Survival in the deep sea is no easy feat. With cold temperatures, darkness, infrequent sightings of prey and the constant threat of being ambushed by a predator, deep sea animals have some interesting and highly evolved adaptations. From eyes the size of dinner plates to glow-in-the-dark flashes, it’s no wonder scientists study the strange features of deep sea creatures!
Recently, biologists from Duke University examined the mismatched eye size of the
“cockeyed” strawberry squid, Histioteuthis heteropsis, a species of deep sea squid that has one normal eye and one giant bulging yellow eye. This small squid lives in the mesopelagic (twilight) zone 200-1000 meters deep, where dim light trickles down from the surface and decreases with depth.
The team of scientists watched more than 150 undersea videos collected over 30 years by the Monterey Bay Aquarium Research Institute (MBARI). Through behavioral surveys, they concluded that the lopsided eyes have an evolutionary function for spotting different sources of light in the deep sea. The large eye of the squid is specifically adapted for gazing upwards where it scans for shadows of other animals against the fading light. The small eye is adapted for gazing downwards through the deeper waters in search of bioluminescent flashes.
Resources in the deep sea are sparse so organisms such as H. heteropsis have evolved energetically inexpensive forms, body plans and adaptations. It is energetically costly to make and maintain eyes, so the squid’s mismatched eye size appears to be winning the constant gamble of cost versus benefit of adaptations.
Journal reference:
Thomas, K., B. Robison and S. Johnsen. 2017. Two eyes for two purposes: in situ evidence for asymmetric vision in the cockeyed squids Histioteuthis heteropsis and Stigmatoteuthis dofleini. Philosophical Transactions of the Royal Society B, 372: 20160069. DOI: 10.1098/rstb.2016.0069.
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