It's well reported that oil spill disasters can have a catastrophic biological and environmental impact. But to date, few studies have examined the effect of whole crude oil on senses of a marine vertebrate. A new study is the first to confirm physiological effects on stingrays – and their sense of smell in particular.
Smell is vital to the very existence of marine animals. It helps them to find food and detect predators among other things. Reduced sense of smell could lead to premature death, with a potential knock-on effect on population levels and the food chain as a whole.
Now, Researchers at Florida Atlantic University have looked at the effect of crude oil on the olfactory function (i.e. sense of smell) of an elasmobranch, which are fish with cartilage skeletons such as sharks, rays, and skate. "Elasmobranchs are renowned for their well-developed sensory systems, which are critical to alert them of the presence of predators, prey, mates, and unfavorable environmental conditions," says Stephen M. Kajiura, a professor of biological sciences and co-author of the research. "Any impairment of these sensory systems could have a damaging effect on their survival and fitness." The Atlantic stingray, Hypanus sabinus, was chosen for the study.
The researchers used the example of the Deepwater Horizon oil spill of 2010. It was the largest marine oil spill in history – an industrial disaster on an unprecedented scale causing extensive damage to marine habitats. Around 5 million barrels of oil poured into the Gulf of Mexico directly affecting coastal habitats along 1,773 km (1,102 miles) of shoreline. Crude oil contains many chemicals that are considered pollutants, such as polycyclic aromatic hydrocarbons (PAHs) and heavy metals which are known to cause considerable environmental damage.
To measure the effects of this type of disaster, stingrays were exposed to oil levels similar to those measured at the shoreline after the spill. They were compared with a control group of stingrays in clean water. Results showed that the olfactory response in the stingrays exposed to crude oil was impaired in terms of both size and duration of the response when compared with the stingrays in clean seawater. In other words: exposure to oil spills is bad for stingrays' sense of smell.
"Unlike other sensory systems in which the receptor cells are not in immediate contact with the environment such as the eye, inner ear, lateral line, and electroreceptors, the chemo-sensory cells of the olfactory organ are directly exposed, through the mucus, to the seawater," says Kajiura. "As a result, environmental pollutants have the ability to directly damage the receptor cells and affect olfactory function."
The study also floats the theory that deep-water elasmobranch species may also be susceptible to crude oil exposure. Despite extensive cleanups over the years, a vast amount of oil remains in the ocean floor. It's believed that oil embedded in the sediment may well be more toxic than at the surface. This could affect those animals which breed on the seafloor by being exposed to high concentrations of crude oil for long periods, particularly during their reproductive cycles. And this may be made worse by their typically slow metabolisms.
A final consideration is that in field conditions animals are naturally exposed to varying levels of pollutants rather than the fixed concentrations used in this study, which could have other implications. "This acute exposure has the potential to induce other physiological responses, potentially compounding the adverse effects of the altered olfactory function," Kajiura explains. "Even if the oil does not cause immediate or direct death, sub-lethal effects could still reduce fitness or contribute to premature death."
The team's paper has been published in the journal Nature, and is available to read online.
Source: Florida Atlantic University
