Something strange is going on in the deep sea. Luminous bacteria have teamed up with predatory anglerfish, which may use the glowing microbes to help catch prey. The bacteria have evolved to depend on the anglerfish 鈥� yet they spend much of their time floating free in the water.
Anglerfish are predatory fish with large teeth. On their forehead they have a long, thin growth called an esca, which resembles a fishing line. The tip of the esca is often luminous. Many anglerfish live in the pitch-black deep sea and may use the glow to lure in prey.
Little is known about them, says Tory Hendry of Cornell University in New York. For instance, there is no evidence that the esca is a lure, 鈥渙ther than it looks like a lure鈥�, she says.
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However, we do know that many anglerfish have bacteria living in their escas, which supply the light. Hendry鈥檚 team has previously found that these , and with it many abilities. 鈥淭hey rely on glucose from the host,鈥� she says.
This discovery implied that the bacteria spend all their lives inside anglerfish. In line with this, the team found that two anglerfish species each .
Mysterious bacteria
Hendry鈥檚 team has now overturned this. The researchers sequenced the DNA of the bacteria found in the escas of seven anglerfish species, belonging to six genera. One species of anglerfish had its own unique bacteria, but the others all shared the same species of bacteria.
The only explanation is that the bacteria live in the water and the anglerfish collect them, says Hendry. This implies the bacteria are widespread, as anglerfish from both the North Atlantic and Gulf of Mexico had the same species. 鈥淭here has to be an environmental pool that all these fish are getting their symbionts from,鈥� says Hendry.
But in that case, why have the bacteria lost so many genes, making it harder for them to survive alone? The team found signs of rogue pieces of DNA called transposable elements, which can disrupt genes and may have triggered the losses. 鈥淚t may not have been adaptive,鈥� says Hendry.
The secret to the bacteria鈥檚 survival may lie in a chemical called polyhydroxybutyrate (PHB). Many bacteria use it to store carbon for lean periods. 鈥淲e found the genes in the genome, so we know they have the capacity to synthesise and degrade this molecule,鈥� says Hendry. She suggests that the bacteria might accumulate glucose from anglerfish, then store it as PHB and live off that when they return to the water.
Hendry speculates that some anglerfish may even seed the water with bacteria. They have mysterious knobs on their back that also hold bacteria, but which are opaque so they cannot glow. 鈥淲e know they have a pore on them, so presumably the bacteria can leave the host and go out into the environment,鈥� says Hendry.
eLife
