...despite the claims of your local pet shop, most species are predatory (feed on animal prey) rather than herbivorous (feed on algae). There are literally hundreds of species of Cypraea, and their diet range is just as wide as the number of species -- ranging from general scavengers to generalized omnivores to highly specific predators. Many different species are commonly imported for the reef trade, and most look so similar that it takes an expert to be able to specifically identify them.
Likewise the "true" cowries (genus Cypraea) are frequently specialists on colonial invertebrates, such as tunicates, hydroids and especially sponges â€“ if you count the number of cowries that consume a given prey item, sponges are certainly the winner. Although I say that these animals prefer a certain prey item, they should probably be considered omnivorous, really.
For all of the cowries for which the diet range is well known, they naturally consume a variety of animal and/or plant matter. Most species will graze anything from algae to sponges and cnidarians, but the majority of them are definitely eating a large proportion of animal prey in their normal diet.
According to "Reef Invertebrates", pages 197 & 198 by Anthony Calfo and Robert Fenner,The bottom line is that cowries have highly variable diets and there are even individual variation that comes into play. You never really know until you have the animal in your tank how it will react, or if that will change when anything new is added to the tank.
They are quite variable in suitability for aquarium life, but mostly should be avoided. Some are carnivorous and even the safe herbivorous species tend to grow large and clumsy.
....some cowries feed on sponge, corals, ascidians, or colonial anemones and cannot realistically be kept at all. For many others, their diet is not even known at this time.
Aquarists seeking reliable herbivores in this category should look first to species like: Cypraea moneta (the Money Cowry), C. annulus (the Gold-Ringed Cowry), and C. caputserpentis (the Snakehead Cowry).
Gosh, sorry for being so long winded .... I hope some of this information helps.It should also be mentioned that cowries tend to be rather strict in regards to water quality requirements (including high saturated oxygen levels) and can be difficult to ship.