Why the 'cockeyed squid' is cockeyed
April 10, 2017
Behind every strange-looking feature on a strange-looking animal, there is a story of evolution and a need for that attribute.
The ridges and folds of the human ear are precisely calculated to funnel sound toward the eardrum. Hummingbirds have long, curved beaks to drink the nectar from specific flowers. Puffer fish blow up like balloons to protect themselves from predators.
The strawberry squid is stranger-looking than most, but like other species, its odd looks have a purpose. Its bright red skin is covered in hundreds of tiny spots—hence, strawberry—and its two eyes are vastly different. One is large and yellow while another is more average-sized and dark.
The squid’s spots are photophores: biological light bulbs that flash signals to other squids and help light up the dim layer of the ocean in which it lives. Now a new study from Duke University has found the need for the squid's different eyes. The bulbous, yellow eye looks upward to see light from the surface and shadows of things swimming above while the smaller, black eye looks for glowing fish from below.
Duke biologist Kate Thomas, the lead author of the study, watched through videos of more than 150 strawberry squid sightings going back 30 years. The videos were collected by underwater drones from the Monterey Bay Aquarium Research Institute in California. You can watch videos of the squid here.
Thomas found that in every video, the squids, also called cockeyed squids because of their differently-sized eyes, seemed to drift with their heads facing the ocean floor and their tentacles facing the surface. The yellow eye was always turned toward the surface while the dark one was always turned toward the ocean floor.
Using simulations, Thomas found that the eye pointed downwards would almost never be able to pick up shadows from the surface. Simulations also showed that making the upward pointing eye larger makes it much better at seeing sunlight and shadows, while making the downward eye bigger would not improve its ability to see glowing fish by much.
Ocean animals have to make adjustments for the amount of light in their habitat. Anglerfish use bright lights to lure in prey because they live too deep for light to come down from the surface. Many fish, including sharks have countershaded skin—light on bottom and dark on top—to blend in when they are seen from above and below.
Strawberry squid live in the mesopelagic zone, roughly 200-1000 meters deep. This depth is also known as the twilight zone because very little light is able to reach that depth. Below is the pitch-black bathypelagic or midnight zone while above is the bright epipelagic zone, and the squid need to respond to predators and food in both areas. Thomas says that eyes cost more energy to produce than most other limbs or organs, so the squid actually saves energy by always looking up with one eye and down with the other because they only need to make one large eye.
This research, published in the journal Philisophical Transacations of the Royal Society B, may solve a mystery involving this species. With 7.7 million animal species on Earth, however, there are many more strange looks to decipher.
Daniel Lane covers science, medicine, engineering and the environment in North Carolina.