Like many land animals, marine organisms follow daily and seasonal clocks—in the water, those clocks are set by the cadence of the sun and the moon. But researchers hadn’t known if deep-sea creatures also exhibit biological rhythms, tucked away in remote and sunless environments. They do, confirms a recent study in Nature Communications. Observations of deep-sea mussels, both at depth and in the lab, confirm regular and rhythmic patterns in their behavior and their gene expression, for instance, tracking the roughly 12-hour cycles of the tides.
Until submersible exploration in the 1970s, “people thought the deep sea was dark, extreme, with no life,” says coauthor Audrey Mat, a marine biologist and chronobiologist who led the new work as a postdoc working jointly with Université de Bretagne Occidentale and Ifremer, France. The 1977 discovery of hydrothermal vents upended conventional visions of a desolate deep sea, suggesting one rich in biomass. But researchers still know little about how life in these ecosystems changes over daily or seasonal timeframes, nor what environmental cues could drive biological cycles such as migration or spawning.
Mat and coauthors probed these questions through both field and lab work. First, they used video recordings of deep-sea mussels to look for cues of possible rhythmic behavior. The recordings, collected between July and August 2014, showed the mussels opening and closing in a regular pattern, roughly every 12 hours. The team then used a remotely operated submersible to collect mussels from a hydrothermal vent nearly 1,700 meters deep. The sub collected and preserved mussels on the seafloor under red light roughly every two hours over the course of about 24 hours. Back onshore, the researchers compared each time stamped sample to see how gene expression changed over the course of the day. They observed regular oscillations in gene expression every 12 hours and 24 minutes, aligned with the tides.
Then, in a second experiment, the team brought the same species of deep-sea mussels into aquariums in the lab and exposed them to 24-hour cycles of daylight and darkness over a 72-hour period. Again, they found regular oscillations in gene expression. But this time, the oscillations tracked day and night rather than the tides, Mat says, suggesting the mussels can perceive light.
In addition to confirming that these organisms follow biological rhythms, the findings also suggest deep-sea mussels, thought to have evolved from a shallow-water ancestor, may have retained the ability to see light even as they moved into deeper water, Mat says. Importantly, the laboratory findings suggest that submersibles using a full-spectrum white light may disturb the animals’ rhythms; better to use red light as they did in this study.
Annie Mercier, a marine ecologist at Memorial University in Newfoundland, Canada, who was not involved in the new work, notes that these findings add to pioneering research over the last decade that’s already revealed biological rhythms in other deep-sea organisms, such as crustaceans, corals, and echinoderms. Evidence for deep-sea biological rhythms in general, she says, “has been gaining traction for a number of years already.” But the use of molecular tools to track these rhythms through gene expression is quite novel, Mercier says.
The fundamental question for the field now, she adds, is why marine organisms are rhythmic or periodic in the first place. Some benefits of life in rhythm are intuitive, such as males and females coordinating their spawning times. But whether rhythm genes in deep-sea mussels are a vestige of shallow-water evolution, or whether the mussels may actually be responding to something they perceive at depth besides light and tidal currents, has yet to be resolved. Teasing this out is one future research direction. “There could be cues,” Mercier says, “we’re not considering yet.”