A walk on the beach: exploring iNaturalist observations of the Atlantic horseshoe crab, Limulus polyphemus, as a tool for conservation

Abstract

The Atlantic horseshoe crab, Limulus polyphemus, is an ecologically and commercially important species occurring across the south and northeastern coast of the United States. However, populations of this species are experiencing declines due to overcollection, use as bait, and other anthropogenic factors. Citizen science holds great potential as a useful complementary tool to traditional monitoring efforts, so we evaluated horseshoe crab observations uploaded to the citizen science platform iNaturalist. Across 10,094 observations, the majority of which occurred primarily during summer months, the number of observations increased annually from 2008 to 2022, presumably as this platform is gaining popularity in overall use by citizen scientists. Analyzing a subset of images from this dataset, we found evidence of scavenging, ranges in the number of individuals, and additional organisms (molluscs or barnacles) attached to specimens, with 52.7% live versus 38.9% dead, and most images showing a complete specimen (61.6%) and adult life stages (80.3%). Most observations were taken on sand (75.7%), with some in shallow water (9.4%) or in vegetation (7%). We recommend researchers incorporate this freely available database as a complementary conservation tool to document presence and natural history and to monitor population trends both in this species and other marine species.