Sensitivity of juvenile lobsters, Homarus americanus L., to different concentrations of malachite green during various states of the intermoult cycle

Abstract. Survival of the early stages (egg, larva and juvenile) of the American lobster, Homarus americanus L., reared under controlled conditions has often been threatened by a variety of fungi and microbial epibionts. A preliminary study was conducted for the purpose of defining toxicity levels of malachite green for 5th stage juveniles during each of the following intermoult (or fused intermoult) states: A-B, C₄, D₀ and D₂-D₃ Irrespective of exposure time or concentration, all the lobsters treated during intermoult state A-B died. This contrasts with seemingly good survival of lobsters in intermoult states C₄, D₀ and D₂- D₃ treated at concentrations of 900 mg/1 malachite green for exposure periods of 10 min. The findings suggest that an effective treatment combination for lobsters immersed in malachite green may be one which is employed during intermoult state C₄, for about 6min and at concentrations of about 700–800 mg/l.