Neoplasms in the swim bladder of juvenile cod

Abstract

340 The mariculture of anadromous and marine fish steadily develops in many countries worldwide. The cod is one of promising species for mariculture. The improvement of biotechnology for mass propagation of this species necessitates clarification of the causes of poor quality of juvenile cod. The disturbances of metabolism and growth resulting in the formation of neoplasms in the swim bladder are the factors that may affect survival of juvenile fish. To date, various neo� plasms have been revealed in more than 40 wild and farmed fish species. Neoplasia does not depend on the fish phylogenetic status [1–6]. Some tumors may be of virus origin [7]. The goal of this study was to use histological tech� niques to determine at least some reasons for the low survival rate in juvenile cod. The study was carried out in the framework of the NorthCod International Project sponsored by the European Union. Juvenile Atlantic cod Gadus morhua was studied. Cod larvae were sampled from fish farms in Nor� way, Iceland, and Scotland (United Kingdom). The larvae were fixed at the ages of 1, 3, 5, 7, 10, 15, 20, 30, 40, 50, and 60 days post hatching. Boin’s fixative, 10% neutral formalin, and Karnovski’s fixative were used. Histological specimens were prepared following the commonly accepted routine [8]. The description of the specimens was based on putative histopatholog� ical alterations caused by waterborne pollutants [9]. The swim bladder of adult cod is closed and lacks the air duct connecting the bladder with the digestive tract. However, in juvenile cod, the swim bladder is connected with the esophagus through the air (pneu� matic) duct. Morphogenesis in cod larvae at the stage of the for� mation of the definitive swim bladder is a metamor� phosis and is coupled with immunological changes. Degeneration (apoptosis) of some cells of the pneu� matic duct starts early, between the seventh and the ninth days after hatching. Chronic inflammatory pro� cesses in juvenile fish entailing the proliferation and hyperplastic growth of the swim bladder epithelium are facilitated by the transitional type of the swim bladder epithelium. Normally, this tissue includes the flat, cubic, and prismatic types of epithelium. No alterations were found in the notochord at the prelarval stage before the resorption of the yolk sac. Deformation of the notochord occurs upon the transi� tion from endogenous to exogenous feeding. This begins approximately on the seventh day after hatch� ing and continues for five to eight weeks, when the for� mation of the first bone components of the backbone starts. During this period of time, a curvature of the notochord behind the scull, above the swim bladder, was recorded in most fish. Four types of malformations of the swim bladder development were revealed in juvenile cod from all fish farms in the studied regions. These malformations were expressed in overfilling the bladder with gases both during the initial period of its filling and during various stages of early development (as long as two months) as a result of progressing inflammation. Pre� tumor processes in the swim bladder of juvenile cod consist in an altered proportion of inflammatory reac� tions and regeneration (excessive and atypical prolif� eration) in combination with the initial transudate accumulation (Fig. 1) followed by exudate accumula� tion and inflammatory alterations in the connective tissue surrounding the swim bladder.