Huber, J. H. | bioRxiv Science

bioRxiv2022

The use of alternative spawning habitats by the American horseshoe crab, Limulus polyphemus

For animals that develop externally, habitats where environmental conditions are optimal for embryonic development are sometimes assumed to represent the highest recruitment potential and thus support the majority of reproductive output for a species. However, organisms may spawn in areas considered sub-optimal for embryonic development. Thus, understanding spawning habitat selection decisions and their potential impacts on recruitment and ecological interactions is necessary for predicting population status and identifying critical habitats to inform sustainable conservation decisions and effective management approaches. The American horseshoe crab, Limulus polyphemus, is ecologically, economically, and biomedically important. Females come ashore to spawn in the sediment where eggs develop for 2 - 4 weeks. Horseshoe crabs have been thought to primarily use sandy beach habitat for spawning in part because this habitat has been shown to be optimal for embryonic development. Horseshoe crab eggs on sandy beaches are an essential part of the diet of many organisms, including shorebirds such as the rufa red knot which requires the eggs to fuel their migration to arctic spawning grounds. While horseshoe crabs have been observed spawning in alternative habitats such as salt marshes and peat beds, this behavior has been assumed to be rare and non-adaptive. In this study, we compare the use of beach and alternative habitats by horseshoe crabs for spawning. To do so, we conducted adult horseshoe crab spawning surveys and horseshoe crab egg surveys in beach and Spartina-dominated salt marsh alternative habitats in South Carolina, Connecticut, and New Hampshire, U.S.A. While spawning horseshoe crabs were more likely to be observed on beach habitats than in alternative habitats, potentially due to logistical constraints surveying alternative habitats, we found similar densities of spawning horseshoe crabs in both habitat types. We also tended to find more eggs in alternative habitats than on beaches. Taken together, these results suggest that alternative habitats likely represent a significant source of horseshoe crab spawning activity and recruitment that had not previously been quantified. We recommend this information be incorporated into horseshoe crab population assessments, habitat protections, and more directed research at understanding variability in habitat-specific horseshoe crab spawning and its relationship to migratory shorebirds.

ecology↗

bioRxiv2017

Seasonal temperature variation influences climate suitability for dengue, chikungunya, and Zika transmission

Dengue, chikungunya, and Zika virus epidemics transmitted by Aedes aegypti mosquitoes have recently (re)emerged and spread throughout the Americas, Southeast Asia, the Pacific Islands, and elsewhere. Understanding how environmental conditions affect epidemic dynamics is critical for predicting and responding to the geographic and seasonal spread of disease. Specifically, we lack a mechanistic understanding of how seasonal variation in temperature affects epidemic magnitude and duration. Here, we develop a dynamic disease transmission model for dengue virus and Aedes aegypti mosquitoes that integrates mechanistic, empirically parameterized, and independently validated mosquito and virus trait thermal responses under seasonally varying temperatures. We examine the influence of seasonal temperature mean, variation, and temperature at the start of the epidemic on disease dynamics. We find that at both constant and seasonally varying temperatures, warmer temperatures at the start of epidemics promote more rapid epidemics due to faster burnout of the susceptible population. By contrast, intermediate temperatures (24-25{degrees}C) at epidemic onset produced the largest epidemics in both constant and seasonally varying temperature regimes. When seasonal temperature variation was low, 25-35{degrees}C annual average temperatures produced the largest epidemics, but this range shifted to cooler temperatures as seasonal temperature variation increased (analogous to previous results for diurnal temperature variation). Tropical and sub-tropical cities such as Rio de Janeiro, Fortaleza, and Salvador, Brazil; Cali, Cartagena, and Barranquilla, Colombia; Delhi, India; Guangzhou, China; and Manila, Philippines have mean annual temperatures and seasonal temperature ranges that produced the largest epidemics. However, more temperate cities like Shanghai, China had high epidemic suitability because large seasonal variation offset moderate annual average temperatures. By accounting for seasonal variation in temperature, the model provides a baseline for mechanistically understanding environmental suitability for virus transmission by Aedes aegypti. Overlaying the impact of human activities and socioeconomic factors onto this mechanistic temperature-dependent framework is critical for understanding likelihood and magnitude of outbreaks.\n\nNon-Technical SummaryMosquito-borne viruses like dengue, Zika, and chikungunya have recently caused large epidemics that are partly driven by temperature. Using a mathematical model built from laboratory experimental data for Aedes aegypti mosquitoes and dengue virus, we examine the impact of variation in seasonal temperature regimes on epidemic size and duration. At constant temperatures, both low and high temperatures (20{degrees}C and 35{degrees}C) produce small epidemics, while intermediate temperatures like 25{degrees}C and 30{degrees}C produce much larger epidemics. In seasonally varying temperature environments, epidemics peak more rapidly at higher starting temperatures, while intermediate starting temperatures produce the largest epidemics. Seasonal mean temperatures of 25-35{degrees}C are most suitable for large epidemics when seasonality is low, but in more variable seasonal environments epidemic suitability peaks at lower annual average temperatures. Tropical and sub-tropical cities have the highest temperature suitability for epidemics, but more temperate cities with high seasonal variation also have the potential for very large epidemics.

ecology↗