Introduction Fecundity and spawning of the Atlantic horseshoe crab, Limulus polyphemus,

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The Atlantic horseshoe crab, Limulus polyphemus, native to the east coast of North America from the Gulf of Maine to the Yucatan, is important ecologically and economically. Limulus eggs are a major food source for migrating shorebirds along the Atlantic coast (Castro and Myers, 1993; Clark et al. 1993, Botton et al., 1994, Tsipoura and Burger 1999). Horseshoe crab adults are commercially harvested for the blood-clotting compound Limulus Amoebocyte Lysate (LAL), which is widely used to detect endotoxins on surgical instruments and implants (Novitsky, 1984; ASMFC, 1998; Rutecki et al., 2004). The eel and whelk fisheries harvest Limulus, especially the large, egg-laden females (Manion et al., 2000; Ferrari and Targett, 2003), for use as bait. Horseshoe crabs are also used as biomedical models to study vision, cell biology, neurobiology, drug development, and immunology (Rutecki et al., 2004).

Populations of horseshoe crabs are thought to be declining due to a combination of harvest pressure and habitat destruction (Rudloe, 1982; Swan et al., 1996; Widener and Barlow, 1999). The importance of this ancient species and concern that its numbers are dwindling has prompted demand for information on horseshoe crab populations and life history variables that can be useful in management and conservation efforts (Berkson and Shuster, 1999; Eagle, 2001). While much research has been done on Limulus, lack of reliable information on their fecundity has constrained the ability of scientists and resource managers to develop models and other management tools. For example, data on ‘net fecundity’ (defined here as the total annual contribution of eggs by a given size class of females) could be used to determine which crabs contribute most to a population’s reproductive success. In areas such as Delaware Bay, where Limulus eggs help nourish shorebirds on their annual migration, fecundity data can be used in estimating how many shorebirds can be sustained from this food source. Basic information would also aid development and refinement of models, currently in demand to predict population growth rates, effects of size-selective harvest, reproductive value, and stable stage distribution of populations.

Previous research in Delaware Bay suggested that, on average, each adult female horseshoe crab produces 88,000 mature and immature eggs (Shuster and Botton, 1985). There are several issues still to be addressed regarding the interpretation of this number. First, in related invertebrates such as spiders, insects, crustaceans, and the Indian horseshoe crab (Carcinoscorpius rotundicauda) larger females carry greater numbers of eggs than smaller individuals (Chatterji and Parulekar, 1992; Stella et al., 1996; Sukumaran and Neelakantan, 1997; and many others). It is not known whether a similar relationship between female size and number of eggs occurs in Atlantic horseshoe crabs. If so, the mean number of eggs measured among Delaware Bay horseshoe crabs, which are among the largest in their range (Shuster, 1955, 1982), may not be representative of all populations. To understand the relative contribution of reproductive effort by assemblages of female horseshoe crabs of different sizes, it is necessary to more clearly define the relationship of egg production to female size in this species.

Second, there is also little known about the pattern of egg maturation in horseshoe crabs and how this may relate to the number of eggs laid by an assemblage of females each year. Some fish and insects continuously develop and replenish eggs laid throughout the course of the breeding season while others mature all of their eggs prior to each season, with remaining immature eggs not maturing until the following year or later (‘determinate’ spawners) (Wallace and Selman, 1981; Hunter et al., 1985, 1992; Watanabe and Adachi, 1987, Murua et al., 1996; Jervis et al., 2001). Determinate spawners generally contain only mature and immature eggs during the breeding season – there are no intermediate developmental stages present. Also, some of these animals retain and/or resorb mature eggs at the end of the breeding season (Bell and Bohm, 1975; Rivero-Lynch and Godfray, 1997; Rosenheim et al., 2000). An understanding of the strategy used by horseshoe crabs would help determine which eggs should be counted when measuring fecundity and refining estimates of reproductive potential.

Breeding patterns of horseshoe crabs, including length of spawning season, size-frequency distribution of spawning females, clutch size, and patterns and timing of egg release, also affect net fecundity. Much is already known about horseshoe crab breeding from previous studies (e.g. Rudloe, 1980; Cohen and Brockmann, 1983; Barlow, 1986; Brockmann 1990, 1996; Brockmann and Penn, 1992; Penn and Brockmann, 1994, and many others). For example, pairs of crabs in amplexus (male clasping posterior of female’s carapace) typically come ashore with the high tides onto protected beaches in spring to breed (Sekiguchi, 1988). Females deposit eggs in multiple small clutches in nests 10-20 cm deep in the sand. As the eggs are laid, they are fertilized externally by the male in amplexus, and often by aggregations of satellite males as well (Rudloe, 1980; Sekiguchi, 1988; Brockmann and Penn, 1992; Brockmann, 1996). Females return to the beach to spawn more eggs over several days (Rudloe, 1980; Cohen and Brockmann, 1983). Though these behaviors have been studied elsewhere, these generalized spawning habits vary by location, making it essential to gather information about breeding habits in our study location to provide a behavioral context to corroborate fecundity estimates. We therefore examined these patterns as an ancillary, confirmatory test of our fecundity results.

We addressed the lack of information on horseshoe crab fecundity by examining a representative population that has received study in Pleasant Bay, Cape Cod, Massachusetts. This Bay sustains a large, actively breeding population of horseshoe crabs with known spawning areas (Shuster, 1982; James-Pirri et al., 2002, 2005; Carmichael et al., 2003). In this study, we quantified size-specific potential fecundity (number of eggs carried by females of different sizes during the breeding season), and realized fecundity (number of eggs actually laid by different sized females). We delineated the breeding season. To corroborate realized fecundity estimates with spawning patterns, we determined number of eggs laid per spawning episode (i.e., visit to the beach) by individual females, and patterns of returns to the beach. Size frequency distribution of spawning Limulus females in Pleasant Bay was determined. We then used this information together with data on abundance and fecundity to calculate net fecundity (F_x).