Uptake of iodide from water in Atlantic halibut larvae (Hippoglossus hippoglossus L.)

The natural diet of marine fish larvae, copepods, contain 60–350 mg I kg− 1, while live feed used in commercial hatcheries have iodine concentrations in the range of 1 mg kg− 1. Seawater is also considered to be an important source of iodine for marine fish. The question asked in this study is whether Atlantic halibut larvae are capable of absorbing iodide from the water and if so, can the seawater sustain the iodine requirement during larval development and metamorphosis.Levels of iodide and iodate in seawater samples from four different rearing facilities were analysed. All samples contained relative low levels of iodide (0–22 nM) and except for samples from one site; the levels of iodide and iodate were in agreement with previously published data. The uptake of iodide from seawater was measured by incubating Atlantic halibut larvae in water with a constant level of radioactive iodide (125I−) and increasing levels of cold iodide (127I−). To evaluate whether the uptake of iodide would change during metamorphosis, three different developmental stages (pre metamorphic, metamorphic and post metamorphic) were examined. The uptake was similar for all three stages, increasing with increasing concentration of iodide in the water. The highest level of iodide used was 2000 nM, 100 times higher than what was measured in the seawater samples. The uptake curves did not seem to reach equilibrium. This may be due to a constant non-specific uptake or that the equilibrium level is higher than 2000 nM. The uptake was partly blocked by perchlorate (ClO3−) which is a known inhibitor of the sodium iodide symporter. This indicates that the Atlantic halibut larvae accumulate iodide through both specific and non-specific uptake pathways.