HEAVY-METALS INHIBIT LIMB REGENERATION IN HORSESHOE-CRAB LARVAE

We studied the effects of heavy metals on the regeneration of walking legs in horseshoe crabs (Limulus polyphemus). The second walking leg w as amputated in embryos (stage 20 and 21) and first instar (trilobite) larvae, and the length and morphology of the regenerated appendage wa s observed after molting to the second instar stage. Regeneration foll owing continuous exposure to TBT (0.001-100 mg/L), mercury (0.001-100 mg/L), cadmium (0.01-100 mg/L), chromium (0.1-100 mg/L), lead (0.1-100 mg/L), and copper (1-100 mg/L) was measured relative to regeneration in seawater. Although regeneration was incomplete in controls, treatme nt with heavy metals led to smaller and/or malformed legs. The impacts of heavy metals on survival, molting, and regeneration of horseshoe c rab larvae were ranked as follows: organotin > Hg > Gd > Cr > Zn > Pb greater than or equal to Cu. Cu and Pb did not inhibit regeneration, e ven at 100 mg/L. TBT, Hg, Gd, Gr, and Zn inhibited the regeneration of appendages, although first instar larvae successfully molted into sec ond instars even after treatment. Regeneration was comparable to seawa ter controls in less than 2.5 mg/L Zn. In 5.0 and 10.0 mg/L Zn, regene ration was inhibited and the length of regenerated appendages remained shorter in all second instars. Larvae treated with 10 mg/L Zn for 1-w eek intervals during the molt cycle showed similar patterns of regener ation. The regeneration of claws was not all or none, and formation of the claw was proportional to the length of regenerated appendages. Li mb regeneration in horseshoe crab larvae may be a useful model system for the study of pollutant impacts.