“New findings on hyperpigmentation in common dab (Limanda limanda) (7358)”

New findings on hyperpigmentation in common dab (Limanda limanda)

Introduction: In recent years, the increasing incidence of hyperpigmentation in the common dab (Limanda limanda) has been investigated in connection with environmental stressors, pollution and pathogens. This pigment anomaly has been linked to hyperplasia of pigment cells (melanophores and iridophores). Heavily pigmented dab exhibited a higher incidence of an active immune response, characterised by dermal lymphocytic infiltration. However, this immune response could not previously be linked to the infectious agents investigated, and the underlying cause remains unclear. Therefore, we have used this currently idiopathic disease to trial new diagnostic approaches based on metabarcoding of bacteria and immune response profiling.

Methodology: Skin samples were taken from dab in the North Sea during a scientific expedition and preserved in RNAlater. Where possible, samples were taken from both hyperpigmented and normal skin on the same individuals. The bacterial microbiome was investigated and determined using nanopore sequencing of the ribosomal genome (complete 16S rRNA). To enable comparison, markers for immune responses and red/black discolouration (hypermelanism) were examined using RT-qPCR in focal changes in the muscles of farmed Atlantic salmon (Salmo salar).

Results: An initial investigation of the microbiome of paired (hyper-pigmented/normally pigmented) skin samples from selected individuals revealed over 450 bacterial species. The most commonly found bacteria were Staphylococcus warneri, Photobacterium carnosum, P. iliopiscarium, P. phosphoreum, P. piscicola, Vibrio sifiae, V. logei, V. wodanis, V. salmonicida and Mycobacterium montefiorense. In some of the examined fish, the microbiome was dominated by a few species, indicating random bacterial infections. The microbiomes of individual fish were highly unique, with hypermelanistic and non-melanistic skin samples from the same animal showing similarity. Differences in the microbiome of individual fish were greater than differences between the two skin samples from one fish. Diversity indices (e.g. the Shannon and Simpson indices) also confirmed the lack of differences in the microbiomes of the two skin samples from the same individual. Furthermore, there were no indications of an antiviral response in the fish’s immune system. As expected, immune markers for red and hypermelanistic focal muscle changes in salmon did not yield conclusive results.

Conclusions: Our study showed that there are no differences in the microbiomes of normal and hyperpigmented dab skin. Furthermore, there were no signs of an immune response to viral infection. Further sampling and investigative methods are therefore necessary to solve the mystery of the causes of hyperpigmentation in dab; however, the evidence increasingly suggests that it is not pathogen-related.

1. ADAMEK, MIKOLAJ, UNIVERSITY OF VETERINARY MEDICINE HANNOVER, Presenter
2. Krebs, Torben, UNIVERSITY OF VETERINARY MEDICINE HANNOVER, Author
3. Średniawa, Filip, Department of Evolutionary Immunology, Institute of Zoology and Biomedical Research, Faculty of Biology, Jagiellonian University, Author
4. Gabel, Michael, Abteilung Fischkrankheiten, Thünen Institut für Fischereiökologie, Bremerhaven, Germany, Author
5. Jung-Schroers, Verena, UNIVERSITY OF VETERINARY MEDICINE HANNOVER, Author
6. Scharsack, Jörn Peter, Abteilung Fischkrankheiten, Thünen Institut für Fischereiökologie, Bremerhaven, Germany, Author