Effects of Body Size and Month of Collection on Total Mercury and Methylmercury Concentrations in Orb-weaver Spiders: Implications for the Use of Riparian Spiders as Sentinels

Orb-weaver spiders (Family Araneidae) build webs on shoreline vegetation and human-made structures and feed on methylmercury (MeHg)-contaminated emerging insects from aquatic ecosystems. Orb-weavers have been proposed as sentinels of MeHg contamination in aquatic systems because the concentration of MeHg in the tissues of riparian spiders can reflect the level of MeHg contamination in nearby aquatic ecosystems. Effectively using orb-weaver spiders as sentinels requires an understanding of how their level of MeHg contamination vary with factors such as spider body size and time of collection (i.e., month, season). The objective of this study was to examine the effect of spider size and month of collection on total mercury (Hg; inorganic Hg + MeHg) and MeHg concentrations in orb-weavers. We collected a size array of orb-weavers (Metazygia sp. and Larinioides sp.) from a floating boat dock on Eagle Mountain Reservoir (near Fort Worth, Texas USA) from May to September 2019. Total Hg and MeHg both increased with spider size, measured as average leg length (tibia + patella), but the monthly patterns in concentrations of total Hg and MeHg differed. Total Hg increased strongly from May through September, whereas MeHg did not vary significantly among months in a size-adjusted model. The percentage of total Hg that was MeHg (%MeHg) declined sharply from May through September, indicating that the proportion of total Hg present as MeHg decreased monthly as total Hg increased. Stable isotope results showed strong monthly variation in δ15N values, suggestive of seasonal changes in diet, and isotope-adjusted models indicated that some of the variance in total Hg and MeHg concentrations was related to δ15N values. However, δ15N values explained a relatively low proportion of the variance in total Hg concentration, suggesting that dietary shifts alone could not explain the increase in total Hg over the course of the study. The different effects of month of collection on concentrations of total Hg and MeHg and resultant decline in %MeHg with season has important implications for using orb-weaver spiders as sentinels of Hg contamination of aquatic systems and suggests that total Hg should not be used a proxy for MeHg without accounting for seasonal variation in %MeHg.