The Remarkable Telescopefish

The ocean is home to a myriad of otherworldly creatures with faces and bodies that capture the imagination. The telescopefish is one such fish with a widespread reputation for being bizarre. With magnified eyes and a toothy grin, this deep-sea fish has garnered a die-hard fan club across the internet. Most people have never seen one in real life or even on video. In fact, the telescopefish is largely known to non-scientists from just a couple of photos.  Here to dispel the misinformation and to quell the need for all things telescopefish, this article dives into what we do know about this alluring animal.  

Telescopefish live in tropical and subtropical areas around the globe and include two species that differ by a few fin rays, Gigantura chuni and Gigantura indica. Their name can be a bit misleading—neither one grows to sizes larger than a foot (26 cm) in length and it is, in fact, their noticeably large eyes that inspired their namesake. Both species have elongated bodies, tubular eyes, ribbon-like tails, silvery coloration, and lack scales.  This is the telescopefish most people have come to know and love. But telescope fish begin their life as a remarkably different fish. As synchronous hermaphrodites (having both male and female reproductive organs), adults spawn both eggs and sperm into the open ocean where a lucky few fuse and grow into larval fish. The tiny larvae then undergo one of the most dramatic transformations among fish. As larvae, or the “Rosaura” stage, they are small, globular, and stout—larval telescope fish look so different from their adult form that for over 80 years they were considered two different species. Upon maturing, they undergo a major transformation that involves changes to bone structures and body shape. Scientists still do not know how this remarkable transformation occurs. 

The goofy appearance of an adult telescope is the result of millions of years of evolution. The midnight zone (aka mesopelagic) is a dark place nearly void of light, and for those living there having sharp eyes is key to survival. Telescopefish live between 1,600 and 6,500 feet (500 to 2,500 m) deep and are known for their tubular eyes that look like binoculars. These remarkable eyes likely capture even the smallest amount of light. Looking through the vastness of the sea, a shimmer or pulse of bioluminescence can mean a potential meal and detecting that light requires acute eyesight. Looking toward the sea surface can also reveal unsuspecting prey—prey up above can appear silhouetted against the filtered sunlight. It is likely that telescope fish use both methods to find prey. In two recent expeditions, two telescopefish were filmed drifting parallel to one another, both with their heads aimed at the surface. 

Telescopefish are also remarkable due to their jaw anatomy, with fish experts noting they are “among the most specialized and distinctive” of the bony fish. With limited prey in the vast sea, it is advantageous to consume all available prey, regardless of size. Telescopefish do just that. With a special jaw anatomy, the telescope fish can eat prey larger than their own bodies. Without normal bones in their jaw, they can expand their mouth to seemingly impossible widths. Scientists refer to this type of feeding as “raptorial feeding”, and it is similar to the extreme capabilities of moray eels.  In one instance, scientists discovered prey two times the size of the telescopefish folded inside its stomach. Also, since most prey are bioluminescent, the telescopefish has a black and opaque stomach to shield the light emitting from the consumed prey. Otherwise, the light could attract other predators.    

With only two live encounters of telescopefish, one of G. chuni and one of G. indica, there is still much to learn about this fascinating fish. Both encounters witnessed two telescopefish side by side, heads pointed to the surface, and there is now ongoing research to determine if telescopefish tend to live in pairs.  Perhaps, living with another helps with survival. The ocean is a vast and lonely place, yet the telescopefish has found a way to thrive despite these challenges. Now, those adaptations capture the imagination of millions of people from across the globe.