The Hyper Eyes of Hyperiids: How Some Shrimp-Like Creatures See Light in the Deep Sea
Hyperiid amphipods are small crustaceans related to sand fleas and distantly related to shrimp. They range in size from very tiny to more than 7 inches long, and are found at all depths of the ocean from the surface to the deep sea. Roughly 320 species have been identified and some have very interesting eyes—evolutionary solutions to the challenge of seeing in the nearly absent light of the mid-ocean and complete darkness of the deep sea.
Hyperiids typically have two pairs of compound eyes. Each compound eye has many photoreceptors—sometimes numbering in the thousands—each of which captures an image of a small part of the scene. The brain pulls these many images together into one single image, like a panorama.
But the variation in hyperiid eyes is staggering, making them a fascinating group to study. Some hyperiids have greatly enlarged eyes that cover the entire head, while others have very tiny eyes. In some species, fiber optic cables connect the lenses to the retinas, which pass the images on to the brain. Some species have cone-shaped retinas that allow them to see almost 360 degrees. Still others have retinas with mirrors that boost light collection, helping them to see in the dark. Some have a single giant pair of eyes and others lack eyes altogether.
Jamie Baldwin Fergus, a postdoc at the National Museum of Natural History, is looking into how exactly hyperiid amphipods see in the light-limited mid-ocean. Enjoy the photos of the animals she’s researching in this slideshow, and read more about her research on the Smithsonian's Department of Invertebrate Zoology No Bones blog.
In the Cold and Dark Depths
This hyperiid is one of seventeen species in the genus Lanceola, which are found between 200 and 7,000 meters below the ocean’s surface. Some species, like this one, are found exclusively below 3,000 meters and are adapted to the cold and dark that comes with that depth. Meanwhile, others may vertically migrate closer to surface waters at night. Because they spend so much time in the dark, Lanceola species tend to have very small eyes—or none at all. In this image, the tiny bright red dot on the right side of the head is a tiny compound eye.
This hyperiid is one of twenty-four species in the genus Vibilia, distinguished by the paddle-like antennae on the front of its head. Vibilia are small (only 5-20 millimeters long) and their eyes are the irregular oval shapes in this image. “We typically find them in large numbers scurrying around inside colonies of salps, planktonic barrel-shaped tunicates that filter seawater as they drift through the mid ocean,” explains Karen Osborn, a researcher in the Department of Invertebrate Zoology. “They essentially function as benthic animals since they live their whole lives crawling over some other animal," Osborn says. Instead of crawling on the sea floor, as is typically the case for benthic animals, these hyperiids live their lives on the surface of another animal in the middle of the water column.
Detecting Green Light
Scypholanceola aestiva looks like an armored alien of the deep. It doesn’t have compound eyes like other hyperiids, but instead sees variations in light using reflective cups embedded in the exoskeleton on its head. This light detection works best with green wavelengths. Why would it want to see green light in the deep sea? It is most likely looking for signs of bioluminescence—bright green and blue flashes of light made by animals—that could signal that food or a predator is nearby.
This hyperiid (in the genus Cystisoma) has only one pair of eyes—but they are very big. You can see them here as the entire surface of its head and the convex orange sheet of retinal cells in the bottom-right of the photo. Its eyes look almost exclusively upwards, possibly to watch out for animals overhead.
This species is completely transparent. Even its gut (the bubble-like blue outline you can see in the picture) is see-through. This transparency keeps it from being seen by predators in the open ocean, where there is no place to hide. These hyperiids are much larger than most, reaching lengths of up to 7 inches. It helps to be see-through when you’re that size.
This much smaller hyperiid (in the genus Paraphromina) has eyes that make up 45 percent of its body! Its many retinas (it has 32!), which researchers believe are used to maximize light detection, are the small red dots attached to long cones. These eyes look upwards and are uniquely adapted to look for silhouettes of other animals illuminated by the dim light that filters down from the surface of the ocean.
At only 1 to 2 cm, these fast-moving hyperiids can be difficult to catch in the small bucket full of animals from a trawl. They typically live at depths of 100-400m on siphonophores, colonies of animals related to Portuguese Man-o-war that are distant relatives of jellyfish.
These Themisto hyperiids live in the top 200 meters of cold waters around the world. Each of their two large eyes has an upward-looking zone and a downward-looking zone. Each zone can see at different resolutions: the upward-looking zones tend to see better than the downward-looking zones. Scientists think this could be because the animals need to see possible prey or mates that would be found overhead. What would you do with an almost 360 degree view of the water column?
Phronima Female and Young
This female hyperiid (Phronima sedentaria) is surrounded by her young, residing in the hollowed out barrel-shaped body cavity of a salp. It is thought that the mother Phronima captures and kills the salp to build a nursery. She lays her eggs inside and then paddles it around while caring for her young.
Phronima eyes see blue light best and are well-suited to look for other animals horizontally from the inside of their barrel. They use large appendages to hold onto the sides of the salp and to steer its movement through the water.
This is another view of Phromina from the side. The eyes take up most of the head, with one pair looking to the side (the red spot) and one pair facing upwards. Phronima live anywhere from 200-1100 meters deep and migrate towards the surface at night. Rumor has it that these strange looking creatures were the basis for the monster in Alien; fortunately in real life they are only up to 4 cm long!