Being young and hot: Fitness-related implications of thermal variation associated with retreat site use across ontogenetic stages in the spider, Morebilus plagusius

Background/Question/Methods

The thermal environment an ectotherm experiences affects its physiology, behavior and ultimately fitness. For nocturnal ectotherms, diurnal temperatures experienced are limited to those available within their diurnal retreat sites. When diurnal retreats are limited in number, competition for retreats with favourable thermal characteristics will typically be intense. However, the traits defining favourable retreats and the ability of an organism compete for retreats will vary ontogenetically. We aimed to see how retreat site usage changed with ontogeny in Morebilus plagusius (Trochanteriidae), an Australian spider that uses crevices underneath exfoliated rocks as diurnal retreats, and identify the fitness implications of different thermal environments on growth and development. We surveyed rock outcrops for M. plagusius and characterised their retreat rocks using a suite of habitat variables which influence the thermal environment underneath the rock. We compared occupied retreat rocks to nearby rocks within and among ontogenetic stages. We also identified how different constant temperatures affected eggsac survival and spiderling growth, developmental rates and survivorship. We incubated recently emerged spiderlings at one of five constant temperature treatments (14, 21, 28, 35 and 42°C), monitored them every two days for molting or death and measured body size after molting. 

Results/Conclusions

Morebilus plagusius used rocks with greater surface area on a rocky substrate (compared with dirt/sand or leaf litter) as retreats compared with other nearby rocks. However, juvenile M. plagusius used smaller rocks than the adults. These smaller rocks reach higher maximum temperatures and experience more rapid temperature changes than larger rocks. As such, juvenile spiders would experience hotter and more rapidly fluctuating temperatures which would directly impact their survivorship and development. We found that eggsac success (emergence of spiderlings from the eggsac) and spiderling survival was highest at 28°C. No eggsacs were successful at 14, 35 or 42°C. Spiderling developmental rates were fastest at 28 and 35°C. Body size after each moult was not affected by temperature. Most spiders at 14 and 42°C died before they reached their first moult. Our results show that the type of retreat rock juvenile flat rock spiders occupy will affect life history and fitness-related traits. Our study highlights the importance of incorporating ontogeny into studies of thermal ecology and the need to include ontogenetic changes in thermal physiology into predictions of the impacts of future temperature changes associated with climate change.