Lots of critters glow in the dark, but most of them aren’t found in just any back yard…unless that back yard happens to be the beach. The ocean is full of bioluminescent critters that use light to attract prey (possibly like the “glowing sucker octopus” Stauroteuthis syrtensis), find mates (like Odontosyllis undecimonda, aka fireworms), or act as a defense mechanism. Organisms might produce a startling flash to scare off a potential predator or use bioluminescence for counterillumination, making it harder for a predator to clearly see the outline of its next snack. My favorite example is the absolutely ADORABLE Hawaiian bobtail squid, you can’t deny the cuteness…there are articles about it.
While it’s possible for animals to actually harbor the chemicals needed for a lovely glow, the light itself is often produced by symbiotic bioluminescent bacteria. Just five years ago, researchers determined that it has been at least 17 times (yes…that’s a lot!) that ray-finned fishes evolved symbioses with glowing bacteria buddies (Davis et al., 2016). However, a broader picture defining patterns across these symbioses had not yet been carefully defined, and I suppose it was time to find out.
In a study just out February 4th, Gould and colleagues aimed to characterize the symbiotic relationships of one genus (Siphamia) in the cardinalfish family, with their bioluminescent bacteria. The bacteria of interest, in this case, are in the family Vibrionaceae and genus Photobacterium. The taxonomy of species in Photobacterium is a little difficult to parse out, but some are clearly defined. Specifically, P. mandapamensis (which isn’t too picky when it comes to hosts) and P. leiognathi (partial to a specific family of fish, the Leiognathidae). These two species are distinct and do not have the same set of genes responsible for light production.
It turns out that all 25 species in Siphamia have bioluminescent symbionts absolutely packed in a light organ connected to their intestine. Before this study, the light organ from only one species of cardinalfish (S. tubifer) had been carefully assessed from samples from just one area in the Okinawa Islands of Japan…even though this fish is found from eastern Africa all the way to French Polynesia.
Gould and colleagues were able to access 59 museum specimens, which included 14 Siphamia species from a range of geographical locations that were collected from 1963 through 2018. They were successfully able to recover DNA from samples that had been fixed in formalin, which is a problematic chemical since it degrades DNA. Ultimately, the authors gathered data that allowed them to ask questions about symbiont diversity and how that linked with what was going on in the host (e.g. geographic range and environmental temperature in which it was found).
The authors found that from their samples, across the genus Siphamia, the symbiotic relationship was always with P. mandapamensis. These symbionts are not passed vertically, but instead are transmitted from the environment, so perhaps this is a bit surprising given the broad geographic range that was sampled. Further studies and careful assessment of any other potential roles that these bacteria play, could help clarify their ecological role both in the host and in the environment.
The authors point out that it’s not even just any old P. mandapamensis that seems to be found in association with Siphamia fishes, but that of the Clade II variety, which you can think of as a subgroup within that bacterial species. This might prompt the question as to what exactly the differences are between this clade and any other clade, and how that impacts their ecology. The authors note that they did see some differences that appeared to be temperature-dependent within the bacteria that were sampled, with more SNPs identified in the samples from relatively chilly waters. While no evidence for codivergence for these fishes and their little friends was found, the remarkably high specificity of the relationship was notable.
Leveraging the wealth of samples harbored by museum collections in order to understand complex symbiotic relationships is a step towards better understanding the evolution of both the host and the bioluminescent bacteria….you might even say…it’s illuminating.
Gould, A., Fritts-Penniman, A. and Gaisiner, A., 2020. Museum genomics illuminate the high specificity of a bioluminescent symbiosis across a genus of reef fish. Front. Ecol. Evol., 04 February 2021 | https://doi.org/10.3389/fevo.2021.630207
Davis, M.P., Sparks, J.S. and Smith, W.L., 2016. Repeated and widespread evolution of bioluminescence in marine fishes. PloS one, 11(6), p.e0155154. | https://doi.org/10.1371/journal.pone.0155154