Environmental factors and occurrence of horseshoe crabs in the northcentral Gulf of Mexico

Abstract

This study provides regional-scale data on drivers of horseshoe crab (Limulus polyphemus) presence along the northcentral Gulf of Mexico coast and has implications for understanding habitat suitability for sparse horseshoe crab populations of conservation concern worldwide. To collect baseline data on the relationship between environmental factors and presence of horseshoe crabs, we surveyed four sites from the Fort Morgan peninsula of Mobile Bay, Alabama (AL) to Horn Island, Mississippi (MS). We documented number, size and sex of live animals, molts, and carcasses as metrics of horseshoe crab presence and demographics for two years. Data were compared to in situ and remotely sensed environmental attributes to assess environmental drivers of occurrence during the time of study. Overall, greater evidence of horseshoe crab presence was found at western sites (Petit Bois and Horn Islands) compared to eastern sites (Dauphin Island, Fort Morgan peninsula), mediated by a combination of distance from areas of high freshwater discharge and interannual variation in weather. Higher sex ratios also were found associated with higher occurrence, west of Mobile Bay. Land cover, particularly Bare Land and Estuarine Emergent Wetland classes that are common to western sites, was most predictive of live animal and to some extent carcass occurrence. Our findings suggest that small-scale variation in habitat quality can affect occurrence of horseshoe crabs in sparse populations where density is not a limiting factor. Data from molts and carcasses were informative to supplement live animal data and may be useful to enhance ecological assessment and support conservation and management in regions with sparse populations.

Assessment of recovery potential for the American horseshoe crab (Limulus polyphemus): An application of the IUCN green status process

“…Mortality pressures are limited primarily to natural forces such as storm events, predation and stranding; there is no known harvest. Habitat loss associated with urbanization/shoreline armouring and changes in hydrology that enhance freshwater discharge and erosion, as well as climate change and SLR, are the major threats to populations because they lead to declines of both the quantity and quality of spawning and foraging areas (Darrow et al, 2017; Smith et al, 2020a; Estes et al, 2021). No conservation measures are currently in place, and none are expected in the foreseeable future.…”

“…• Development: Habitat loss from coastal development and shoreline armouring is occurring in this spatial unit but with unknown scope and severity. High levels of freshwater discharge and associated low salinity in spring may limit spawning and distribution, particularly near Mobile Bay (Smith et al, 2020a;Estes et al, 2021). The effects of storms/hurricane activity, SLR and beach nourishment activities are a concern.…”

“…• Natural system modification: Moderate to high scope with unknown severity is expected to be associated with freshwater inputs and opening of spillways (e.g., Smith et al, 2020a). Those modifications in Louisiana could limit the westward distribution of horseshoe crab populations (Estes et al, 2021).…”

“…Genetic analyses are currently unavailable for this area, and phenotypic characteristics are consistent with the Eastern Gulf of México. The patchily distributed populations in this area define the western extent of the known range along the Gulf of México coast (Fulford & Haehn, 2012; Estes et al, 2015; Estes et al, 2021).…”

High-throughput amplicon sequencing datasets of coastal sediments from three locations of the Gulf of Mexico, USA

“…Suspended sediments reduce visibility, and high nutrient levels stimulate algal blooms and exacerbate hypoxia [9] . Over the past several decades, abundant submerged aquatic vegetation has disappeared, and commercially important benthic fauna, including oysters, crabs, and brown shrimp, no longer support commercial harvest in much of the Bay [10] , [11] , [12] , [13] . Over this period, most of the shore has been hardened with sea walls with little remaining marshes [14] .…”