The Story Of Barreleye Fish

                                                        

                                          The Story Of Barreleye Fish

        In the ocean's shadowy twilight zone , between 600 and 800 metres beneath the surface , there are fish that gaze upwards through their transparent heads with eyes like mesmerising emerald orbs . These domes are huge spherical lenses that sit on a pair of long , silvery eye tubes - hence its common name , the barreleye fish (Macropinna microstoma) .

         The green tint ( which actually comes from a yellow pigment ) acts as sunglasses , of a sort , to help them track down their prey . There's nowhere to hide in the open waters of the deep ocean and many animals living here have glowing bellies that disguise their silhouette and protect them - biolumi-nescent prey is hard to spot against the dim blue sunlight trickling down . But barreleyes are one step ahead . 

        Their eye pigment allows the fish to distinguish between sunlight and bioluminescence , said Bruce Robison , deep - sea biologist at the Monterey Bay Aquarium Research Institute in California . It helps barreleyes to get a clear view of the animals trying to erase their shadows . 

                                                                               Sensitive eyes

        The barreleye's tubular eyes are extremely sensitive and take in a lot of light , which is useful in the inky depths of the twilight zone . But Robison was initially mystified that their eyes seemed fixed upwards on the small spot of water , right above their heads ." It always puzzled me that their eyes aimed upward , but the field of view did not include their mouths , " said Robison . Imagine trying to eat scraps of food floating in front of you , while fixing your eyes on the ceiling . 

The Story Of Barreleye Fish

        But , after years of only seeing dead , net - caught specimens , Robison and colleagues finally got a good look at a living barreleye through the high definition cameras of a remotely operated vehicle . " Suddenly the lightbulb lit and I thought ' A - ha , that's what's going on !, " he said . " They can rotate their eyes . " This means the fish can track prey drifting down through the water until it is right in front of their mouth . 

        Seeing a barreleye alive in the deep , Robison saw something else that scientists had previously missed . " It had this canopy over its eyes like on a jet fighter , " he said , referring to the transparent front part of the barreleye's body , which had been torn off all the specimens he had previously brought to the surface . 

                                                                            Stinging tentacles 

        He thinks this canopy probably helps protect their eyes as they steal food from among the stinging tentacles of siphonophores - animals that float through the deep sea in long deadly strings , like drift nets. Barreleyes have been found with a mix of food in their stomachs , including siphonophores ' tentacles , as well as animals that siphonophores feed on , including small crustaceans called copepods .        

 Their tactic may be to swim up to siphonophores and nibble on the small prey snagged in their tentacles , using the transparent shield to protect their green eyes from stings . But encountering barreleyes in the wild is not easy . In his 30 - year career , Robison said he has only seen these 15cm - long fish alive maybe eight times . " We do spend a lot of time exploring down there , so I can say with some confidence that they're quite rare , " he said .