Branchial and extra-branchial ammonia excretion in goldfish (Carassius auratus) following thermally induced gill remodeling

Under cold acclimated conditions, goldfish (Carassius auratus) express an interlamellar cell mass (ILCM) which limits diffusive ion loss but may also impede branchial ammonia excretion (Jamm). In the present study, goldfish were subjected to a 2-week 5 or 25 °C acclimation in order to modulate the degree of ILCM gill coverage and determine potential effects on Jamm. 25 °C-fish displayed gill coverage which was significantly lower than the 5 °C-fish, though the ILCM was not completely absent in these fish. 5 °C-fish demonstrated Jamm values approximately 60% lower than those of 25 °C-fish. The magnitude of anterior (branchial) Jamm strongly correlated with gill coverage (r2 = 0.83), suggesting that the ILCM may impede branchial Jamm. Divided chamber experiments demonstrated that relative to the 25 °C-fish, 5 °C-fish relied more upon posterior routes of excretion. In response to high external ammonia (HEA; 1.5 mM NH4HCO3) exposures, 25 °C-fish displayed ammonia uptake while 5 °C-fish maintained excretion against HEA, suggesting that the ILCM may act as a barrier preventing ammonia uptake. In summary, the ILCM appears to impede branchial Jamm, such that 5 °C-rely more on extra-branchial routes of excretion. We hypothesize that gill remodeling in these fish may be intimately tied to physiological adjustments on the whole-body scale.