The relationships between spawning horseshoe crabs and egg densities: Recommendations for the assessment of populations and habitat suitability

Abstract

1. Horseshoe crab population sizes and trends have been previously studied using surveys of spawning adults and counts of eggs from surface (top 5 cm) and deep (20 cm) sediment samples. The correlations between surface and deep eggs were studied at two locations, Delaware Bay and Jamaica Bay, USA, and the correlations between egg densities and spawning counts were examined in Jamaica Bay.2. There were significantly higher densities of eggs in deep sediments than in surface sediments. Only about 10% of the variability in surface egg density was explained by deep egg density. The numerical patterns between surface and deep eggs were similar between Delaware Bay and Jamaica Bay and across sampling dates.3. Nearly 20% of the deep samples in the combined data from Delaware Bay and Jamaica Bay with an egg density of ≥100,000 m −2 had zero surface eggs.Therefore, the use of surface eggs as an indicator of habitat suitability and spawning intensity may seriously underestimate the importance of a beach for spawning horseshoe crabs. 4. When paired with nearest survey date, Jamaica Bay spawning indices did not predict deep or surface egg densities. This may be related to a temporal mismatch between survey methods, the extreme overdispersion (patchiness) of the eggs, and/or the dynamics of egg distribution after exhumation. 5. Both egg density and spawning surveys can provide useful data on habitat suitability for horseshoe crabs and can offer excellent opportunities for student and citizen scientist engagement. More labour is required for egg surveys than spawning surveys because of the time required to sample, sort, and enumerate the eggs.

The use of alternative spawning habitats by the American horseshoe crab,Limulus polyphemus

“…Despite grain sizes not differing significantly between beach and alternative habitats in CT and NH, and wave energy being generally similar between beach and alternative habitats in NH, we nevertheless tended to find more eggs in samples from alternative habitats than those from sandy beaches in these states as well. Egg density measurements are often used as a proxy for habitat suitability in horseshoe crabs (Smith et al 2002b;Pooler et al 2003;Botton et al 2006Botton et al , 2018Botton et al , 2021. By this metric, the alternative habitats surveyed in this study are equally, if not more, suitable for horseshoe crabs than sandy beach habitats.…”

“…For instance, numerous potential salt marsh habitats in SC are only accessible by boat, and, during our surveys in SC, CT, and NH, the high water level on the salt marsh platform made it difficult to see spawning horseshoe crabs, increasing the likelihood of underestimating spawning horseshoe crabs in these alternative habitats. These survey challenges may explain the somewhat disconnected results from our spawning and egg surveys, although densities of adults on spawning beaches do not always directly correlate with egg abundance (Botton et al . 2021, but see James-Piri et al .…”

“…2003; Botton et al . 2006, 2018, 2021). By this metric, the alternative habitats surveyed in this study are equally, if not more, suitable for horseshoe crabs than sandy beach habitats.…”