Crude oil can prompt ocean larvae to metamorphose

Many marine invertebrates, like sand dollars and sea snails, spend their larval stage floating through the sea. Eventually, they detect chemical signals indicating that it’s time to metamorphose. Some of these critters can delay this transformation for months, even years, until they find the perfect spot. However, new research shows for the first time that crude oil and its burnt residues can prompt marine invertebrates to metamorphose within a few days (Environ. Sci. Technol. 2023, DOI: 10.1021/acs.est.3c05194). This potentially early metamorphosis could impact ocean creatures’ survival, because they may not be able to reach an ideal spot to transition.

Rodrigo Almeda, a marine biologist at the University of Las Palmas de Gran Canaria and one of the study’s authors, and his colleagues first observed the transformation of sea snail larvae by accident. They had added soot or burnt oil residues to vessels with seawater. Later, they used some of this water for tests of lethal effects on larvae caught nearby off the coast of Crete. They didn’t expect to observe metamorphosis. Neither soot nor oil caused high mortality, but after 3 days of exposure to the seawater containing oil residues, up to 87% of the larvae had transformed from their free-floating state to their juvenile form, in which they can settle on the seafloor where they live into their adulthood. Only 10% of those exposed to soot morphed, similar to the level in the unexposed control larvae.

The team also exposed sand dollars to crude oil, not its burnt residues, that had been suspended in seawater. Within three days, about 84% percent of the sand dollar larvae transformed as compared with around 3% in unexposed control larvae. These transformations occurred at crude oil concentrations far below those that occur during oil spills and even below levels that are allowed in discharges from oil drilling platforms, says co-author Sinja Rist, a marine biologist at the Technical University of Denmark.

Additionally, some of the sand dollars that had undergone metamorphosis after they had been exposed to crude oil were deformed. Depending on the oil concentration and whether they were simultaneously stressed by warm water, which animals could experience during a marine heat wave, up to some 50% had shrunken or regressed arms. “They would not be able to survive,” Rist says.

It’s not clear yet what in the crude or its burnt residues prompts the transformation. Crude contains hundreds or thousands of different chemicals. It’s also possible that the crude or burnt residues may influence environmental bacteria, which exude chemical cues that induce invertebrates to morph. “They’re [the researchers] working with mixtures that are beyond our ability to comprehend analytically,” says Dan Rittschof, a marine biologist at Duke University. “The solution isn’t finding out who the culprit is—it’s figuring out how to keep those mixtures from being put in the environment.”

Rittschof isn’t surprised that crude oil can induce metamorphosis. He and his colleagues have observed that compounds in plastics can prompt marine algae to morph. Like some of those chemicals, crude oil has a biological origin. Its chemicals can interact with membranes and cause changes to creatures’ neural pathways, including those that are part of metamorphosis, he says.

Changes to metamorphosis could shift population sizes and their connectivity, Rist says. It’d be hard to untangle this effect of crude oil from other stressors such as marine heatwaves and food availability. But she expects that this induction of metamorphosis is happening in the environment.

Like sand dollars and sea snails, most marine invertebrates live floating in the ocean’s water before settling on the seafloor, Almeda says. So it’s possible that crude oil may induce metamorphosis in more creatures than the two studied. “This is completely changing the perception of the effects of crude oil,” he says.