Underwater Creature

Deep-sea gigantism
The term deep-sea gigantism describes an effect that living at such depths has on some creatures' sizes, especially relative to the size relatives that live in different environments. These creatures are generally many times bigger than their smaller counterparts. The Giant Isopod (related to the common pill bug) exemplifies this. Scientists haven't been able to explain deep-sea gigantism, with the exception the giant tube worm. Scientists believe these creatures are much larger than shallower-water tube worms because they live on hydrothermal vents that expel huge amounts resources. They believe that, since the creatures don't have to expend energy regulating body temperature and have a smaller need for activity, they can allocate more resources to bodily processes.
There are also cases deep-sea creatures being abnormally small, such as the lantern shark, which fits in an adult human's palm.