Inducibility of metallothionein mRNA expression and cadmium tolerance in larvae of a marine teleost, the turbot (Scophthalmus maximus).

An acute (48-hr) lethal toxicity study of turbot at various stages from hatch to juveniles showed that prior to first exogenous feeding, larvae of turbot were an order of magnitude more sensitive to cadmium toxicity than later developmental stages (LC50 of 0.18-0.23 ppm Cd versus 2-5 ppm Cd). To investigate the possible role of metallothionein (MT) in Cd tolerance, the concentrations of MT mRNA were determined in control and Cd-exposed animals using a plaice MT cDNA probe. MT mRNA was expressed throughout embryonic and early larval development at levels approximately twice those in juvenile turbot liver apart from the period around hatching, when there was a decreased level of expression. During endogenous feeding, despite larval drinking, MT mRNA levels did not appear to be inducible by exposure to Cd in the water within 3 days of hatching but were induced three- to fivefold after 48 hr exposure to waterborne Cd at all later stages of development. The results suggest that Cd tolerance is associated with metal induction of MT gene transcription and that the enhanced sensitivity of very early stage larvae is attributable to a deficiency in MT gene transcription.