Generations of exposure to polycyclic aromatic hydrocarbons appears to lead to adaptation in killifish, accompanied by higher mortality rates, according to a study out of Duke University. Published in the journal Neurotoxicology and Teratology, the findings, from a Duke Superfund Research Program (SRP) Center study led by Richard Di Giulio, Ph.D., shed light on the evolution of resistance to environmental chemicals, and the environmental effects and associated costs of chronic exposure in killifish.
Resistance accompanied by higher mortality
Killifish in the Elizabeth River, near the Atlantic Wood Industries Superfund site in Portsmouth, Virginia, are chronically exposed to sediments contaminated with PAHs. They have developed resistance to the toxic, behavioral, and developmental effects seen in other killifish exposed to these chemicals.
The study showed that developmental exposure to PAHs significantly altered behavior and locomotor activity in previously unexposed, or naive, killifish. The PAH-adapted Atlantic Wood killifish population did not show these behavioral effects. However, long-term data revealed greater mortality levels in fish from the Atlantic Wood site, raising questions about the environmental impacts and costs associated with PAH exposure.
“Evolutionary toxicology has important ramifications for environmental science and management,” said Di Giulio. “A risk assessment based on an analysis of the population that evolved to resist pollution may lead the assessor to underestimate risks, without taking into account other costs of the adaptation.”
PAHs are widespread environmental contaminants. They are a byproduct of burning organic substances, such as coal, petroleum products, trash, and wood, and are also manufactured for use in products such as asphalt, coal tar, and creosote. In Virginia, several wood treatment facilities contaminated the Elizabeth River with creosote. Although creosote is no longer manufactured in the area, high levels of PAHs remain in the river, particularly in sediments.
Studying adaptation in polluted environments
Fundulus heterolitus, or killifish, are common to coastal estuaries. Although they can be found all along the East Coast, they don’t travel far from where they hatch, making the species ideal for studying the effects of local contamination.
Adult killifish from the Atlantic Wood site and from Kings Creek, a relatively uncontaminated tributary of the Severn River in Virginia, were collected and maintained in the laboratory. Embryos were collected following breeding, and were exposed to Elizabeth River sediment pore water, water that fills the spaces between grains of sediment.
PAH exposure led to short-term and persistent behavioral impairments in naive killifish, or offspring from the uncontaminated Kings Creek site. Behavioral tests showed that the killifish were impaired in locomotor activity, or movement from one place to another; tank diving and exploration; and sensorimotor response, or receiving messages from the senses, such as sight, and responding to them. On the other hand, the PAH-adapted killifish, or offspring from the contaminated Atlantic Wood site, did not show behavioral alterations.
Exposure to PAHs also caused increased mortality over time in the Kings Creek killifish, but not in the Elizabeth River killifish.
Understanding risks associated with adaptation
According to the authors, because of the evolutionary relationship between the Atlantic Wood killifish and contaminated sediment at the site, the sediment may be protecting the killifish by regulating bacteria and parasites, and potentially influencing microbes in the gut. However, there may be other costs associated with the adaptation, such as reduced genetic diversity, that need to be further explored.
Duke SRP Center researchers continue to study how the resistance to PAHs may affect the killifish, including health impacts when they are reared in clean water. They are also looking at the mechanisms underlying the resistance and PAH-mediated toxicity, which may help inform human health studies.
Citation: Brown DR, Bailey JM, Oliveri AN, Levin ED, Di Giulio RT. 2016. Developmental exposure to a complex PAH mixture causes persistent behavioral effects in naive Fundulus heteroclitus (killifish) but not in a population of PAH-adapted killifish. Neurotoxicol Teratol 53:55-63.
(Sara Mishamandani is a research and communication specialist for MDB Inc., a contractor for the NIEHS Superfund Research Program and Division of Extramural Research and Training.)