Temperature and food quantity effects on the harpacticoid copepod Nitocra spinipes: Combining in vivo bioassays with population modeling

Abstract

The harpacticoid copepod Nitocra spinipes has become a popular model species for toxicity testing over the past few decades. However, the combined influence of temperature and food shortage, two climate change-related stressors, has never been assessed in this species. Consequently, effects of three temperatures (15, 20 and 25°C) and six food regimes (between 0 and 5 × 105 algal cells/mL) on the life cycle of N. spinipes were examined in this study. Similarly to other copepod species, development times and brood sizes decreased with rising temperatures. Mortality was lowest in the 20°C temperature setup, indicating a close-by temperature optimum for this species. Decreasing food concentrations led to increased development times, higher mortality and a reduction in brood size. A sex ratio shift toward more females per male was observed for increasing temperatures, while no significant relationship with food concentration was found. Temperature and food functions for each endpoint were integrated into an existing individual-based population model for N. spinipes which in the future may serve as an extrapolation tool in environmental risk assessment. The model was able to accurately reproduce the experimental data in subsequent verification simulations. We suggest that temperature, food shortage, and potentially other climate change-related stressors should be considered in environmental risk assessment of chemicals to account for non-optimal exposure conditions that may occur in the field. Furthermore, we advocate combining in vivo bioassays with population modeling as a cost effective higher tier approach to assess such considerations.

Making Sense of Life‐History Effects of the Antidepressant Citalopram in the Copepod Nitocra spinipes Using a Bioenergetics Model

“…The cryptophytic microalga Rhodomonas salina was used as the sole food source during the experiments. This alga was provided at 2.5 × 10 5 cells mL −1 , which corresponds to ad libitum feeding conditions (Koch et al 2017). Water renewals, during which 70% of the test medium in each well was replaced with fresh medium, food, and the test chemical, were performed every first, third, and fifth day of a week.…”

“…As is typical of copepods, N. spinipes grows by molting 11 times (passing through 6 naupliar and 5 copepodite stages) before it reaches sexual maturity and stops growing (Abraham and Gopalan 1975). Once fertilized by a male, females can produce multiple broods that are typically released at intervals of 3 to 4 d (Koch et al 2017). The strain of N. spinipes used in the present study was isolated from Tvären Bay (near Studsvik, Sweden, in the Baltic Sea) in 1975 and has been kept in continuous laboratory culture ever since.…”

Investigating Population‐Level Toxicity of the Antidepressant Citalopram in Harpacticoid Copepods Using In Vivo Methods and Bioenergetics‐Based Population Modeling

“…The microalga Rhodomonas salina was used as a food source and was provided twice a week (as detailed below) at 2.5 × 10 5 cells mL −1 . This concentration was found to conform to optimal food availability when fed to N. spinipes at densities of up to four animals per milliliter three times a week (Koch et al, 2017) but may become limiting at higher population densities. Water renewals, during which 70% of the test medium in each cup was replaced with fresh medium, containing the appropriate concentration of fresh algae and the test chemical, were performed on every Tuesday (which was also the initiation day) and Friday.…”

“…To reproduce the (unimpaired) life history of the individual copepods as accurately as possible, we used physiological DEB parameters and parameters that capture interindividual variability, which were estimated specifically for N. spinipes (Koch & De Schamphelaere, 2019, 2020). Life‐history processes that are not explicitly covered by the DEB theory, such as the timing and total number of reproductive events per reproducing female, were implemented ad hoc based on laboratory results presented by Koch et al (2017). Furthermore, because not much is known about the species' strategy to cope with severe food scarcity, a rather simple starvation submodel was developed which, in accordance with literature findings for other copepods (Finiguerra et al, 2013; Holm et al, 2018), gives females a higher starvation tolerance than males.…”

“…d Metamorphosis is morphological only and has no implications for DEB rules. e Reproduction parameters based onKoch et al (2017). f Toxicity parameters for N. spinipes exposed to the citalopram hydrobromide estimated byKoch and De Schamphelaere (2021).…”