Hi,
As we know about Caves that they are hollow places on the ground that are big enough for the entry of a human, but not great places to live in for him. They are basically formed through some natural processes that occur on land and on ground parts of mountains. The natural processes may include weathering of rocks etc.
So if not humans then who lives in caves? Well, there are some some living organisms like cave shrimp, isopods, some cave salamanders,amphipods, millipedes, and insects that can be found in Caves.
What can be their unique feature?
Their adaptability to live in Cave conditions is important aspect unique feature which can manifest in several forms, such as Loss of eyesight (because there is always dark in the caves and they donot need light anymore), , loss of body pigments (because they are almost never exposed to light so donot need pigments for protection or other related purposes) development of special sensory organs for sense perception like long antennae and hair (in troglobites) are some unique features of cave living organisms.
Hope it helps!