Is it legal for an employer to take money from my paycheck without my permission?

Hello, I recently have run into an issue with my paycheck. So, I normally receive $1013.91 on the 15th and 31st each month. I am a salaried employee. Today, I checked my paycheck only to find out that I had received $960.47. Now, the company I work for recently merged with another company and from speaking with a co-worker regarding this situation, she said that they may have removed the 5% as part of an auto opt in for 501k. Now, I was completely unaware of this, and have not signed any sort of documentation allowing this to happen. She also said that you can change the percentage, but I was not told this. I never agreed to setting up any sort of 501k nor do I want to with this company. I need that amount in full. Is this legal for them to take $53 from my paycheck without my consent? As a recent college grad with near $800 per month in student loans, every dollar is important. I appreciate your help. Thank you.