Coastal ecosystems, especially near densely populated areas, are often impacted by pollution, including metals, that enter the sea via run-off. Copper is a common component of polluted run-off, coming from a variety of sources including building materials, paints and wood preservatives, algaecides and brake pads.
The exposure of marine species to copper and other metals can result in impacts on their health with long-lasting consequences, and one of the directions research has been headed recently is to better understand not only how exposure to coastal pollution directly affects individuals in that impacted environment, but their offspring as well.
It has long been known that exposure to water-borne metals can harm eggs and reduce fertilization success in aquatic animals. Invertebrates are also often used as “bio-indicator” species in environmental monitoring programs to detect pollution in the field because metals can accumulate in the gonads and other tissues. Nevertheless, the relationships between pollution in the water, the metal burden in adult gonads, and effects on their offspring are unknown. Further, there is evidence from some species that parental exposure to metal pollutants actually increases their offspring’s tolerance to that same pollutant later; thus, effects of parental exposure to pollution can sometimes be positive for the next generation.
Sea urchins are common coastal species and often used as bio-indicators. They also play a dominant ecological role in their communities, so it is important to understand factors that can affect their populations. In a recent paper (Phillips and Rouchon 2018) we examined these ideas by exposing adult sea urchins to elevated copper in seawater (similar to a very polluted environment) for a month, alongside those that stayed in clean seawater. We then spawned males and females from these two treatments and fertilized them with each other and with mates in the opposite treatment. These fertilization experiments were also done in either clean or polluted seawater. Our expectation was that if parental exposure to copper conferred resistance to copper in their offspring, then embryos in the copper contaminated water should do better if from copper-exposed parents than from parents that had been in clean seawater.
We found that month-long exposure to elevated copper in the seawater increased copper in gonads of both males and females and reduced gonad size by almost half. There was a negative relationship between adult size and copper burden, such that smaller animals had a higher metal burden.
We also found that although fertilization was not affected by either parental or gamete exposure to copper, normal larval development and larval size four days after fertilization were strongly affected by the amount of copper in female gonads such that female urchins with higher copper burdens produced more abnormal and smaller larvae. Interestingly, and unexpectedly, this was a dose-dependent effect. In other words, there was a lot of variability in the amount of copper in gonads of urchins that had been side by side in the same polluted environment for a month. Some had similar amounts of copper as those that had been in clean seawater, and some had relatively high copper levels. If the copper in the gonads was elevated over a threshold amount, there was a sudden drop in the success of offspring from those mothers, but if the amount of copper in their gonads was below that threshold (whether they had been in contaminated water or not) then their offspring had a high probability of developing normally.
So, in our experiment, the dominant effect on larval development was not the copper in the environment as such, but the copper burden in gonads of individual mothers, with smaller mothers generally worse off. We also found no evidence for or against parental exposure to copper conferring resistance to their offspring, and also no evidence of strong paternal effects.
Our study demonstrated a strong effect of maternal exposure to copper pollution on offspring performance that was mediated by individual traits of females. This means that knowing the broad scale levels of environmental pollution (i.e. how much of a contaminant is in the water), is only part of the picture. To understand the long-term effects of pollution on marine populations, we need to take into account the high degree of inter-individual variability in responses to pollution and how those responses affect the success of the next generation.
These findings are described in the article entitled A dose-dependent relationship between copper burden in female urchin gonads and developmental impairment of their offspring, recently published in the journal Marine Environmental Research. This work was conducted by Nicole E. Phillips and Agnes M. Rouchon from the Victoria University of Wellington.