Neuromag May 2017 - Page 22

Even the early investigators at the end of the 19th century have noted that most deep-sea fish have strikingly large eyes. Demersal (deep bottom liv- ing) species are relatively large (up to >1m), and the general body shape is not affected by big eyes. However, fish living in the water column between 500 and 1,000m (called mesopelagic) are considerably smaller (5-35cm) and their large eyes and consequently large heads may conflict with a more delicate body shape and their ability to swim agilely. Tubular eyes are found in many mesopelagic fish species (Fig. 1) and may be considered as a mecha- nism to overcome this dilemma. They may be interpreted as cylindrical cen- tral segments of the ordinary hemi- spherical eyes. This design conserves the large pupillary opening, and the lens projects a focused image onto the well-developed retina at the bot- tom of the cylindrical eye. The vertical walls of the tubular eye are lined with an “accessory“ retina characterised by thinned layers receiving unfocussed light. The advantage of saving volume comes at a serious drawback, how- ever, being that Tubular eyes have a much-reduced visual field. In most spe- cies, the optical axes are directed dorsally making them optimally suited to detect silhouettes of ani- mals above that block the residual sunlight. On the other hand, this eye design would make their bearers vulnerable to attacks from other directions. as details, the diversity of evolution- ary adaptations in eye design found in deep-sea fishes with their different optical environment, have fascinated researchers for ages. However, the functions of some structural modifi- cations, though known for decades, have not been understood. On one of my previous deep-sea expeditions with the German research vessel “FS Sonne,” we were lucky to make some progress in this respect. In the area of the Tonga trench, in a trawl from between 600 and 800m, we discovered a f