Cod and related species in the family Gadidae are susceptible to a variety of diseases and parasites.
"Laenaeocera branchialis".
"Lernaeocera branchialis", the "cod worm", is a copepod that infects gadoids. The first host used by cod worm is a flatfish or lumpsucker, which it captures with grasping hooks at the front of its body. It penetrates the lumpsucker with a thin filament, which it uses to suck the host's blood. The nourished cod worm then mates with another one on the lumpsucker. The female worm, with her now-fertilized eggs, then finds a cod, or a cod-like fish, such as a haddock or whiting. There, the worm clings to the gills while it metamorphoses into a plump, sinusoidal, worm-like body, with a coiled mass of egg strings at the rear. The front part of the worm's body penetrates the body of the cod until it enters the rear bulb of the host's heart. There, firmly rooted in the cod's circulatory system, the front part of the parasite develops like the branches of a tree, reaching into the main artery. In this way, the worm extracts nutrients from the cod's blood, remaining safely tucked beneath the cod's gill cover until it releases a new generation of offspring into the water.
Parasites of Atlantic cod.
Atlantic cod act as intermediate, paratenic, or definitive hosts to a large number of parasite species; 107 taxa are listed by Hemmingsen and MacKenzie (2001) with seven new records by Perdiguero-Alonso "et al." (2008). The predominant groups of cod parasites in the northeast Atlantic were trematodes (19 species) and nematodes (13 species), including larval anisakids, which comprised 58.2% of the total number of individuals. Parasites of Atlantic cod include copepods, digeneans, monogeneans, acanthocephalans, cestodes, nematodes, myxozoans and protozoans:
Monogenea
Trematoda – metacercariae
Trematoda – adult
Cestoda – larval forms
Cestoda – adult
Nematoda – larval forms
Nematoda – adults
Acanthocephala – post-cystacanths
Acanthocephala – adults
Hirudinea – adults
Copepoda - larval forms
Copepoda – adults
Amphipoda
Isopoda
References.
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This article incorporates CC-BY-2.0 text from the reference: Perdiguero-Alonso D., Montero F. E., Raga J. A. &amp; Kostadinova A. (2008). "Composition and structure of the parasite faunas of cod, "Gadus morhua" L. (Teleostei: Gadidae), in the North East Atlantic". "Parasites &amp; Vectors" 2008, 1: 23. 