Abstract

Fishes living in fresh water counter the passive loss of salts by actively absorbing ions through specialized cells termed ionocytes. Ionocytes contain ATP-dependent transporters, are enriched with mitochondria, and therefore ionic regulation is an energy-consuming process. The purpose of this study was to assess the aerobic costs of ion transport in larval zebrafish (Danio rerio). We hypothesized that changes in rates of Na⁺ uptake evoked by acidic or low Na⁺ rearing conditions would result in corresponding changes in whole-body oxygen consumption (ṀO₂) and/or cutaneous oxygen flux (JO₂), measured at the ionocyte-expressing yolk sac epithelium using the scanning micro-optrode technique (SMOT). Larvae at 4 days post-fertilisation (dpf) that were reared under low pH (pH 4) exhibited a higher rate of Na⁺ uptake compared to fish reared under control conditions (pH 7.6) yet displayed a lower ṀO₂ and no difference in cutaneous JO₂. Despite a higher Na⁺ uptake capacity in larvae reared under low Na⁺ conditions, there were no differences in ṀO_{2 ­}and JO₂ at 4 dpf. Furthermore, although Na⁺ uptake was nearly abolished in 2 dpf larvae lacking ionocytes after morpholino knockdown of the ionocyte proliferation regulating transcription factor foxi3a, ṀO_{2 ­}and JO₂ were unaffected. Finally, laser ablation of ionocytes did not affect cutaneous JO₂. Thus, we conclude that the aerobic costs of ion uptake by ionocytes in larval zebrafish, at least in the case of Na⁺, are below detection using whole-body respirometry or cutaneous SMOT scans, providing evidence that ion regulation in zebrafish larvae incurs a low aerobic cost.