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Trilobite larvae of the horseshoe crab Limulus or unusually heavy wave action. Within 2 additional polyphemus L. develop from eggs deposited within intertidal sediments. Most of these larvae emerge from sediments during the summer. We have found that a smaller component of the larval population on a Delaware Bay (New Jersey, USA) beach can delay emergence, and remain alive within the sediments until the following spring. Overwintering trilobites were distributed in a 3 m wide band in the mid-tide region, at sediment depths > 15cm. 103 to 104 live trilobites have been found throughout winter and into early spring, This phenomenon may be ecologically significant, since emergence of overwintering larvae in early spring occurs at a time when predation by birds is minimal. Sediment disturbance by winter storms may be the major factor limiting the survivorship of overwintering cohorts. Adult horseshoe crabs Limulus polyphemus L. deposit eggs within intertidal beaches from Maine, USA, to Yucatan, during spring and early summer (Shuster 1982). A large concentration of spawning L. polyphemus occurs in lower Delaware Bay (New Jersey, USA) (Shuster & Botton 1985, Botton et al. 1988), first appearing in late April. Peak numbers of horseshoe crabs are generally associated with spring high tides during May and June, although a small spawning population persists through July and August (Botton 1984a, Shuster & Botton 1985). Eggs develop through the larval stage ('trilobite') within the sand. The developmental rate of embryos is temperature dependent (French 1979, Jegla & Costlow 1982, Sekiguchi 1988), but under typical field conditions in Delaware Bay, trilobites are common within beach sediments 3 to 4 wk after fertilization. Rudloe (1979) found that tnlobite emergence was correlated with full moon ' Addressee for correspondence weeks, trilobites molt into the benthic first posthatch juvenile stage (Sekiguchi et al. 1988). Based on these findings, one might predict that no trilobite larvae should remain within beach sediments by late summer. In this paper, we present field evidence that tnlobite larvae of Limulus polyphemus overwinter on sandy beaches of Delaware Bay. This previously unknown life-history phenomenon is discussed with respect to the ecology of these beaches and the population biology of this species. Material and methods. The study area was located near the Rutgers University Shellfish Research Laboratory in Cape May County, New Jersey (cf. Fig. 1 in Botton et al. 19881, an optimal area for horseshoe crab spawning along the western shore of Delaware Bay (Shuster & Botton 1985). The beach is ca 12 m wide with a 6" slope. The beach was sampled at low tide on 33 dates between January 1989 and January 1992 at ca 3 to 4 wk intervals, with the exception of the period from 6 July 1990 to 14 January 1991. Replicate transects were established 1 m apart on the beach from the spring high water mark to mean low water. Only the mid-beach region is discussed in this paper, because no overwintering trilobites were found above or below this zone. Three stations on the transect, within the midbeach region, were established at 1.5 m intervals. The exact position of each station was fixed relative to a reference marker located in a vegetated area above the high tide line. Transects were established north or south of this reference marker in order to avoid sampling the same region consecutively. Plots within a station along the transect were 35 cm on a side (area 0.1225 m'). We sequentially excavated 5 cm slices of O Inter-Research 1992 290 Mar. Ecol. Prog. Ser. 88: 289-292, 1992 sediment within the plot frame, from the surface to a maximum depth of 25 or 30 cm. Sand from the entire slice was mixed thoroughly, and a 1 1 sediment sample was placed into a plastic bag. Samples were refrigerated (5°C) for laboratory analysis, which commenced within a few days. In all, 682 samples were taken. Three separate aliquots of sand (usually 80 ml, but occasionally less in samples containing large numbers of eggs) were washed through a 1 mm mesh. Horseshoe crab eggs, embryos and trilobite larvae, and other macrofauna were counted. To compute trilobite density (expressed as tnlobites m-2), the average of the aliquots for a particular depth was summed over all depths from surface (0 to 5 cm) to bottom (down to 25 to 30 cm). Densities for replicate transects were averaged for purposes of plotting. Throughout this study, samples of live trilobite larvae were separated from the sand and maintained in plastic Petri dishes with filtered sea water in order to determine their competency to molt to the first post-hatched juvenile stage. Results. Overwintering trilobite larvae of Limulus polyphemus were found during 3 of the past 4 winters in Delaware Bay (Fig. 1). In January 1989, live trilobite larvae of the 1988 year class were discovered in beach sediments at depths of 15 to 25 cm. These larvae persisted throughout March, but were absent from the sediments by mid-April 1989. The 1989 year class of horseshoe crab trilobite larvae were first detected in mid-June, ca 1 mo after adult horseshoe crabs were observed to begin their spawning activity on the study beach. By October 1989, there were still over 7000

Overwintering by trilobite larvae of the horseshoe crab Limulus polyphemus on a sandy beach of Delaware Bay (New Jersey, USA)