Morphological and molecular identification of Cryptocotyle lingua metacercariae isolated from Atlantic cod (Gadus morhua) from Danish seas and whiting (Merlangius merlangus) from the English Channel

Maureen Duflot,Mélanie Gay,Kurt Buchmann,Per Walter Kania,Graziella Midelet

doi:10.1007/s00436-021-07278-6

Abstract

Trematode larvae (metacercariae) causing black spot disease occur frequently in gills, fins, skin and the superficial muscle layers of marine fish. Species within the genus Cryptocotyle Lühe, 1899 are frequently associated with this disease. Descriptions of the metacercarial stage are relatively limited and none has hitherto been reported from fish from the English Channel. The present study reports the morphological and molecular identifications of encysted black spot-inducing parasites from whiting (Merlangius merlangus) and Atlantic cod (Gadus morhua) caught respectively from the north coast of France (English Channel) and from Danish sea waters. Metacercariae were characterised morphologically based on microscopic observations and molecularly using Sanger sequencing of fragments of the mitochondrial cox1 gene and rDNA ITS region. Morphological data were compared with available data in the literature. Phylogenetic trees including reference sequences were built to confirm morphological and molecular identifications. This survey constitutes the first description of C. lingua metacercariae in the English Channel ecosystems.

Highlights

Marine fish are sometimes speckled with “black spots” induced by a host response to a trematode infection (Aalvik et al 2015; Borges et al 2015; Kristoffersen 1991; Sindermann & Farrin 1962)
The metacercariae presented in this study belong to Cryptocotyle according to their morphological traits and organ sizes, and their molecular identification
Morphological identification of trematode larval stages in fish requires experimental infections to grow the parasite to its adult stage

Summary

Introduction

Marine fish are sometimes speckled with “black spots” induced by a host response to a trematode infection (Aalvik et al 2015; Borges et al 2015; Kristoffersen 1991; Sindermann & Farrin 1962). Cercariae are released from the snail and actively swim to the host, a fish, penetrating its skin and infecting the skin and the subcutaneous muscle of the fish host It encysts and develops into the metacercarial stage. Melanophores concentrate at the infection site; melanisation is visible to the naked eye (Mazzi 2004) Metacercariae protect their soft tegument against this fish immune response by developing a proteinaceous cyst wall resistant to pepsin and acids (El-Mayas & Kearn 2009). The juvenile parasites attach to the avian intestinal wall and develop into the adult stage (Buchmann et al 2009)

Methods

Results

Conclusion