One of the MOST important things to be skillful at when owning hermit crabs is the ability to read their body language. For me, this took a while, but has become gradually easier as time goes on, and as I gain in experience. You'll need to be able to simply read their habits and body language in order to tell if you need to make any changes. For example, one morning I went to check on my crabs. (Now, here's a fun fact: my crabs have a small, tucked-off room in my attic. This means that the temperature in their tank often reflects the temperature outside. Ex: if it's hot outside, it's hot in their tank. In the summer, I need to turn off ALL their heat sources- and in the winter, add extra reflective sheets to keep them warm.) Now, the temperature in their tank that morning was running a bit low. I could tell that the crabs noticed- they were all just barely sticking their legs outside their shells, and the most were huddled around the back/corner edges of the tank, in order to stay close to the heat.
This attention to detail is vital for us owners. It can prevent your crabs from dying. So, what should you be looking for?
Sluggish movement (too cold, too hot)
Resource guarding (Not enough food, water, shells, or space- can look like physical attacking, shoving, mounting) (See below for more information.)
Faded colors, cloudy eyes, excess eating/water drinking- Crab is about to molt
Fast and sudden movement- Fear/stress (often seen when you interact with your crabs, or when they see/notice you)
Digging behavior- Crab either wants to molt or just wants to dig. Do not disturb or dig up your crab, except in EXTREME emergencies!
Cannibalism- Not enough protein, shells, space, or other resources. Double-check your current care and diet.
These are just some of the behaviors, not an exhaustive list. Either way, I hope this helps give an insight to your crab's behavior.
RESOURCE GUARDING EXAMPLE:
Food aggression can occur with the lack of proper nutrition, or within a tight hierarchy. Here is a picture example of how food aggression can present itself:
Rapid antenna greeting. Pumpkin (blue shell) turns her attention to Koda (brown shell). Starts to become defensive.
As Koda approaches the food bowl, Pumpkin shoves her away aggressively.
In turn, I have added an extra food bowl in the back of my tank. In doing so, all four crabs will be able to eat their fill and hopefully cut down on food aggression.
Please, ensure you are feeding your crabs a well-rounded diet. Never leave out protein and calcium- in my eyes, the two most important staples!