Abundance and population structure of the Atlantic horseshoe crab Limulus polyphemus in Pleasant Bay, Cape Cod

Populations of horseshoe crabs Limulus polyphemus are widely distributed from Maine to the Gulf of Mexico, are commercially harvested, and are thought to have decreased in abundance in recent decades. To provide information needed to manage exploited populations of horseshoe crabs, we conducted comprehensive field sampling in a major shallow estuary containing preferred habitat 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 growth and mortality of juvenile and adult horseshoe crabs. We found that juveniles were more abundant and suffered greater mortality than adults. Adults were largely found on the sediment surface, but 20% were buried shallowly in sediments. The male:female sex ratio in juveniles was 1.4: 1, but the adult sex ratio was 2.3:1. Juveniles grew faster than adults, and adult crabs may plausibly molt as frequently as once per year rather than have a terminal molt. Spawning appeared to span late March to mid-July, and juveniles hatched at semilunar intervals during Year 0 and grew to 16.6 +/- 0.9 mm prosomal width by the start of Year 1. The distinct semilunar cohorts of Year 0 coalesced into annual cohorts after Year 0. Females deposited large numbers of eggs, but only 0.001% survived to the end of Year 0, and approximately 78% of these juveniles reached adulthood.