Feeding and reproduction of a tropical coastal copepod across warming and copper gradients

Why I highlight this preprint:

While there are many studies on the toxic effects of copper on copepods, this paper caught my attention because it also incorporates increased temperatures and how these variables may in combination impact reproduction and feeding. As there are multiple growing threats to marine life, it is essential to understand how marine populations may change over time and how this may affect ecosystem function.

Background:

Copper pollution in the ocean has been increasing due to expanding aquaculture and shipping in Southeast Asia, which harms the health and survival of marine organisms. Ocean temperatures are also rising, with more frequent and prolonged record highs. Many cold-blooded marine animals already live close to their thermal limit, meaning that warmer waters may push them beyond what they can tolerate. Increased toxin levels can also impact an animal’s sensitivity and ability to deal with warmer waters, emphasizing the importance of studying these stressors together.

Calanoid copepods are a major food source in their ecosystems and have short generation times, with changes in their population size having a large impact on their communities. In this preprint, the authors investigated how these copepods might be affected by combined higher temperature and copper levels to determine how their populations may change over time.

Approach:

The research team raised copepods under different combinations of copper and temperature levels for 7 days. The temperature ranged from 26-35°C, and copper concentrations varied from 0 to 40 µg/L to represent the low and high extremes the copepods may experience (Fig. 1). They measured fecal pellets to approximate energy intake in addition to tracking survival and offspring production.

Figure 1: Experimental design used to test the effects of temperature (26, 29, 32, and 35 °C) and copper concentrations (0, 10, 20, 30 and 40 µg L−1) on the tropical copepod Pseudodiaptomus annandalei. This figure was made available under a CC-BY-NC-ND 4.0 International license.

Key findings:

Survival did not change under higher temperatures, but offspring and fecal pellet production decreased

Survival of the copepods did not differ between any of the different temperature and copper combinations, which the authors suspected may be due to the copepods using more energy to tolerate higher temperatures for short periods of time. However, fecal pellets decreased, indicating that warmer waters may increase energetic demand that the copepods cannot meet by feeding alone. Reproduction may have also decreased under the most extreme temperatures, as more energy had to be allocated to survival. Offspring production peaked at 32°C, suggesting that while higher temperatures increase reproduction rates to an extent, extreme conditions can push copepods past their limits and decrease overall production. These findings support the idea that many organisms live near their upper temperature limits and could explain why climate change may reduce their abundance in the future.

Copper alone had little effect on the copepods, but temperature stress and copper combined, as well as an extended experimental duration, showed mixed results

Copepods’ survival, reproduction, and feeding were highly variable under any of the different copper concentrations, but no consistent pattern was found. The authors speculated that this may be due to differences in copepod age, physical health, and reproductive stage. However, adding temperature stress to the mix changed how the copper effects appeared in the copepods. At the optimal temperature of 32°C, there was no difference between any of the copper levels, and all of the copepods seemed to behave the same. When the water was either colder or warmer than their optimal temperature, the responses became inconsistent, with copper seeming to benefit, harm, or have no effect on the copepods. These results indicate that copper concentrations are not the only factor that determines how toxic copper is. The different groups showed no differences for the first 5 days, but diverged in days 6-7, with these changes also being inconsistent. These findings show that both reproduction and feeding are more sensitive to combined copper exposure and temperature stress than survival. They also stress the importance of examining longer-term effects.

Questions for the authors:

1. The number of offspring produced was used as a proxy for reproductive health in copepods. Would the offspring of the parents similarly show reduced survival, development, or reproductive success? It would be interesting to see how future generations are impacted by their parents experiencing adverse conditions.
2. Since combined high copper and temperature levels had mixed, unpredictable results, how might other environmental variables interact with either copper or temperature and affect the copepods? For example, would UV and copper combined produce different results than UV and temperature?

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