Ecology of Horseshoe Crabs in Microtidal Lagoons

Abstract

The American horseshoe crab, Limulus polyphemus, typically inhabits estuaries and coastal areas with pronounced tides that have a significant influence on their physiology, behavior, and ecology. Most notably, changes in water level associated with the tides have been shown to underlie temporal and spatial patterns in spawning. However, horseshoe crabs also inhabit areas that lack significant tides and associated cues that are known to mediate reproduction. Our investigations of the potential impact of conditions in microtidal lagoon habitats on the timing and frequency of spawning of L. polyphemus adults and the development and survival of embryos and larvae have focused on populations inhabiting the Indian River Lagoon (IRL), a large, shallow estuary along the east coast of Florida, USA. Extensive surveys of adult activity along beaches indicated that spawning occurs year-round but is aperiodic. Larval abundances in the plankton are low and more closely associated with changes in water level than adult spawning activity. Laboratory experiments indicated that embryos and larvae are able to tolerate extreme salinities and temperatures that occur in temperate and tropical microtidal lagoons in the summer. Embryos complete development and molt at salinities below 60, yet fail to develop at temperatures ≥ 35°C. Larvae tolerate salinities of 10–70, but survivorship declines significantly at more extreme salinities (i.e., 5, 80, and 90). Results indicate that both temperature and salinity influence the rate of development, but only the extremes of these environmental factors have an effect on survival. In general, the reproductive ecology and early life history of L. polyphemus in microtidal lagoons differ markedly from those documented for populations inhabiting tidal areas. This difference is likely due to extreme salinity and temperature conditions during summer reproductive periods and the lack of tidally related synchronization cues for spawning and regular, periodic beach inundation.