Respirometry and cutaneous oxygen flux measurements reveal a negligible aerobic cost of ion regulation in larval zebrafish (Danio rerio)

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 and are enriched with mitochondria; 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-fertilization (dpf) that were reared under low pH (pH 4) conditions exhibited a higher rate of Na+ uptake compared with fish reared under control conditions (pH 7.6), yet they 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 ṀO2 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, ṀO2 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.

Glucose uptake as an alternative to oxygen uptake for assessing metabolic rate in Danio rerio larvae

“…This was omitted in favour of reducing the total volume of the respirometer, improving detection of small changes in oxygen saturation and limiting bacterial respiration, as reported for other low volume respirometry studies ( Folkerts et al, 2017 ; Baker et al, 2018b ; Rodgers et al, 2016 ). The recirculation pump in the ambient water bath in which the chambers were placed ensured consistent water temperature and caused turbulence (i.e., agitation of the chambers) similar to the gentle shaking used by ( Parker et al, 2020 ).…”

“…An approximate larval volume was instead used to calculate the net respirometer volume (net volume to animal volume ratio 2999:1). As such, oxygen consumption data was calculated per larvae rather than per unit mass ( Parker et al, 2020 ).…”

Physiology and aquaculture: A review of ion and acid‐base regulation by the gills of fishes

“…Babikian et al, 2017; Giacomin, Bryant, et al, 2019; Urbina & Glover, 2015) and there is a lack of consensus regarding the cost of ion regulation in fishes (Ern et al, 2014). Furthermore, the cost of maintaining ion balance in developing fish larvae whose ionoregulatory physiology differs dramatically from that of adults due to a lack of gills (cutaneous ionoregulation) and large surface area‐to‐volume ratios (Brauner & Rombough, 2012; Rombough, 2007) may be even lower than that estimated for adults (Parker et al, 2020). Nevertheless, a general trend has been observed whereby FW fishes held at a slightly higher salinity and SW fishes held at lower salinities (i.e.…”

Chemical niches and ionoregulatory traits: applying ionoregulatory physiology to the conservation management of freshwater fishes

“…Basic ionoregulatory traits such as ion fluxes ( Giacomin et al , 2020 ), blood/tissue ion content ( Blanchard and Grosell, 2006 ), transepithelial potential ( Wood et al , 2020 ), stress indicators (e.g. cortisol/glucose responses; Kammerer et al , 2010 ), behaviour ( DeLonay et al , 1993 ; Ikuta et al , 2003 ), metabolic rate/metabolic status ( Parker et al , 2020 ) or general fitness traits (i.e. mortality, growth, development, reproduction) can underlie more complex traits like substrate affinity for ion uptake (e.g.…”

“…Virtually all freshwater fishes regulate concentrations of major ions (Na + , Cl − , Ca 2+ , K + , Mg 2+ , SO 4 2− ) within a narrow range in the blood plasma. Maintaining ion balance involves a co-ordinated response of many organ systems (gills, gut, kidney) and imposes a significant metabolic cost, although estimates of this cost vary substantially across different studies ( Kirschner, 1995 ; Boeuf and Payan, 2001 ; Ern et al , 2014 ; Parker et al , 2020 ). Furthermore, disruption of ion balance in freshwater fishes can have detrimental effects, culminating in osmotic disturbances and consequent cardiovascular failure in severe cases ( Milligan and Wood, 1982 ; Grosell et al , 2002 ).…”