Anglerfish are famous for their head-mounted bulbs that light up the inky blackness of the deepest oceans, but exactly what's going on in that process is still relatively unknown. To find out, researchers from Cornell University have now sequenced the genome of the bacteria that live inside those lightbulbs, and found a strange story of symbiosis still in progress.

Anglerfish aren't winning any beauty contests – but they don't really need to, since they live in almost perpetual darkness. Females of the freaky fish attract prey and potential mates with a glowing bulb, made possible by a colony of bioluminescent bacteria. The fish and bacteria have a symbiotic relationship: the bacteria glow to attract food for the fish, while the fish supplies the bacteria with necessary nutrients.

But their relationship has remained murky, given how difficult it is to catch and study the fish from the bottom of the ocean. To shed light on the mystery, the Cornell researchers sequenced and analyzed the genomes of bioluminescent bacteria from anglerfish bulbs, with these particular ones coming from specimens collected from the Gulf of Mexico.

On the surface the symbiosis sounds pretty straightforward, but the team found that the story isn't quite so simple. The bacteria aren't locked to their host, like many other examples of symbiotic relationships, they're still evolving.

"What's particularly interesting about this specific example is that we see evidence that this evolution is still underway, even though the fish themselves evolved about 100 million years ago," says Tory Hendry, lead author of the study. "The bacteria are still losing genes, and it's unclear why."

When the team compared the genomes of these microbes to related species that swim freely, they found that the anglerfish bacteria had lost 50 percent of their genes, and had large numbers of "pseudogenes" – those that are likely to vanish over time. That leaves the bugs struggling to make amino acids, break down nutrients or find food by sensing chemical cues. The fish are probably providing their guests with these vital nutrients, the team surmised.

To explain how the bacteria are still evolving so long after the fish, the team suggests that the bugs are relative newcomers to the relationship. Anglerfish might have had a similar setup with another species in the past, but has for whatever reason, recently recruited the new microbes in the role.

Interestingly, the researchers took bacteria from two different species of anglerfish and found that they had different types of bacteria inhabiting their bulbs, suggesting these same changes were taking place independently.

The research was published in the journal mBio.

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