LAWRENCE — University of Kansas researcher Trevor Rivers recently had a hand in the making of the BBC-TV documentary “Super Senses: The Secret Power of Animals” for his work with ostracods.
“We came up with a plan for me to come join the BBC crew to consult with them as well as collect the appropriate predatory fish and the luminescent ostracods,” said Rivers, a lecturer and academic program associate in the KU Undergraduate Biology Program and research affiliate in the Department of Ecology & Evolutionary Biology. “My wife, Jessica, and I drove across Puerto Rico and spent a couple of nights snorkeling along a reef in the dark finding and catching our organisms.”
Rivers then met with the BBC film crew and host on Vieques, where they were filming bioluminescent bays. “We got some incredible footage on the beach, and then the cameraman and I, using a very sensitive low-light camera, spent an additional six hours in a dark hotel room recording other displays,” he said.
Also known as seed shrimp, ostracods are teeny crustaceans that beckon potential mates or confound would-be predators with a flash of luminescence. A GIF taken from the BBC program that involved Rivers — showing a cardinalfish trying, and miserably failing, to chomp an ostracod — went viral on web sites such as Reddit.
“Ostracods are a very old group of crustaceans — think shrimp, lobsters, crab, etcetera — that are found in an incredible range of aquatic and marine habitats,” said Rivers. “The ostracods I study are only found in marine systems. They are less than two millimeters in length, and they look a bit like swimming sesame seeds. In the Caribbean, there is a suite of ostracod species that have developed the most complex luminescent courtship patterns found to date in marine systems.”
Rivers said that ostracods’ use of bioluminescence was similar to fireflies, “but bright blue, with glowing orbs being left in the water column in very specific patterns in both time and space, and with only males signaling. They also have bright luminescent predation defense displays. Their displays are only occurring during the night when the moon is down, and these displays typically start at sunset and last for only about an hour.”
The researcher said that, in spite of their evening-time light shows, seed shrimp are typically low-key in their habits. In fact, for most of the day, they effectively disappear.
“The rest of the time these ostracods are presumably crawling on the ocean bottom or buried in the ocean substrate,” Rivers said. “We have tried to find where they are during the day on multiple occasions and have come up short.”
Bioluminescence is common in many species, with fireflies being the most familiar example to most Americans. Unlike with fireflies, however, ostracods produce “extracellular luminescence.”
“Bioluminescence is the creation of light via chemical reaction,” said Rivers. “The best analog is to think about glow sticks — you crack a vial inside the plastic tube, and the chemicals then mix, creating light. Most people assume that symbiotic bacteria are the major source of luminescence, but many more organisms are capable of producing their own luminescent chemicals. The chemicals used vary in nature, with some examples requiring only oxygen for the reaction to occur, and others requiring other triggers such as calcium. In contrast to firefly luminescence, which takes place inside cells in light organs, with extracellular luminescence the chemicals are secreted from cells. In this case, ostracods squirt out the luminescent chemicals into the water.”
Rivers’ research into ostracods has appeared in the journals Behavioral Ecology, Animal Behavior and the Journal of Experimental Biology. In the latter, Rivers examined the differences between powerful luminescent displays intended to fend off attackers versus subtler displays meant to lure mates.
“Not only are the luminescent displays meant to startle their predators, it might also be a signal that brings in nearby larger predators to attack the ostracod’s attacker,” he said. “This is called the ‘burglar alarm effect,’ where presumably the brighter the signal the better. However, for courtship, male ostracods display multiple times a night over multiple nights, so are likely hedging their bets by being able to display more times. In addition, there is an incredible amount of male-male competition. In one species, Photeros annecohenae, for every female found in the water during mating, there are 175 males. Perhaps the intensity of the displays is dimmer to limit the amount of other males that join in on the displays.”
Rivers said he could trace his fascination with bioluminescence back to a fourth-grade teacher in his home state of Washington.
“She had me write a report on bioluminescence,” he said. “I doubt she realized the impact she had on me, but it was great to reconnect with her and tell her she was in my dissertation acknowledgements. I saw my graduate adviser Jim Morin’s talk on ostracods during a bioluminescence conference when I was an undergraduate student and asked if he was taking any graduate students. The rest is history.”
According to Rivers, researching marine biology from the Midwest isn’t as odd as it may sound to some. “I have to travel,” he said. “As a field biologist, that’s just fine with me. Even if I were at an institution along the coast, very few places in the U.S. are near where my organisms are found, as they are only in the Caribbean. I’d need to be traveling regardless of where my home base was. There are many researchers whose work spans the globe here at KU, with actually a respectable number of marine folk here like Paulyn Cartwright, Daphne Fautin, Kirsten Jensen and Leo Smith.”
Photo: Martin Dohrn, Ammonite Films.