Populations of horseshoe crabs Limulus polyphemus are widely distributed from Maineto the Gulf of Mexico, are commercially harvested, and are thought to have decreased in abundancein recent decades. To provide information needed to manage exploited populations of horseshoecrabs, we conducted comprehensive field sampling in a major shallow estuary containing preferredhabitat for horseshoe crabs. The data we obtained are the first to comprehensively define abundance,determine sex ratios of horseshoe crabs across an entire estuary, and estimate cohort-specific growthand mortality of juvenile and adult horseshoe crabs. We found that juveniles were more abundantand suffered greater mortality than adults. Adults were largely found on the sediment surface, but20% were buried shallowly in sediments. The male:female sex ratio in juveniles was 1.4:1, but theadult sex ratio was 2.3:1. Juveniles grew faster than adults, and adult crabs may plausibly molt as frequentlyas once per year rather than have a terminal molt. Spawning appeared to span late March tomid-July, and juveniles hatched at semilunar intervals during Year 0 and grew to 16.6 \pm 0.9 mm prosomalwidth by the start of Year 1. The distinct semilunar cohorts of Year 0 coalesced into annualcohorts after Year 0. Females deposited large numbers of eggs, but only 0.001% survived to the endof Year 0, and approximately 78% of these juveniles reached adulthood.
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