Eye size of deep-sea shrimp reveal secrets of the deep

Different species of deep-sea shrimp have evolved to have different sized eyes to best see one another, according to new research.
 
FIU marine scientists Heather Bracken-Grissom and Lorian Schweikert — a former FIU distinguished postdoctoral scientist and current assistant professor at the University of North Carolina Wilmington — found that in the case of deep-sea sergestid shrimps, the eyes are windows to the secret of life in the deep sea.
 
More than other ecological factors, like habitat, migration patterns, depth and even sunlight level, bioluminescence was the strongest indicator for how big the shrimp’s eyes are — a finding that emphasizes the connection between vision and bioluminescence, and provides more evidence that making light is, in fact, a deep-sea form of communication.
 
Schweikert collaborated with co-lead author University of Texas postdoctoral scientist Kate Thomas and a team of researchers to examine 454 sergestid shrimps from 16 species. Eye size directly correlated to specific types of light organs.
 
“I think I expected depth to have the strongest effect on eye size, because sunlight can be such a powerful entity in marine environments,” Schweikert, the study’s lead author, said. “That’s what is so incredible about our findings. They suggest these shrimps evolved to have different eye sizes to better see the bioluminescence of their own species.”
 
In a place as huge and expansive as the ocean, some animals in the deep sea evolved to produce light. Bright blue bioluminescence is a way to communicate. Like all communication, this language of light is about transmitting and receiving signals. Vision and bioluminescence are intertwined.
 
“Such a simple and elegant part of nature, like eye size, can tell you how an animal might behave and survive in its environment,” Schweikert said. “Vision is so important to understanding the lives of many animals, especially those in the deep sea. I wanted to know what it was about their lives underwater that caused these shrimps to have different eyes and sensitivities in vision.”
 
Deep-sea sergestid shrimp each have a sort of unique pattern of bioluminescence, almost like a fingerprint. That’s because the light organs, called photophores, vary. Deep-sea shrimp in this study have three different variations — lensed, non-lensed and internal light organs called organs of Pesta. However, even across this species, the light organs are an array of different sizes, structures and arranged in different ways.

The largest and brightest type of photophore is the organs of Pesta, most common in shrimp that migrate to shallower waters where light levels are higher. Species with these photophores had the smallest eyes.

The deep-sea shrimp with the biggest eyes had smaller photophores dispersed along their bodies. Because they can migrate and use more deep and dark habitats, evolving to have larger eyes is a way to compensate and better detect fainter or weaker light.

“There is still so much to learn from studying these deep-sea marvels and we are only beginning to understand how and why animals use bioluminescence. I am hopeful this work will inspire others to think about all the possible ways animals may be using the language of light in the deep sea,” Bracken-Grissom said. 

This research was published in Frontiers in Ecology and Evolution.