Bioaccumulation and Metabolic Effects of the Endocrine Disruptor Methoprene in the Lobster, Homarus americanus1

Although we did not observe altered swimming behavior, we did find that low levels of methoprene had adverse effects on lobster larvae. Methoprene concentrations of 1 ppb or higher significantly affected survival for Stage II larvae. Stage IV larvae were more resistant, but did exhibit significant increases in molt frequency beginning at exposures of 5 ppb methoprene.

Our studies of juvenile lobsters revealed variations in tissue susceptibility to the effects of methoprene. The hepatopancreas appears to be the most vulnerable, with environmental concentrations of methoprene inhibiting almost all protein synthesis in this organ. The hepatopancreas is critical to homeostasis in crustaceans, being involved in digestion of food, absorption of nutrients, production of hemolymph proteins and host defense against infectious agents. The in vivo consequences of hepatopancreatic compromise could include, therefore, both increased morbidity and mortality of juvenile lobsters.

Although phylogenetically related, insects and crustaceans have many obvious anatomic and physiologic differences. One difference is the necessity of many crustaceans to molt not only during embryonic and larval stages, but also to continue to molt throughout adulthood. Studies by Laufer et al. (1987), Chang (1993) and others have indicated that MF influences the crustacean molt in part by increasing the synthesis of the molt-related hormone, ecdysone. Laufer et al. (1998) have also shown that MF stimulates ovarian maturation in the crayfish Procambarus clarkii.

The increased frequency of molts in the stage IV larvae and the historical observation of berried females dying while attempting to molt raise the possibility that methoprene could be responsible for endocrine disruption in larval and adult lobsters. Other investigators have shown an interplay between MF and ecdysone in other crustaceans. In 1975, Demeusy suggested that large premolt increases in the hemolymph ecdysteroid liter were due to an ecdysiotrophic action of methyl farnesoate. Then, in 1977, Hinsch demonstrated that, during the premolt, the mandibular organ in Libinia emarginata undergoes ultrastructural changes indicative of increased synthetic activity. In Cancer magister, Tamone and Chang (1993) showed that ecdysteroid synthesis by Y organs is increased both by incubation with mandibular organ-conditioned medium or with methyl farnesoate. If methoprene is acting as an MF analog, it is reasonable to suggest that methoprene could affect the synthesis of ecdysone, and accordingly influence the timing and frequency of molts. Molting is a stressful and vulnerable time for all crustaceans, larval and adult; thus, increased numbers of molts lead to increased periods of vulnerability to predation, trauma and infection. Moreover, our studies also indicate that methoprene has an effect on the quality of the post-molt shell.

Our studies in blue crabs revealed that methoprene interrupted chitin production in the adult postmolt blue crab as evidenced by decreased incorporation of 3Hglucosamine into methoprene-treated expiant cuticle. We also found an effect on protein synthesis as shown by diminished incorporation of radiolabeled amino acids into the expiant shells (Horst and Walker, 1999). In the present investigation, we observed that methoprene alters the synthesis and incorporation of both buffer and urea soluble chitoproteins into adult postmolt lobster expiants. Presumably, these two fractions represent intermediate stages of glycosylation.