| Summary: | The mesopelagic zone, often referred to as “the twilight zone” can be defined as the area of the ocean at 200-1000 m depth. Here, light levels vary greatly and provides a varying visual scene. At shallow depths, down-welling daylight is the dominant light-source, but at greater depths, bioluminescent point-sources take over. Mesopelagic fish, therefore, have a variety of eye-sizes and other visual adaptations for better vision in the zones they inhabit. Several adaptations are designed to increase the eyes sensitivity to light because it is so sparse in the mesopelagic, and being able to take advantage of the light that is there is crucial. A larger eye provides extra sensitivity but is also costly. A smaller eye will have less sensitivity, but in a dark environment, it might be enough to distinguish a bioluminescent flash from the background. Therefore, I hypothesize that fish with relatively small eyes select bioluminescent prey because they are easier to see. In this thesis, I compare the diets of mesopelagic fishes of different eye-sizes and other visual adaptations to see whether there is a link between the size or type of eye they possess and whether they have a selectivity for bioluminescent prey in the two hatchetfishes Sternoptyx diaphana and Argyropelecus hemigymnus and the four lanternfishes Protoyctophum arcticum, Benthosea glaciale, Notoscopelus kroyeri and Lampanyctus macdonaldi. I use samples and data from two separate cruises done by The Institute of Marine Research (IMR) on R.V G.O. Sars. I determine the eye-body-ratio, do stomach content and diet composition analyses, and use Ivlev’s electivity index to compare the diet of the studied fish with plankton data from the water column. My aim was to determine whether a pattern can be found of small-eyed fish selecting bioluminescent prey in larger proportions than their large-eyed counterpart. I find that although there is a difference in selectivity especially between the hatchetfishes and the lanternfishes, the small-eyed lanternfish in this study ...
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