Paper: Baker, L. J., Freed, L. L., Easson, C. G., Lopez, J. V, Sutton, T. T., Nyholm, S. V, & Hendry, T. A. (2019). Diverse deep-sea anglerfishes share a genetically reduced luminous symbiont that is acquired from the environment. Elife, 8:e47606, 1–21.
Organisms living in the inky black waters of the deep-ocean see no sunlight – they live at depths far greater than warm, life-breeding sunlight can penetrate. Yet, these animals are not condemned to a life devoid of light. Many animals make their own light in a process known as bioluminescence.
In fact, bioluminescence is common in our world and found even in shallow waters and on land. A wide variety of organisms have evolved the capacity to create light: bacteria, insects, fungi, and fish are just some of the individuals that use bioluminescence. These organisms can either produce their own light (as is the case for lanternfishes and dragonfishes) or they rely on help from other organisms, called symbionts. Oftentimes, symbionts are bacteria capable of producing their own bioluminescence. An animal that cannot inherently produce its own bioluminescence may instead develop a symbiotic relationship with bacteria – the animal provides a safe home for the glowing bacteria somewhere in its body and, in exchange, the bacteria provides the animal with the bioluminescent glow it could not otherwise produce.
The wonders of the weird and wacky anglerfish
Of course, one of the most notorious bioluminescent critters is the anglerfish, named for the glowing lure which protrudes from its head. There are over 160 species of deep-sea anglerfishes living in the deep, bathypelagic waters of our ocean (between 1000-4000 m below the surface). While their striking (and, admittedly, slightly spooky) appearance has garnered the fish widespread attention, we still do not know a whole lot about anglerfish biology because they live in waters that are often too deep for scientists to reach. Only a few anglerfish have been caught on video in their natural habitat and most of our knowledge of these fish comes from specimens that are caught in nets and preserved for later examination.
The little that we do know about these fish is pretty dang cool, though. Anglerfish live in the deep-ocean where there is no sunlight, extremely high pressures, and extremely low temperatures. Only female anglerfish are bioluminescent and rely on bacterial symbionts to produce their light. Anglerfish appear to live mostly solitary lives; except, of course, once they have found a mate. It can be hard to find a suitable mate in a vast, dark ocean (especially without the help of an anglerfish tinder) so when a male anglerfish finds his preferred female companion, he grabs on and doesn’t let go. Literally. Male anglerfish, which are much smaller than their female counterparts, will bite onto the female’s body, attaching itself for the rest of its life. In some species, the attachment is so complete that the male’s body fuses to the female and he becomes a parasitic mate. When the female is ready to breed, the male is conveniently already there to fertilize her eggs, which she lays in an egg raft that will float up into the sunlit waters of the upper ocean. The baby anglerfish will hatch and grow, eventually making their way back into the deeper ocean waters and females will develop their prominent lure.
How do you turn this thing on?
While scientists have long-known that female anglerfish are bioluminescent and rely on bioluminescent bacterial symbionts to, they still do not know exactly why anglerfish produce light – whether to attract prey and mates or to avoid and confuse predators – and how the anglerfish acquire the help of their glowing bacterial symbionts in the first place.
Organisms (the host) can obtain bacterial symbionts either by 1) coming in contact with bacteria in the environment and then assimilating these bacteria into their own body or 2) through direct transmission from other individuals – usually when a parent directly passes the bacteria on to its offspring (called “vertical transmission”). Because some bacteria are directly passed down from parent to offspring through the generations they can co-evolve with their host species. The bacteria may become specifically adapted to living within a host and might lose the ability to function independently in the environment (termed obligate symbionts). For instance, they might lose genes that allow them to grow cell walls or structures that can help them move around. On the contrary, bacterial symbionts that are typically acquired through environmental contact are usually perfectly capable of living on their own, but are also able to live symbiotically within a host (termed facultative symbionts).
Scientists do not know how anglerfish obtain their symbionts or if the bacteria are obligate or facultative symbionts. There are two species of bacteria which form symbiotic relationships with anglerfishes – both within the genus Enterovibrio (side note: while the bioluminescent symbionts are beneficial, other species of bacteria in this order cause human illnesses, including cholera). Interestingly, both of these bacterial species have small genomes (50% smaller than their relatives) and lack some common genes found in species that live independently in the environment (like those used for motility). This small genome size suggests these bacteria would be obligate symbionts that are handed down vertically from parent to offspring, but what we know about anglerfish life-history seems to preclude this possibility. Larval and juvenile anglerfish have little to no contact with adults and do not even have a lure to house these bioluminescent bacteria until later in life. So how and when do anglerfish acquire their bacterial symbionts and achieve the ability to glow?
Have bioluminescent bacteria evolved within their anglerfish hosts?
A team of scientists headed by researchers at Cornell and Nova Southeastern Universities had to take a novel approach to answer this question. Given the serious challenges of studying anglerfish and their bacterial symbionts in the field or lab, the team turned to genetic tools to investigate how different anglerfish species and their symbionts were related to one another.
The team examined 6 different groups of anglerfishes and the bioluminescent bacteria living within their lures. The researchers expected that if bacteria were handed down from parent to offspring (acquired through vertical transmission), the bacteria would have evolved within their specific host species. Therefore, different lineages of the bacterial symbiont would be different from one another but their evolution would mirror the evolutionary lineages of the anglerfish species. On the other hand, if the bacterial symbionts were acquired through the environment, these bacteria would not have evolved within a specific host species and the bacterial lineages would be more similar to one another, regardless of the anglerfish host they had colonized.
After analyzing the genetic sequences of the anglerfish and their symbionts, the scientists were able to see how they were all related to one another. They found that the bacteria did not drastically differ from each other, despite the evolutionary differences of their anglerfish hosts. This finding suggests that the bacterial symbionts do not evolve within their hosts and are not vertically transmitted from parent to offspring, but rather that anglerfish must be acquiring their symbionts directly from the environment. To verify this possibility, the scientists took water samples at locations where anglerfish were found. They discovered the species of symbiotic bacterial present in the water, further supporting the hypothesis that anglerfish obtain their symbionts from the environment
While this discovery helps to answer some questions about anglerfish biology, it uncovers just as many about their symbiotic bacteria. It is astonishing that these symbiotic bacteria, which appear to lack some of the critical machinery to live independently in the ocean, are able to persist in the environment for long enough and over a wide enough range to be picked up by anglerfish hosts. Scientists must conduct more research to illuminate the complicated relationship between anglerfish and their glowing bacterial symbionts.
I received my Master’s degree from the University of Rhode Island where I studied the sensory biology of deep-sea fishes. I am fascinated by the amazing animals living in our oceans and love exploring their habitats in any way I can, whether it is by SCUBA diving in coral reefs or using a Remotely Operated Vehicle to see the deepest parts of our oceans.