Of things in the Waters, you may eat anything that has Fins and Scales
Shell-fish are not kosher one suspects at least partly because of their capacity to filter and quarantine marine microbes. Specific marine microbes are now being cultured and used to create an artificial feed (Novacq) that allows for a 30% faster growth rate than compared to the usual pellet-fed prawns (the main ingredient of the traditional pellets fed to farm prawns being wild fish). The metabolism of these 'filters of the ocean' is enhanced by adding a lot more microbes to their feeds; perhaps mimicking their more natural oceanic environ. It's a dirty animal to begin with - then we try and grow it in (cleaner) farms - and then we add buckets of microbes to grow them faster. Just how nature intended... NOT! Source: Scientists create prawn super food Kashrut: Jewish Dietary Laws