Vision is a vital component of many species’ sensory toolboxes. Animals utilize sight to pinpoint prey, spot predators and check out potential mates. But even eagle-eyed critters struggle to see in the dark, which is why animals in the deepest and darkest depths of the ocean have developed some of the most extreme eyes on the planet.
According to Karen Osborn, a research zoologist at the National Museum of Natural History, deep-sea denizens usually approach vision in two very different ways. Many species, including toothy anglerfish and eyeless cavefish, invest little in sight, utilizing heightened senses like smell and touch to guide themselves through inky water.
Other species do the exact opposite. “They devote a ton of energy to make really cool eyes that can use the little bit of light that makes its way down here,” said Osborn, who specializes in studying how animals have adapted to life in the deep sea. Giant squids, for example, sport eyes the size of soccer balls. Other creatures possess peepers that are stranger than science fiction. For example, the barreleye fish has bulbous, bright green eyes lodged deep within their transparent, fluid-filled heads while cockeyed squids sport a pair of drastically mismatched eyes — their left eye is twice as big as their right eye and is perpetually looking upwards.
Another class of critters employing extreme eyes are hyperiid amphipods. Many of these small, shrimp-like crustaceans inhabit the ocean’s twilight zone, a gloomy swath of the water column sandwiched between the light-flooded surface and the perpetually dark abyss.
To peer through the dimness, hyperiid amphipods sport a stunning diversity of different eyes. “The other 10,000 species of amphipods basically have the same type of eye with some slight variations, and then there’s this little group that has nine totally different types of eyes,” Osborn said. Some sport enlarged eyes that cover their entire head while others have cone-shaped retinas that allow them to see nearly 360 degrees. Some lack eyes altogether. “To me, that’s particularly interesting,” said Osborn. “Why are there so many kinds of eyes within this small group of animals?”
To get a better glimpse of how these dramatically different eyes developed, Osborn teamed up with several colleagues including Zahra Bagheri, a bioengineer at the University of Western Australia, to analyze the extraordinary eyes of the hyperiid amphipod Phronima sedentaria. These tiny crustaceans reside up to 3,600 feet below the ocean’s surface and are well-known for their grisly habit of killing gelatinous zooplankton called salps and hollowing out their barrel-shaped bodies to serve as floating nurseries for their young. This parasitic penchant, combined with their extraterrestrial appearance, is the reason why many people think Phronima amphipods inspired the design of the Alien film franchise’s xenomorph monster.
Like other hyperiid amphipods, Phronima have two pairs of compound eyes that take up most of their heads. The smaller pair is oriented towards the side of the head while the larger pair is positioned upwards at the top of the animal’s head. To understand how Phronima visualizes the shadowy world around it, the researchers needed to digitally examine each eye in minute detail.
“The first time you look at their eyes, it doesn’t make sense,” said Bagheri, who specializes in studying the biomechanics behind vision. “But when we do the computational modeling, it starts to become clearer why these eyes developed like this.”
The researchers micro-CT scanned two Phronima specimens and used a custom made computer program to map the fine structure of the eyes and take exact measurements for the thousands of parts that make up all four of the amphipod’s eyes. They then plugged the measurements into a series of equations to model what the animals can actually see. These allowed them to determine the distances over which Phronima’s compound eyes can detect bioluminescent and transparent objects at different depths.
They discovered that Phronima’s smaller eyes were built for peripheral vision. While the vision was blurry, these eyes allowed the crustacean to see nearly 360 degrees save for a small blindspot directly behind its head. The larger eyes at the top of its head were much more focused and powerful, allowing the amphipod to see surprisingly far in the dark. The combination of broad and focused vision was unique according to Osborn. “Usually you have to balance having really good vision with how much vision you have,” she said. “Phronima has managed to develop two very different sets of eyes that give it both peripheral vision and super detailed vision.”
The researchers published their new Phronima eye model earlier this week in the journal PLOS Computational Biology, allowing scientists to finally peer through the extraordinary eyes of these otherworldly crustaceans. “This is a super powerful tool to understand what these animals are seeing and how they use their vision to interact with other animals.” Osborn said.
She and her colleagues are currently utilizing similar models to understand how other amphipods see the world with their many different eyes. While Phronima has always intrigued scientists and beach-combers alike with their bizarre eyes, there’s a slew of other deep-sea inhabitants just waiting for a closer look.
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