The Power of Citizen Science: 20 Years of Horseshoe Crab Community Research Merging Conservation, Education, and Management

Long-term observations of the distribution, mating behavior, and movement patterns of horseshoe crabs are important for understanding their ecology. Population genetic analyses are useful to define ecological management units. Combined, this information informs the management of the species. Citizen science, the active public involvement in scientific research, can result in the production of “big data” both spatially and temporally especially when volunteers are involved with tagging (mark/recapture) studies and spawning counts. In this study, involving Limulus polyphemus, citizen scientists placed over 76% of our 95,567 tags put out on horseshoe crabs over 20 years in Long Island Sound (LIS) and reported nearly 85% of our recaptures. Of the 15,655 marked and recaptured horseshoe crabs, 18% had moved more than 10 km and 547 individuals crossed the Sound from Connecticut (CT) shores to the northshore of Long Island, New York (NY). Our genetic analysis of 11 microsatellite loci from 186 individuals from five areas across the Sound found no population substructure and no inbreeding depression. This supported the results of the tagging study. Thus, the horseshoe crab population of the New York region is panmictic. It is known from observations in Delaware Bay during May and June that horseshoe crabs are a dominant species (spawning index ~1 female/m2) and their egg-laying activity directly increases biodiversity and biomass on beaches and in estuaries where they are abundant. When horseshoe crab populations decline, as observed in LIS, so do the number of species and the populations of dependent predator species that feed on their eggs. The spawning index is low 0.009–0.036 female/m2 and generally has been declining over the past 10 years in LIS. The trend from over 15 years of tagging shows 20–30% of the females come up to the beach without a mate. In LIS, a shared resource of Connecticut and New York, horseshoe crab populations continue to be overharvested and continue to decline, which has a negative impact on biodiversity. How these results relate to the three Asian horseshoe crab species will be discussed. The horseshoe crab population in the New York region is breeding well below its intrinsic rate of growth. A moratorium on the harvest of horseshoe crabs is needed; investment in coastal habitat restoration and establishment of marine protected areas in LIS are also required for recovery of the population