Monthly Archives: March 2020

by Piter Kehoma Boll

Japanese sea waters can emit a beautiful blue light at night, especially when disturbed. This phenomenon is caused by bioluminescent organism. One of the most famous species that produce light in the sea is the sea sparkle, a dinoflagellate. Here, however, the light is caused by a crustacean, the ostracod Vargula hilgendorfii, known in Japan as 海蛍 (umihotaru, literally “sea firefly”). Thus, the name sea firefly is commonly used to refer to the bioluminescent ostracods of the genus Vargula, most of which live along the North American Pacific coast and the Caribbbean, with Vargula hilgendorfii being the only species to occur in Japan.

As all ostracods, the Japanese firefly is a very small crustacean. Their body measure about 2–3 mm in length and have proportionally large eyes with about 0.2 mm in diameter. They live in the sandy substrate of shallow waters up to 5 m deep, being, therefore, benthonic, and have nocturnal habits.

The areas where the Japanese sea firefly is found are marked by very strong currents. Since they have a very poor swimming ability, they are not very good to disperse to new areas but at least are able to swim well enough to avoid being carried far away by the currents. Nevertheless, the strong Japan Current seems to have slowly moved the species northward since the last ice age.

Female Japanese sea fireflies are slightly larger than males. When they copulate, they pair by touching their ventral sides while lying in opposite directions, kind of like a yin yang. Being an ovoviviparous species, the eggs remain inside the mother until they hatch, so that the female gives birth to live juveniles. Newborns are much smaller than adults, of course, but otherwise behave exactly like them, so that this species does not have a planktonic larval stage like most crustaceans.

The diet of the Japanese sea firefly is composed mainly of debris of all sorts, including dead animals. One of the easiest ways to capture them is simply by placing dead fish as traps in the water at night and waiting for them to come.

When the Japanese sea firefly is disturbed or attacked, it releases a luminous blue cloud. This happens by the ejection of two compounds, the substrate luciferin and its enzyme, luciferase. Luciferase catalyzes the reaction of luciferin with molecular oxygen, which emits the characteristic blue light. There is a reflecting organ near the posterior end of the animal that seems to increase the power of the emitted light, which may improve their ability to escape from predators.

Luciferase enzymes are of research interest especially as reporter genes. When scientists want to know whether a specific gene is being expressed in an organism, they can attach another gene right after it so that this gene will necessarily be expressed together with the gene of interest. The luciferase of the Japanese sea firefly, usually named Vargula luciferase, has been studied as a reporter gene in mammals. After creating the chain formed by the gene of interest + luciferase and inserting it into a cell, one can know whether the gene is being expressed by applying luciferin to the cells. If they glow blue this means that the gene is indeed being expressed.

As you can see, even a tiny sea creature can have a profound influence on scientific research.

– – –

Like us on Facebook!

Follow us on Twitter!

– – –

More Ostracods:

Friday Fellow: Sharp-Toothed Venus Seed Shrimp (on 22 June 2018)

Friday Fellow: Stonewort Seed Shrimp (on 19 July 2019)

– – –

References and further reading:

Abe K, Ono T, Yamada K, Yamamura N, Ikuta K (2000) Multifunctions of the upper lip and a ventral reflecting organ in a bioluminescent ostracod Vargula hilgendorfii (Müller, 1890). Hydrobiologia 419: 73–82. doi: 10.1023/A:1003998327116

Abe K, Vannier J (1995) Functional morphology and significance of the circulatory system of Ostracoda, exemplified by Vargula hilgendorfii (Myodocopida). Marine Biology 124: 51–58. doi: 10.1007/BF00349146

Kobayashi K, Ohmiya Y, Shinohara D, Nabetani T, Niwa H (2001) Purification and properties of the luciferase from the marine ostracod Vargula hilgendorfii. Proceedings of the 11th International Symposium on Bioluminescence & Chemiluminescence: 87–90.

Maeda Y, Ueda H, Hara T, Kazami J, Kawano G, Suzuki E, Nagamune T (1996) Expression of a Bifunctional Chimeric Protein A-Vargula hilgendorfii Luciferase in Mammalian Cells. BioTechniques 20: 116–121. doi: 10.2144/96201rr01

Ogoh K, Ohmiya Y (2005) Biogeography of Luminous Marine Ostracod Driven Irreversibly by the Japan Current. Molecular Biology and Evolution 22(7): 1543–1545. doi: 10.1093/molbev/msi155

Thompson EM, Nagata S, Tsuji FI (1989) Cloning and expression of cDNA for the luciferase from the marine ostracod Vargula hilgendorfii. PNAS 86(17): 6567–6571. doi: 10.1073/pnas.86.17.6567

Thompson EM, Nagata S, Tsuji FI (1990) Vargula hilgendorfii luciferase: a secreted reporter enzyme for monitoring gene expression in mammalian cells. Gene 96(2): 257–262. doi: 10.1016/0378-1119(90)90261-O

Vannier J, Abe K (1993) Functional Morphology and Behavior of Vargula Hilgendor Fii (Ostracoda: Myodocopida) From Japan, and Discussion of Its Crustacean Ectoparasites: Preliminary Results From Video Recordings. Journal of Crustacean Biology 13(1): 51–76. doi: 10.1163/193724093X00444