Horseshoe crabs in the Inland Bays of Delaware: Exposure to endocrine disrupting compounds, the impacts of those compounds on larval development and horseshoe crab populations

The horseshoe crab, Limulus polyphemus, is found along the east coast of North America. This species is of great economic, biomedical and ecological importance. For these reasons, it is important to better understand the population ecology of the horseshoe crab, as well as how this species may respond to chemical stressors. My research examines the horseshoe crab populations of Delaware from 3 perspectives: ecologically, chemically and toxicologically.;Ecological perspective: Part I. The population of L. polyphemus in Delaware Bay is world-renown and well-studied, however there are no published studies of the population in the nearby Inland Bays of Delaware. I studied distributions of spawning adult horseshoe crabs at 6 sites within the Inland Bays. Spawning densities within the Inland Bays averaged to 3.34 individuals/m beach. However, the Coastal Kayak site within the highly enclosed Little Assawoman Bay had a significantly lower spawning density than any other site studied (0.82 individuals/m beach). I determined the overall spawning density of the Inland Bays is similar to that of the 3.5 individuals/m beach in Delaware Bay.;Chemical perspective. Horseshoe crab eggs, embryos and larvae and the sediments surrounding them were chemically analyzed using a gas chromatography-mass spectrometry method to detect evidence of endocrine disrupting compound (EDC) exposure. Six EDCs were examined in my study: p-Nonylphenol (NP), alachlor, 17beta-estradiol (E2), 17alpha-ethinylestradiol (EE2), methoprene and triclosan (TCS). Samples were taken from 3 beaches within the Inland Bays (James Farm, Tower Road and Bay Colony) and 3 beaches within the Delaware Bay (Woodland Beach, Kitts Hummock, Prime Hook). When comparing the Inland Bays with the Delaware Bay, there were no significant differences in average contaminant levels for any compound.;Toxicological perspective. After determining that larval horseshoe crabs were indeed exposed to EDCs in the environment, a suite of physiological experiments were performed to examine how EDC exposure might impact larval horseshoe crab development. The EDCs methoprene, NP and EE2 were selected for these exposure experiments. Exposure to these compounds did not result in significant alterations in biomass or respiration rate in larval horseshoe crabs. However, individuals exposed to 1000 microg/L of methoprene caused inhibition of normal development of all larvae from the 1st into the 2nd instar stage. The effective dose however was beyond the levels typically reported in the natural environment, which are 2-4 orders of magnitude lower. It is surmised that developmental delays in early life stages would increase the mortality of larvae, which have already been shown to have low survivorship rates.;Ecological perspective: Part II. A mathematical model was constructed in an attempt to understand how developmental delays, such as those caused by EDCs, could impact the horseshoe crab population of the Inland Bays over time. Several scenarios were tested, including no stress, developmental delays (1 week and 4 weeks in duration) and habitat loss. The results of the no stress scenario suggested that the horseshoe crab population of the Inland Bays is relatively stable and will experience a 140 % increase over the next 100 years. When developmental delays resulting from EDCs were assumed to occur in the entire estuary, it resulted in an 84-99 % population decline. Loss of habitat at the Tower Road site had no negative impacts on populations, and actually resulted in population increases (137 % over 100 years). Also, when developmental delays were assumed to occur only at Tower Road, the population was relatively well-maintained over the 100 year simulation (136-137 % increases over 100 years). These results suggest that while care must be taken not to overwhelm the Inland Bays system with EDCs (as it could results in severe declines), the system can handle some level of stress without the horseshoe crab population crashing. (Abstract shortened by UMI.)...