The Effects of Water Quality on Horseshoe Crab Embryos and Larvae

It is well established that horseshoe crab eggs can develop successfully across a wide range of temperatures and salinities. However, many estuaries in which horseshoe crabs spawn have been heavily impacted by pollutants, and degraded water quality may be affecting the survival of horseshoe crab eggs laid in such areas. Laboratory bioassays using Limulus polyphemus embryos and larvae have shown a very high tolerance to a variety of contaminants, including heavy metals, oil, and organic compounds, in comparison to similar stages in other marine arthropods. Of the metals tested, acute toxicity was highest for mercury and tributyltin (TBT), but much lower for cadmium, zinc, and copper. Possible mechanisms of pollution tolerance, including stress proteins (Hsps), are discussed. Sublethal levels of heavymetals (especially mercury and TBT) induced a variety of embryonicmalformations and impaired regeneration of walking legs. The frequency of malformed L. polyphemus embryos was low (ca. 1%) both in relatively pristine habitats (lower Delaware Bay, NJ) and in urban estuaries (Sandy Hook Bay, NJ, and Jamaica Bay, NY). In contrast, a much higher percentage of malformed Tachypleus tridentatus embryos have been found from several locations in Japan, and severely polluted water may be hampering Japanese efforts to re-establish horseshoe crab populations. Pollutants accumulated by adult females may become incorporated into their eggs and could conceivably be passed up the food chain to egg predators.