How Parapriacanthus got its glow back

January 15th, 2020
by Jeremy D. Bailoo, PhD

You’ve probably never heard of the fish, Parapriacanthus ransonneti, and to be honest neither did I until I stumbled upon this recent article in Science Advances. So what makes P. ransonetti unique? P. ransonetti is a bioluminescent fish — that is, it is capable of producing and emitting light.

Bioluminescence, however, does not make P. ransonnetti unique — after all, in the tree of life there are over 800 genera that contain bioluminescent species, and of which ~200 are fishes. Nor is it the use of the protein luciferin and the enzyme luciferase (also a protein) in the production of light. Like many bioluminescent species, the protein luciferin is often obtained from the food that these organisms eat (their diet). In fishes, the organisms that comprise their diet may include marine organisms such as copepods and ostracods. In contrast, however, the enzyme luciferase is thought to come from within the bioluminescent animal (i.e. it’s endogenous), and produced by transcription of DNA. Thus, it is typical for bioluminescent fish to get the bioluminescent protein from their diet and their endogenous enzyme luciferase acts on the protein to cause light production.

But new research shows that P. rasonnetti is different. Rather than producing the enzyme luciferase endogenously, P. ransonnetti obtains both luciferin and luciferase from its diet — and it is this that makes P. ransonnetti unique.

To explore this question, researchers fed P. rasonnetti non-bioluminescent food for up to a year, causing them to lose their “glow”. Then they fed P. rasonnetti an ostracod with bioluminescent properties, Cypridina noctiluca, and found that this diet was sufficient for P. rasonnetti to get its “glow” back. The luciferase enzyme found in the light organs of P. rasonnetti was identical to that of the C. noctiluca that it was fed rather that endogenously produced luciferase derived from DNA transcription. To further confirm that fish physically ingested and used these specific enzymes, the researchers then fed the fish a completely different ostracod, Vargula hilgendorfii, and found the enzyme causing light production within the fish was identical to V. hiligendorfii, not C. noctiluca.

This basic research question is quite stimulating for a number of reasons. First, it highlights that acquisition of traits across species may not simply be always be a consequence of the genetic makeup of the animal. Secondly, it highlights that the same trait across different species may be acquired through multiple pathways and that the maintenance of that trait may be directly related to the kinds of things that are found within its environment. Afterall, without marine organisms such as ostracods or copepods, P. rasonnetti would not be able to produce light. Finally, such research is a good demonstration of the breadth of research for which animals are used. More often than not, we think of animal research in terms of its application to humans and other animals. While those kinds of research are important, we should not forget that the pursuit of knowledge through animal research is an end unto itself.