A study on enrichment of the rotifer Brachionus “Cayman” with iodine from different sources

Iodine is an important micronutrient for the marine fish metamorphosis and rotifers contain relatively low amounts of iodine compared to copepods, the main natural prey for marine fish larvae. In previous studies, rotifers were enriched with iodine from NaI. However, this source of iodine is poorly retained in rotifers during storage. In the present study rotifers were enriched (3 h) with graded levels of thymol iodide (TI), 3-5-diiodosalicylic acid (DISA), chelated iodine (CI), kelp and sodium iodide (NaI) mixed with a commercial enrichment diet (Ori-green®). Thereafter, iodine retention in rotifers enriched with selected iodine sources and concentration and stored (0, 1, 2, 5, 10 and 18 h) at high and low temperatures was studied (room temperatures at 24 and 7 °C, respectively).Linear regression showed a positive relationship between dietary and rotifer iodine concentration for all iodine sources. Results from the first trial showed that 0.36–2.03% replacement of the enrichment diet with TI, gave copepod iodine levels. Maximum iodine level (446.67 mg kg− 1 DW) was observed in the rotifers fed with a diet containing 9.7 g kg− 1 rotifer DW, added iodine from TI. 118 mg kg− 1 DW iodine was observed in rotifers fed with diet containing 9.7 g kg− 1 rotifer DW, added iodine from DISA. Rotifers enriched with kelp, gained maximum up to 196 mg kg− 1 DW iodine in this study. Chelated iodine was a poor source of iodine for rotifer enrichment. Even with 100% replacement of the enrichment diet with CI, rotifers were not able to reach the lower copepod iodine level (50 mg kg− 1 DW). 0.3 g l− 1 NaI gave 370 mg kg− 1 DW iodine in rotifers. Trial 1 indicated that with TI, DISA and NaI, rotifers can be enriched to copepod level with minimal replacement of the enrichment diet.In trial 2, temperature did not affect the rotifer iodine retention. In the first h of storage, rotifer enriched with TI lost 34–62% iodine and rotifers enriched with DISA lost 55–66% iodine while NaI enriched rotifers lost 78–85% iodine. After 18 h storage 79–97% loss was observed in rotifers enriched with NaI and 40–56% loss was observed in rotifers enriched with TI. Loss of the other compounds was intermediate.In conclusion, thymol iodide seems to be a good source of iodine for enrichment of rotifers as only < 2.5% replacement of the enrichment diet gives copepod iodine levels and after enrichment followed by storage at high or low temperature for 18 h, rotifers are able to retain relatively high amounts of iodine in their body.