Trout Lake Station’s Gretchen Gerrish sheds light on the importance of dark

During a recent Science on Tap event, Gretchen Gerrish of the Trout Lake Station spoke about the importance of darkness and her research involving bioluminescent ostracods.

Ostracods, she said, are tiny organisms ranging from one millimeter to four millimeters. They are found in all of the waters across the globe, she said. Even a puddle in a driveway could be home to an ostracod. Much of the work of studying ostracods has been funded by industry. Due to the hard shell of the ostracod, there are good fossil records, Gerrish said, noting that they were very diverse and prolific during the carboniferous period of the Paleozoic Era. Companies looking for oil are especially interested in this time period and have conducted a good deal of the research regarding ostracods. 

The type of ostracods on which Gerrish works are called myodocopids. They are strictly marine ostracods. While most ostracods do not produce light, myodocopids have evolved to do just that. The original evolution of this was a defense mechanism. Once an ostracod gets eaten by a fish, it puts all its energy into lighting up. The fish will them spit the ostracod out. The theory is that the light would light up the outline of that fish, potentially bringing in bigger predator fish. So, to keep itself hidden and keep the fish from being eaten itself, it spits out its potential prey. That dynamic is still being researched, she said.

In the Caribbean, this bioluminescent behavior has evolved into a mating behavior. Only in the Caribbean is that true, she said. There are over 40 different species of organisms that have evolved the ability to produce light, Gerrish explained. Most of this is tied to a defense mechanism. Many of the chemicals used to produce that light are relatives of the chemicals found in the digestive system. 

Gerrish did much of her work on a reef off the coast of Belize. She and other researchers have found over 63 new taxa in the last five sites at islands they have been studying. The hypothesis, she said, is that the unique reproductive signals given out by various taxa of these ostracods has allowed them to speciate and diverge more rapidly than the ostracods that do not use light in their reproductive process.

Gerrish then turned the discussion to artificial light. 

In order to show the bioluminescent colors of the dried ostracods she had with her, she needed the lights to be dimmed. 

Artificial light in aquatic systems, she said, changes the visual system for the organisms that live there. They are very sensitive to light. 

She spoke about a Smithsonian research facility that had a couple boat motors stolen, so in 2017 they put three huge lights that projected out onto the ocean where they kept their boats for safety. The lights wiped out all of the ostracods within 300 meters, she said. Also, for approximately 9 kilometers around Belize City, there were no bioluminescent ostracods, likely due to the light dome from that city.

The two weeks after the full moon is the best time to see these displays because the sky is actually dark just after nautical twilight, before the moon comes up, she added.

 Nautical twilight is approximately 40 minutes after sunset. The displays are easy to miss, Gerrish said, as they may last only 30-40 minutes in total, with each species kicking off at slightly different times. That is one of the main reasons why it can be difficult to catch these displays. One of the main risks she encountered when diving at that time of night was box jellyfish. These ostracods can be seen at any time of the year, she said, and are more dependent on the moon phase.

“Why do we care about ostracods anyway?” Gerrish asked, although they are the beings she has studied for many years. Humans tend to pick flagship species as indicators, such as looking at polar bears as indicators of climate change. Bioluminescent ostracods, she said, can be the leading indicator that something is off in a system. Corals have evolved to spawn along with lunar and solar signals. Most organisms on the sea floor depend on those signals for breeding success. 

In her research, Gerrish said she has used red lights at night, or infrared lights. Those lights do not disrupt the behavior of these marine creatures, as ostracods cannot see the red lights. She said this is one of the simple solutions to lessen the effects of artificial light.

At Trout Lake Station, one of the things Gerrish is working on is the vertical migration of zooplankton in the lake. They move up in the water column at night to feed where there is less fish predation. During the day they move back to the bottom, again to avoid predation which is lessened with less light.

With artificial lights hitting the water, then, it is obvious this could disrupt that feeding cycle. Zooplankton, she said, is the basis of the food web in aquatic systems. With more solar lights on at night now, although they are not as bright, they still tend to disrupt life cycles of many different species.

Artificial light has been shown to be detrimental to many nocturnal species, but is even more detrimental to bioluminescent species such as these ostracods or, more close to the Northwoods, fire flies.

Gerrish said limiting artificial light is one way to limit one more additional pressure on systems that are already experiencing difficulties from other stressors.

Gerrish’s entire Science on Tap presentation can be found on the Trout Lake Station YouTube channel, as can all of the previous presentations in the series. 

The next presentation will be at 6:30 p.m. on Thursday, January 4 and will feature Bart Kotarba and Amanda Schirmer of the Northwoods Wildlife Center.

Beckie Gaskill may be reached via email at [email protected].