Study of shrimp eyes opens window into life in the deep sea

Tiny, glowing shrimp that live in oceans’ darkest depths are shedding light on how life operates in one of the final frontiers, the deep sea.

Research examining the eye size of more than 16 species of planktonic, almost transparent shrimp called sergestid shrimps, is revealing how animals of the deep have adapted to surviving in low light.

“We don’t know very much about the deep sea because it’s incredibly difficult to study,” said Lorian Schweikert, an assistant professor of biology and marine biology at the University of North Carolina Wilmington. “A really good place to start is by looking at vision, light and vision and that’s because, from what we understand, vision and the detection of light is critical to deep sea survival.”

Schweikert was part of a three-week research expedition funded by the National Oceanic and Atmospheric Administration’s Office of Ocean Exploration and Research in June 2019 that afforded groups of researchers to focus on their particular areas of deep-sea study in the Gulf of Mexico. Coincidentally, this was the same expedition where one of those groups of researchers captured a giant squid on video, a sighting that made national news.

Schweikert’s team, which included marine biologists from the University of Texas and Florida International University, were there to study patterns of the eye size of sergestid shrimp pulled from varying depths off the Louisiana coast.

Animals of the deep have evolved the ability to glow and they use bioluminescent signals to communicate with each other.

This means that their vision is crucial to their survival. It’s how they find food and one another in the deep.

“That really is the basis of why we were looking at patterns of eyesight,” Schweikert said.

The research team targeted sergestid shrimp because they swim in large numbers in the water column throughout the world’s oceans.

They’re found in various deep-sea habitats – on the sea floor, in the open deep, and more shallow areas – which makes them particularly ideal to study when comparing patterns of survival in the deep-sea environment.

Sergestid shrimp take part in a nightly phenomenon called diel vertical migration, the largest mass migration of animals on Earth, when creatures of the deep leave the safety of the cold, darkest depths of the ocean to avoid predators and rise closer to the surface to feed in better light conditions.

“These shrimps, like all the animals down there, differ in how much they do this migration behavior, how far they migrate every night,” Schweikert explained.

Researchers used giant trawl container nets, which seal as animals are bought to the surface to keep them in a light-tight environment, to pull shrimp from depths ranging from 200 meters, about 660 feet, at night to between 1,000 meters, or about 3,300 feet, and 2,000 meters during the day.

Captured shrimp were taken to a wet lab aboard the research vessel and released in tanks where they lit up like a constellation, Schweikert said.

In all, the research team examined more than 450 shrimp.

“We looked at eye sight patterns across these shrimps and compared that to different aspects of their life, different aspects of ecology, because we wanted to know what was it about life in the deep sea that drives the importance of vision,” she said.

They compared eye size of shrimp pulled from different depths of the ocean and compared how much they migrated through the water column.

“We found that, above all other aspects of light that we compared, that the light organ patterns on their bodies was the best predictor of how their eyes are going to be,” Schweikert said. “In other words, it told us that the glowing signals that they may send each other is, by far, the most important aspect of their life for determining their visual mobility.”

Shrimp that have larger bioluminescent organs have smaller eyes.

This makes the case, Schweikert said, that species with smaller, and therefore potentially dimmer organs, had to evolve larger eyes to detect subtle glimmers of light at a distance.

“The size of the light organs was, by far, the greatest predictor of how the eyes were going to differ in these animals,” she said. “But, we did we did see weaker relationships for depth and for migration range.”

Researchers found that, overall, species that lived at greater depths – 1,000 meters or so – had larger eyes.

Their findings now give biologists an understanding, because these crustaceans’ eyes match the brightness emitted from their organs, they use their bioluminescence to communicate with one another.

Still, this is a mere glimpse into the mysterious world of the deep sea, the tip of the iceberg.

“We know more about the surface of the moon than the topography of the sea floor,” Schweikert said.

Next up, researchers are set to study how deep-sea creatures produce bioluminescence and how they communicate using the light from their bodies.

Schweikert said continuing research will ultimately help people better manage plans for how humans tap resources of the deep sea, including fish, gas and oil, and rare minerals.

“These are having an impact on the environment that we don’t fully understand,” she said.