Spawning behavior and larval biology of the American horseshoe crab, Limulus polyphemus, in a microtidal coastal lagoon

The American horseshoe crab, Limulus polyphemus (Linnaeus), typically inhabits estuaries and coastal areas with pronounced semi-diurnal and diurnal tides that are used to synchronize the timing of spawning, larval hatching, and emergence. Spawning occurs in the intertidal zone of sandy beaches. Tidally timed activities of L. polyphemus were investigated in a microtidal coastal lagoon [Indian River Lagoon complex (IRL), Florida] to determine the effect of a system that lacks significant tidal cues on spawning and larval hatching. Spawning occurred year-round, but in the absence of tidal cues, was asynchronous. Peaks in larval abundance were independent of spawning events but were associated with changes in water level. These results indicate that the reproductive activity of horseshoe crabs inhabiting the IRL is different from populations that occur in areas that experience regular tidal changes.; Limulus polyphemus typically inhabits estuaries with salinities ranging from 5–34. Previous studies indicate that the optimal temperature and salinity ranges for early development are 25–30°C and 20–30, respectively. While embryos have been shown to develop at salinities ≤40, tolerance of higher salinities has not been investigated. The osmotic concentration of the perivitelline fluid remained isoosmotic to the external medium in all salinities. These results indicate that high temperature and salinity conditions during the reproductive season influence the rate of larval development, yet, have a limited effect on survivorship and do not account for the low abundance of larvae in the IRL.; Horseshoe crab embryos undergo development below the sediment surface, hatch, and enter the water column when the nest is inundated. Thus, hatching may be facilitated by environmental triggers that serve to synchronize larval hatching and emergence with periods of high water. To test this hypothesis, the effects of hydration, agitation, osmotic shock, and hypoxia on the hatching success of L. polyphemus embryos were determined. All treatments facilitated hatching, yet hydration coupled with agitation produced the highest hatching rates.; Endogenous activity rhythms that match predictable environmental cycles may enable L. polyphemus to time swimming activity to prevent stranding on the beach. Autocorrelation and MESA analysis of the activity records revealed a circatidal rhythm with a free running period of ≈12.3 h. In nontidal and microtidal systems, larvae may lack an endogenous rhythm due to the lack of a tidal entrainment cue and may increase their chances of being stranded on the beach when inundated. (Abstract shortened by UMI.)...