Distribution and Development of Limulus Egg Clusters on Intertidal Beaches in Delaware Bay

Abstract

Accurate knowledge of where spawning Limulus females place their egg clusters in beaches is important for sampling egg cluster density, which in turn is important in studies of habitat use, monitoring Limulus egg production, determining potential shorebird forage, and for investigating the effects of beach erosion or replenishment. We examined Limulus egg cluster placement on seven western shore Delaware Bay beaches. Depth to center of recently laid clusters ranged from 3.5–25.5 cm, with an average depth to center of 15.5 ± 3.5 cm (n=533). Centers of 88.0% (469) of all clusters were within 20 cm of the undisturbed beach surface, and 98.1% of all clusters (523) extended 1 cm or more into the 20 cm horizon. Clusters were found only in the upper 85% of the foreshore, beginning at the spring tide high-water mark, even though beach widths varied (n = 6,132 clusters in 80 transects). Intact, recently laid clusters contained from 2,524 to 16,835 eggs, with an average size of 5,786 ± 2,834 eggs (n =26). Beach sediment temperatures in early May ranged from 13.7°C to 24.2°C (mean = 17.0° ± 3.0°C SD); in early June from 16.4 to 29.7°C (mean = 21.6° ± 3.3°C SD); and in early July from 22.4 to 30.4°C (mean = 26.8° ± 1.9°C SD). Under these conditions, the first trilobite larvae (<25 larvae per beach sampled) normally began to appear in our sediment samples during the first week in June, suggesting that about 35 days are required for development of the earliest eggs in a normal spring. Because egg clusters on all beaches were confined to 85% of the upper foreshore, and most clusters were within reach of a 20 cm deep sample, future studies to assess cluster densities should be designed to sample within that portion of a beach.