Characteristics of aquatic animals
Sharks have sensory organs able to detecting electromagnetic fields. They would use these organs to orient themselves according to the earth's magnetic field and to detect their prey.
Many fish can perceive UV light. To lure them, fishermen use UV-reflecting flies.
In photography, there are devices with "full spectrum" sensors that can capture this light invisible to the human eye.
Some jellyfish measure only a few millimeters while the largest can reach nearly 2 meters in diameter and 6 meters in height.