Simanaitis Says

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GETTING A GLOW ON—IN AN EVOLUTIONARY SENSE

“TINY CARIBBEAN CRUSTACEANS and their bioluminescent mating displays are shining new light on evolution.” This, from Elizabeth Pennisi in her fascinating article “Sparkling Waters” in Science, August 22, 2022. 

The long exposure of this stunning image by Kyle McBurnie, Science, August 22, 2022, shows the bioluminescence of ostracods in motion.

Tiny Aquatic Creatures. “No bigger than a grain of sand,” Pennisi says, “ostracods abound in fresh and saltwater.” Wikipedia notes that their name derives from the Greek οστρακόν, ostrakon, shell or tile

Two species of ostracods. Above, Photeros morini; below, Maristella chocoi.

Pennisi cites Timothy Fallon, an evolutionary biochemist at University of California, San Diego, who says, “They are very cute but also sort of bizarre—like a cross between a crab and a tiny spaceship.” Among ostracod characteristics noted by Wikipedia are antennae used for propulsion, and males and females each having dual genitalia. Indeed, some species are partially or wholly parthenogenetic

Bioluminescent Diversity. “Only seagoing ostracods are bioluminescent,” Pennisi observes, “and it’s not their bodies that glow. Rather they spew out glowing mucus. In most of the world’s oceans, ostracods do this for defense—to startle and distract would-be predators. But in the Caribbean, and only in the Caribbean…those bright blue dots can double as mating calls.” 

Researchers believe these signals have driven Caribbean ostracods to diversify into more than 100 species. In a sense, they’re performing evolution in quick time. 

A chase scene: the downward luminescent trail of an ostracod evading a fish. The ostracod is the dot to the lower left of its path. Such marvelous videos are posted at “Sparkling Waters.”

Pennisi says, “In just the past 2 years, researchers have figured out how to grow ostracods in the lab, a development that will allow them to dissect the molecular mechanisms of evolution in a way once possible only in more conventional lab animals such as nematodes and fruit flies.” 

What’s more, Pennisi notes, “The mechanics and biochemistry of their light flashes are relatively simple, and many species of ostracods overlap in small areas. Compared with other animals with complex mating rituals—songbirds, say—they may more readily yield clues about the forces that generate biological diversity.”

How Do Ostracods Luminesce? In general, bioluminescence involves two chemicals, a luciferin and luciferase, both deriving their names from lucifer, Latin for light-bearing, as in the morning star associated with a falling angel, whence Lucifer. We also have the word “lucifer” as a traditional name for a friction match: “…while you’ve a lucifer to light your fag” in “Pack Up Your Troubles in Your Old Kit Bag.”

Ain’t science researching fun? You never know where it’ll lead.

Image by C. Bickel/Science, August 22, 2022

Some 150 species of ostracod have separate storage compartments for luciferin, luciferase, and mucus. On demand, the luciferin and luciferase are squirted into a ring of mucus, thus generating the bluish light. 

Why Do Caribbean Ostracods Do It? “The consortium,” Pennisi writes, “is now using ostracods as a new lens on one of the biggest questions in evolutionary biology: What drives the formation of new species? Female choosiness about mates is one candidate.… but there has been little evidence it can actually split one species into two. Oakley thinks his studies of bioluminescence provide some of the first proof.”

As another example, Pennisi recounts, “At a study site in Belize, Gerrish and her graduate student Nick Reda have also found evidence that distinctive displays might be pushing one species to divide into two. Most males of the species Photeros annecohenae swoop upward as they flash, painting their string of glowing dots in a consistent direction. But a few swoop downward, and this tendency seems to be increasing, suggesting enough females prefer this behavior to cause it to proliferate.” 

My Own Accumulated Trivia. Years ago, when I lived on St. Thomas, we’d make holiday excursions to nearby Puerto Rico. A visit to Parguera in the southeastern portion of the island was memorable: La Parguera Nature Reserve is renowned for its bioluminescent bay. Wikipedia says this particular luminescence is traceable to dinoflagellates, a kind of marine plankton possessing its own particular luciferase enzyme and luciferin. 

On the other side of the world, this is a long exposure image of N. scintallans in a yacht port of Zeebrugge, Belgium, by Hans Hillewaert from Wikipedia. 

Best viewed on a moonless night, I recall the Parguera bay looking like it was strewn with tiny diamonds. 

Another bit of luminescent trivia: Wikipedia says that ostracod bioluminescence “made them valuable to the Japanese during World War II, when the Japanese army collected large amounts from the ocean to use as a convenient light for reading maps and other papers at night. The light from these ostracods, called umihotaru in Japanese, was sufficient to read by but not bright enough to give away troops’ position to enemies.” Elsewhere in the world, ostracods were merely seeking hookups. ds

© Dennis Simanaitis, SimanaitisSays.com, 2022

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