Cruising at top speeds of 93 km h−1 (move over, Usain Bolt!), there aren't many fish that swim faster than mahi-mahi. Like high-performing athletes, mahi-mahi have exceptionally conditioned hearts that allow them to swim so swiftly. Yet, these extraordinary ocean athletes are vulnerable during embryonic development and spend most of this time at the sea surface. Mahi-mahi spawn in the Gulf of Mexico, the site of the April 2010 Deepwater Horizon oil rig explosion, which occurred during the height of the mahi-mahi spawning season. David Attenborough doesn't need to explain what an ecological disaster this was, but to understand its impact, biologists from the RECOVER consortium have been researching the effect of Deepwater Horizon oil. In their recent publication, Prescilla Perrichon and colleagues from the Universities of Miami and North Texas, USA, demonstrate the combined, sinister effects of temperature and certain toxic hydrocarbon components of crude oil – polycyclic aromatic hydrocarbons – on larval mahi-mahi.
To accomplish this, the team artificially weathered Deepwater Horizon crude oil to mimic its naturally altered composition in the Gulf of Mexico (composed of a high proportion of polycyclic aromatic hydrocarbons). Next, the team separated 8 h old mahi-mahi embryos into two groups at different temperatures (26 and 30°C – the normal temperature range over which these embryos develop in the Gulf of Mexico) and subjected them to different concentrations of the artificially weathered oil, for 24 h. Typically, warmer temperatures accelerate embryonic development and increase their energy consumption, so the team wanted to see how the effects of the oil toxins would vary over the two temperatures. Finally, the researchers transferred the embryos to clean seawater and assessed the development of their hearts, at 56 h. They looked at the heart because hotter, faster-maturing embryos are more dependent on it – even as tiny embryos – because this vital organ is responsible for circulating oxygen, nutrients and other materials (including dangerous toxins) to the tissues.
The team found that the oil toxins caused the heart to pump less blood and beat more slowly. Also, the hotter embryos were affected more severely. In short, oil is toxic and even deadlier if you're a fast-growing larval fish.
Having discovered that the hydrocarbons impaired heart function, the researchers investigated whether this might be due to changes in heart structure. Microscopy images of specimens revealed that the oil toxins had caused unusually large amounts of fluid to accumulate around the heart, which places abnormal pressure on the tissue and distorts its shape. Unsurprisingly, as more fluid surrounded the heart, the worse it performed. The team also observed that the hotter embryos consumed their yolk faster than normal. This was an alarming observation, as hotter embryos rely more on their yolk to sustain their rapid growth. So, not only did the toxicity of the hydrocarbons impair heart shape and function but also it left hotter embryos with a perilously depleted energy supply, which they desperately need for their accelerated development and survival.
Although fish possess remarkable adaptations for adjusting to changes in their environment, this study shows that fish embryos might not be as resilient to human pollution when faced with higher temperatures. In the context of climate change, as sea-surface temperatures rise, aquatic organisms that are typically tolerant of higher temperatures might face dire outcomes. The combined effects of high temperature and pollution could impede the development of ecologically and economically important organisms, thereby imperilling their ability to successfully grow and reproduce in the near future.