Gill remodeling in three freshwater teleosts in response to high environmental ammonia

•Modifications in macro- and microstructure of gills were tested in trout, carp and goldfish during 1 mM ammonia exposure.•The first (and non-specific) response in all the fish species was the overproduction of mucus.•Thickening of filaments and lamellae, retraction of lamellae and enlargement of ILCM was prominent in goldfish.•Adjustments in the surface ultrastructure of MRCs, PVCs and MCs were intensified in goldfish and carp.•Gill remodeling in response to ammonia was ‘classic’ in the goldfish, ‘modest’ in carp, and ‘insignificant’ in trout.

The present study aimed to determine whether gill macro- and microstructure show compensatory responses in three freshwater fish differing in their sensitivity to high environmental ammonia (HEA). The highly ammonia-sensitive salmonid Oncorhynchus mykiss (rainbow trout), the less ammonia-sensitive cyprinid Cyprinus carpio (common carp) and the highly ammonia-resistant cyprinid Carassius auratus (goldfish) were used as test species and were exposed for 0 h (control), 3 h, 12 h, 24 h, 48 h, 84 h and 180 h to 1 mM ammonia (as NH4HCO3; pH 7.9). In cyprinids, dramatic alterations were initiated quickly evident by thickening of filaments and lamellae, retraction of lamellae, enlargement of interlamellar cell mass (ILCM), and increase in the water–blood diffusion distance; while in trout, these modifications were absent or developed very slowly. These reorganizations may attempt to reduce the surface area presumably protecting against the water borne ammonia; and were more pronounced in goldfish marked by momentous enlargement of ILCM volume and the presence of rudimental and almost fused lamellae. Extensive mucus production in the gills of goldfish and carp and to a limited extent in trout may have been part of general stress response and/or may have played a protective role. While goldfish and carp showed shrinkage of apical crypts of mitochondrion rich cells (MRCs), probably aiding to regulate ion status, trout showed enlarged apical crypts of MRCs. All species displayed changes in the pattern of the microridges on the surface of pavement cells (PVCs). Overall, the present results connote that the goldfish with its minimal respiratory surface area and a large population of the MRCs with small apical crypts located on the edge of ILCM is better prepared for survival in ammonia polluted water compared to carp which maintain larger lamellae and especially the trout that did not show gill remodeling.