If 'Rock shrimp' are an example of filter feeding shrimp, they are extremely passive. They lack claws, and their mouths are designed for preening food out of their fan like filters. Even newly hatched shrimp would be large and quick enough to get away from a filter feeder. Filter feeders don't hunt, or ambush their food. If they are not stationary, in a fast current, they slowly walk and rub their fans on rocks, scraping off algae and microbial food.