As the oceans acidify due to climate change, scientists assess the ability of marine animals to adapt.
Scientists have become increasingly concerned over the past decade with an indirect, yet alarming, effect of climate change – ocean acidification. The large amounts of carbon dioxide being emitted into the atmosphere must go somewhere, and gradually many of these molecules are dissolving into the oceans. Complex ocean chemistry causes this rise in carbon dioxide concentration to manifest itself as decreased pH of the water and, thus, an increase in its acidity. It has been well established that increased acidity of marine waters has widespread impacts on shelled organisms, which take advantage of current conditions to produce the compounds found in their tough, protective shells.
However, the lack of understanding about the effect of ocean acidification on other marine organisms, specifically commercially important fish species, is a major gap in current scientific understanding of climate change. Because ocean pH will likely continue to drop gradually over at least the next few hundred years, researchers are attempting to address whether or not fish species (many of which have generation times on the order of years) will be able to “keep up” and evolutionarily adapt to these changes.
A recent publication in Evolutionary Applications used quantitative techniques to assess the evolutionary capabilities of a fish species called the Atlantic Silverside (Menidia menidia). These fish are a critical component of coastal ocean habitats in the Eastern U.S. as well as an important food source for larger fish harvested for human consumption, such as striped bass.
The term “evolutionary capability” refers to the ability of the fish to adapt to the changing environment. The researchers used an established technique called genotyping (think, paternity tests) to understand how traits conducive to survival in high carbon dioxide waters are passed from parent to offspring.
Initially, they found a significantly reduced (34% reduction) survival rate under elevated carbon dioxide conditions, in itself a gloomy figure. More importantly, however, the team found that heterozygosity (increased genetic variation within an individual) was positively correlated with higher survival rates under increased acidification. This result highlights the importance of genetic variation in populations of fish species when it comes to survival in a changing environment. Also interestingly, more related individuals showed more similar survival rates (relatedness was based on those fish paternity tests). This shows that there must be a genetic influence on survival rate in high carbon dioxide water.
Based on these findings, the conclusion of the study stated that the Atlantic Silverside could potentially exhibit evolutionary adaptation to ocean acidification. In other words, the fish do possess the evolutionary capability that the researchers were interested in studying. Although initial mortality rate is high in projected future conditions, genetic variation in the form of heterozygosity, coupled with differential survival of interrelated offspring, show that the fish who are best suited to the shifting marine waters should better pass on their genetic information and have a positive feedback on adaptation.
The researchers hope their genetically quantitative techniques and study design will be used to determine the effect of climate change on other marine species and, more generally, on the oceans of the future. Their results show that marine metazoan may possess the capability of not only surviving, but also actually evolving, under conditions that will occur as humans become a substantial force in shaping the environments on Earth.