Ecdysteroid responses to urban heat island conditions during development of the western black widow spider (Latrodectus hesperus)

Abstract

The steroid hormone 20-hydroxyecdysone (20E) controls molting in arthropods. The timing of 20E production, and subsequent developmental transitions, is influenced by a variety of environmental factors including nutrition, photoperiod, and temperature, which is particularly relevant in the face of climate change. Environmental changes, combined with rapid urbanization, and the increasing prevalence of urban heat islands (UHI) have contributed to an overall decrease in biodiversity making it critical to understand how organisms respond to elevating global temperatures. Some arthropods, such as the Western black widow spider, Latrodectus hesperus, appear to thrive under UHI conditions, but the physiological mechanism underlying their success has not been explored. Here we examine the relationship between hemolymph 20E titers and spiderling development under non-urban desert (27°C), intermediate (30°C), and urban (33°C) temperatures. We found that a presumptive molt-inducing 20E peak observed in spiders at non-urban desert temperatures was reduced and delayed at higher temperatures. Intermolt 20E titers were also significantly altered in spiders reared under UHI temperatures. Despite the apparent success of black widows in urban environments, we noted that, coincident with the effects on 20E, there were numerous negative effects of elevated temperatures on spiderling development. The differential effects of temperature on pre-molt and intermolt 20E titers suggest distinct hormonal mechanisms underlying the physiological, developmental, and behavioral response to heat, allowing spiders to better cope with urban environments.

Fig 1. Average ecdysteroid titers during spiderling development.

Ecdysteroid titers were determined for spiderlings reared at 27, 30, or 33˚C from 4 days before to 10 days after the second molt that occurred per family at 27˚C. Error bars represent standard error.

Fig 2. Developmental ecdysteroid profiles at different temperatures.

(A-C) Average daily ecdysteroid titers from spiderlings reared at 27˚C (A), 30˚C (B), and 33˚C (C). Only families that had spiderlings reared at both temperatures were used for analysis. Spiderling ages are relative to the timing of the second molt for each family, at 27˚C. Error bars represent standard error.

Fig 3. Normalized developmental ecdysteroid profiles.

Average daily ecdysteroid titers from spiderlings reared at (A) 27˚C, (B) 30˚C and (C) 33˚C. Only families that had spiderlings reared at all three temperatures were used (N = 6 families). Spiderling ages are relative to the timing of the second molt for each family, at each temperature. The presumptive molt-inducing peak of ecdysone is indicated (arrowhead). Error bars represent standard error.

Fig 4. Phase-specific ecdysteroid titers.

Average daily ecdysteroid titers from spiderlings reared at 27˚C, 30˚C and 33˚C were determined at two time points: two days prior to the second molt (Molt) and days 2–5 after the second molt (Intermolt). Only families that had spiderlings reared at all three temperatures were used (N = 6 families). Spiderlings were aged relative to the timing of the second molt for each family, at each temperature. Error bars represent standard error. (*) indicates a significant difference (p<0.05).

Fig 5. Effects of elevated temperatures on development.

(A) Average molt times for spiderlings reared at different temperatures. (B) Average growth rates were measured from 55–75 days of development for spiderlings reared at different temperatures. (N = 6 families per treatment) (C) Average spiderling size at time of second molt was predicted from molt times and growth rates for each family (N = 6 families per treatment). (D) Spiderling mortality at each temperature was determined from 55–75 days of development for each family. (A-D) Different lowercase letters indicate significant differences (p<0.05). N = 6 families per treatment.