Marine hatchetfish (Argyropelecus lychnus). X-ray of a section through a tropical marine hatchetfish, showing its large jaws (left). Marine hatchetfish live in the dark depths of the ocean. This specimen was collected at a depth of around 850 metres in the Pacific Ocean. As with many other deep-sea fish, their eyes (upper left) are large and their bellies have numerous small cream-colored spots (light organs) that are bioluminescent. Each species emits its own unique pattern of light. This specimen measures 7.5 centimetres in length.