Mysid shrimp. We cant give an exact species ID, they were initially introduced  into our own system tanks via wild live rock collections. They have proven to be very suitable over many years now to be maintained in a captive reef with little extra care. They are excellent as a food item for difficult to keep or raise marines like Mandarin fish, sea horses and other fishes with a diet of small crustaceans. We have also used these to successfully raise small tropical cuttle fishes. Ideally you should provide a pile of rubble or empty sea shells as a refuge to maintain a breeding colony within your system. We supply at least ten individuals in each order  more if they are easily available for us to collect from our system tanks. usually at least some are berried. They are prolific soon establishing within your system.