Gill lesions in the perch, Anabas testudineus, exposed to monocrotophos.

Histopathological effects of sublethal doses of monocrotophos on the gills have been studied by exposing the fish for a period ranging from ten to twenty days. The extent of damage to gills was dependent on the dose and duration of exposure. Histopathological changes in the gills observed were characterized primarily by hemorrhage in the primary and secondary gill lemellae. Degeneration and necrosis of epithelial cells were very prominent. Distortion of the secondary lamellae was very prominent, beginning with disruption of epithelial cells from pillar cells.

Serotonergic vasomotor control in fish gills

Serotonin (5-hydroxytryptamine, 5-HT)-containing neuroepithelial cells (NECs) have been discovered in the gills of fish belonging to different phylogenetic groups, and an additional serotonergic innervation of the gill filaments is present in teleosts. The most apparent effect of serotonin is a bronchial vasoconstriction. For teleosts, it has been postulated that the serotonergic innervation of the proximal portion of the efferent filamental artery (EFA) and adjacent efferent lamellar arterioles (ELA) acts to constrict the vasculature. Thus, as perfusion pressure increases, more lamellae are recruited. In contrast, recent results from rainbow trout show that one precise action of serotonin is a vasoconstriction on the distal portion of the filament vasculature. This may explain why exogenously added serotonin impairs gas exchange in rainbow trout. In addition to constricting the respiratory (arterio-arterial) pathway, injection of serotonin into the Atlantic cod dilates the arterio-venous pathway, diverting blood flow to the arterio-venous pathway. The vasoconstrictory effects of serotonin can be blocked by the 5-HT-receptor antagonist methysergide, whereas the vasodilatory effects cannot.