Diving into the fish pathology of an important commercial fish species: the case of the European hake (Merluccius merluccius Linnaeus, 1758) in the northwest Mediterranean Sea

Abstract

The gross pathology and the histopathological alterations identified in juvenile specimens of the European hake (Merluccius merluccius), one of the most important target species of commercial fisheries in the northwest Mediterranean, are described. A qualitative and semi-quantitative histological approach was performed in specimens from 2007 and 2019. Prevalence and mean intensity of parasites and histopathological changes were calculated in both years. No macroscopic alterations were found in any organ but several parasites (e.g., copepods, nematodes, digeneans) were detected. Microscopically, alterations identified in gills included foci of inflammation and hyperplasia (present in 77.38% of hakes from both years), extensive hyperplasia (33.33%), and inflammation (16.65%) that were potentially related to the mechanical effects of monogeneans and copepods, cysts of unknown etiology (62.69%), and lamellar inflammation associated to Aporocotyle spinosicanalis eggs (8.33%). Granulomas and inflammatory focus were detected in the liver, spleen, and stomach, apparently associated with the presence of nematodes. Coelozoic myxosporean parasites were detected within the renal tubules (66.66%). Most of the pathologies detected were similar to those described in other gadoid species (i.e., Gadus morhua) and were usually related to the presence of ecto and endoparasites. The potential impact of parasites on the health of this fish species is discussed for improving the knowledge and management of these valuable fishing stocks.

Keywords: European hake; gross pathology; histology; histopathology; juvenile fish; parasites.

FIGURE 1

Histological (H&E stain) gill sections in Merluccius merluccius. (a) A female of the copepod Lernaeocera sp. with the head embedded in the gill arch. Note coiled egg sacs (arrow). Scale bar = 500 μm. (b) Cysts of unknown etiology in a gill filament. An eosinophilic central core (1) surrounded by acellular basophilic material of varying thickness (2) is recognized. An inflammatory response of macrophages and connective tissue is observed surrounding the cyst (3). Scale bar = 400 μm. (c) Mild, (d) moderate, and (e) severe alteration foci. Note the presence of aggregation of immune cells infiltration in the gill tissue. Scale bar = 100, 400, and 800 μm, respectively. (f) Higher magnification of (e). Note the presence of inflammatory cell infiltration of lymphocytes inside the filament with hyperplasia (asterisks) and a proliferation of cell activity leading to an increase in the thickness of the lamellar epithelium (arrows). Scale bar = 200 μm. (g) Possible site fixation of a copepod infecting the gill causing an inflammatory response and extended hyperplasia involving two filaments. Scale bar = 800 μm. (h) Gill filaments with some Aporocotyle spinosicanalis eggs with different degree of development. Scale bar = 400 μm.

FIGURE 2

Histological (H&E stain) sections of hepatic parenchyma morphologies and alterations of Merluccius merluccius. Scale bar = 100 μm. (a) Hepatocytes with glycogen stained by periodic acid Schiff (PAS) stain (arrows). Scale bar = 30 μm. (b) Lipidic metamorphosis I with small and non‐aggregated lipid droplets within the hepatocytes, barely displacing the nucleus and cytoplasm towards the periphery of the cell. (c) Lipidic metamorphosis II with different lipid droplet sizes, some of them aggregated in larger droplets, displacing the nucleus and the basophilic cytoplasm to the periphery of the cell, making it difficult to discern the limits of the cell. (d) Lipidic metamorphosis III with large and abundant lipid droplets merging together within the hepatocytes totally displacing the nucleus to the periphery of the cell. Basophilic cytoplasm was totally absent, the cellular limits were diffused and completely undistinguished. (e) Inflammatory focus in the hepatic parenchyma. Scale bar = 200 μm. (f) A granuloma in the hepatic parenchyma. Scale bar = 200 μm.

FIGURE 3

Histological (H&E stain) sections of alterations identified in different organs of Merluccius merluccius. (a) Macrophages aggregates containing ceroid pigment in the splenic tissue. Scale bar = 100 μm. (b) Melanomacrophagic centres (MMCs) within the splenic parenchyma. Scale bar = 200 μm. (c) A renal tubule with coelozoic myxosporean parasites (arrows). Scale bar = 30 μm. (d) A non‐encapsulated digenean metacercaria (arrow) in the stomach muscular layer without causing any host reaction. Scale bar = 200 μm.