a lower prevalence (2.5%), while Micromesistius poutassou and Lepidorhombus spp. had very low infection rates. No infections were found in pelagic species.
Geographic Gradient: Prevalence generally declined towards southern latitudes, consistent with the distribution of grey and harbour seals. L. budegassa showed a sharp decline from >80% in northern areas to 1.4% in southern areas (IXa).
Tissue Tropism: Larvae showed a significant preference for muscle tissues over viscera. Belly flaps accounted for 60.1% of larvae, and loins for 27.4%. This distribution is critical for food safety as these are the primary edible portions.

2. Molecular Identification

Species Composition: Molecular analysis identified two species: Phocanema krabbei (96.4%) and Phocanema decipiens (3.6%).
Host and Geographic Expansion: The study expanded the known host range for both species to include L. budegassa, L. piscatorius, M. poutassou, and Lepidorhombus spp. P. krabbei was the dominant species across all sampled fishing grounds and host species. P. decipiens was found in low proportions across five ICES divisions in L. piscatorius and in specific areas for L. budegassa and M. merluccius.
Species Distribution: The relative proportions of the two species were not significantly influenced by host identity or geographic origin, suggesting a consolidated biogeographic pattern in the NE Atlantic where P. krabbei predominates.

3. Population Genetic Structure of P. krabbei

Diversity Indices: The analysis of 450 P. krabbei sequences revealed high haplotype diversity ( $H_d \approx 0.958$ ) and low nucleotide diversity ( $\pi \approx 0.0065$ ).
Neutrality Tests: Tajima's $D$ and Fu's $F_s$ tests yielded significantly negative values for most groups, indicating an excess of low-frequency polymorphisms consistent with recent demographic expansion and/or positive selection.
Population Structure:
- Across Hosts: AMOVA showed that 99.87% of genetic variation occurred within host species, with only 0.13% among them. Pairwise $F_{st}$ values were non-significant, indicating no genetic structuring based on host species.
- Across Geography: Similarly, 99.80% of variation was found within ICES divisions. $F_{st}$ values were generally non-significant, supporting the existence of a single panmictic population circulating from Scotland to Portugal.
- Across Tissues: Genetic structure was largely absent across infection sites (viscera, belly flaps, loins, tail), with 99.54% of variation within sites. While a slight but significant $F_{st}$ was detected between belly flaps and other sites, the dominant haplotypes were shared across all compartments.
Haplotype Network: Three dominant haplotypes (H4, H14, H2) were shared across all fish species, geographic divisions, and infection sites, alongside a large number of unique, low-frequency haplotypes.

Significance and Claims

The authors claim this study provides the first comprehensive assessment of Phocanema infection in the southern waters of FAO Area 27, significantly expanding the known geographic and host ranges of P. krabbei and P. decipiens.

Biological Insight: The findings support the hypothesis that Phocanema species, particularly P. krabbei, function as a single, highly connected panmictic population within the NE Atlantic marine trophic network. This connectivity is driven by the migratory behavior and trophic interactions of paratenic fish hosts rather than the sedentary nature of the definitive pinniped hosts.
Food Safety Implications: The high prevalence of larvae in edible muscle tissues (belly flaps and loins), particularly in Lophius spp., underscores the limitations of standard gutting procedures for ensuring food safety. The study highlights the need for expanded surveillance and enhanced monitoring of these commercially important species.
Methodological Contribution: The research demonstrates the utility of Phocanema larvae as biological tags for fish stocks, given the genetic differentiation of southern hake stocks relative to northern ones, while simultaneously showing that the parasite population itself remains genetically homogeneous across these distinct fish stocks.

The paper concludes that while Phocanema infections are widespread, the lack of significant genetic structure suggests that control measures must account for the broad connectivity of the parasite across the region's food web.