Increasing the level of selenium in rotifers (Brachionus plicatilis ‘Cayman’) enhances the mRNA expression and activity of glutathione peroxidase in cod (Gadus morhua L.) larvae

Commercially produced Atlantic cod (Gadus morhua L.) larvae are fed rotifers, while in the wild cod larvae feed on copepods. Rotifers contain up to 30-fold less selenium (Se) than copepods, and may contain insufficient Se to meet cod larvae requirements. In this study, rotifers were fed Se-enriched yeast (Se + rotifers) to obtain copepod levels of Se (3 mg kg− 1 dry weight (DW)), and the effects of feeding the Se + rotifers to cod larvae was investigated and compared to cod larvae fed control rotifers containing 0.7 mg Se kg− 1 DW. No differences were observed in growth or survival between groups at the end of the rotifer-feeding period (29 days post hatch (dph)), or in the rate of skeletal deformities at 120 dph. Cod larvae fed Se + rotifers had increased whole-body levels of Se, and expression and activity of Se-dependent glutathione peroxidases during the rotifer-feeding period (3 to 29 dph) compared to controls. Meanwhile no effect of the treatment was seen in the activity or mRNA expression of the deiodinases. There was no difference in thyroid hormone levels at 29 dph, but the T3/T4 ratio was increased in the Se + larvae. Thyroid follicles of Se + larvae had increased total volume, colloid volume and ratios of epithelium to colloid compared to controls. The results demonstrate that Se-dependent enzyme mRNA expression and activities in cod are retained according to a selenoprotein hierarchy as found in other vertebrates, and suggest that rotifers need extra supplementation of Se to meet cod larvae requirements.