Distribution, abundance, and survivorship of young-of-the-year in a commercially exploited population of horseshoe crabs Limulus polyphemus

Horseshoe crab Limulus polyphemus eggs are laid on sandy estuarine beaches, and subsequently develop into first instar (trilobite) larvae that emerge into the plankton before settlement. To obtain estimates of early survival, we collected data on the densities of trilobite larvae within intertidal sediments on a beach in Delaware Bay, New Jersey, and the densities of post-settlement juveniles, through the first 4 instar stages, on the adjacent tidal flats. Over the course of the spawning season in 1999, there were some 761 000 live trilobites produced within a 1 x 3 m band of intertidal beach. Losses (mortality + emigration) were highest during the brief planktonic phase or shortly after settlement to the benthos; we estimated that about 97.5% of the live trilobites on the beaches were lost during their transition to epibenthic trilobites on the intertidal flats. Newly settled horseshoe crab juveniles were not dependent on a specific sediment type, but tended to be more abundant close to shore. After settlement, there was about a log-fold decrease in juvenile density with each molt from the second through the fourth instar. Overall, only about 3 larvae per 100 000 remained as fourth instar juveniles on the tidal flats at the end of their first summer. Our ability to extrapolate future adult year-class strengths based on young-of-the-year densities is limited, because 9 to 10 yr are required for horseshoe crabs to reach sexual maturity.