I just purchased a tiger cowrie yesterday and am wondering what do they eat? I know they have a mantle that covers their shell when they are feeding, and my pet shop told me that they are generalist feeders who make great scavengers in the tank, but what are they actually feeding on? Is it algae? Are they filter feeding with those frills on their back or what?
Let me start by explaining some terms I’ll use before I get into this answer. Snails are members of the Class Gastropoda in the Phylum Mollusca. The molluscs include such familiar groups as the chitons (Class Polyplacophora), the snails & slugs (Class Gastropoda), the bivalves (clams, oysters, mussels and the like all fall into Class Bivalvia), and the squids, octopus and cuttlefishes (Class Cephalopoda). All of these animals share a number of features that unite them as molluscs, but the one of primary importance here is that they have a fleshy fold of tissue (called the mantle) that surrounds the rest of the body, and secretes the shell (in species that have a shell).
Depending on who you ask, "Cowrie" is the common name for any snail that has a shell which is sort of folded around itself, and has a mantle which extends to cover it's shell (or part of it) when the animal is active (this would include both the Cypraeids and the Ovulids) or specifically those snails of the genus Cypraea. In either case, 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. It is common to see gastropods are misidentified in pet shops, and unfortunately for you as a consumer, it is almost impossible to tell what the diet will be from any common name (the suppliers give the animals simple common names, and unless the pet shop staff really knows what the snails look like, they have nothing else to go on). I often see several different species in a tank with a single name on them, and all of this is working against you when you’re trying to figure out what you have and how to care for the animal.
Despite that problem, if you can identify the animal there are a number of generalizations that you can safely make about the different groups. The egg & spindle cowries (Ovulids) are primarily cnidarian predators, typically specializing on one or a few species of soft corals, although many species may accept other foods (even algae) if they get hungry enough in an aquarium. For example, the Flamingo Tongue (Cyphoma gibbosum) may be one of the most commonly photographed snails in the Caribbean, and preys exclusively on the tissue of gorgonians. Likewise the "true" cowries (genus Cypraea) are frequently specialists on colonial invertebrates, such as tunicates, hydroids and especially sponges – 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.
All of the cowries for which the diet range is known 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.
To identify your animal, it should have a white to rich auburn shell with only black spots of variable size and random pattern on it. When crawling, the mantle should completely cover the shell, and there should be an obvious grey and black “finger-print” (for lack of a better description) veining to the mantle. Spread across the surface of the mantle, you’ll see numerous projections (called papillae) which are unbranched and end in a smoothly rounded, opaque white tip. If the papillae are branched or the mantle lacks the veining pattern, then you have not actually been sold a tiger cowrie, but one of the several other species that superficially resemble this best-known of the cowries. If the identification is correct, and you tiger cowrie is really Cypraea tigris, it can reach a full size of roughly 5 inches in length, and the mainstays of its diet are sponges and algae.
The reason that these types of snails are typically popular with aquarists is that both the shell and the mantle extended around it are typically brightly colored and the mantle is often highly elaborated, making them an attractive addition to the aquarium. However, most of these animals are nocturnal (only active at night), so they are rarely seen in the aquarium while the lights are on for viewing. More importantly, however, the biological reason for having that brightly colored and elaborated mantle is thought to be a warning to predators that they taste bad; and when snails taste bad, its almost always because they're eating something that is chemically defended (such as a sponge or tunicate). Like some nudibranchs (one of the groups of Opisthobranch molluscs commonly known as sea slugs), some cowries also share the ability to harvest unfired nematocysts (the stinging cells characteristic of cnidarians) from the corals and anemones they eat and move them onto the frilly extensions on their back, where the snails can use them for their own defense. Although their preferred diet consists of algae and sponges, I have watched my tiger cowrie (Cypraea tigris) eat tentacles off an anemone in one of my tanks. Together with the sequestering of toxic chemicals that make the snails taste bad, and the collection of unfired nematocysts, these snails are well defended against predators in the wild and there are relatively few animals that eat them.
The bottom line, however, is that although cowries are beautiful and interesting animals, they are not well suited to a reef tank, and should probably not be added to one. There is no reliable way to even hazard a reasonable guess at what some unidentified cowrie eats when you come across it in the pet shop, and other than the fact that it is certainly not a filter feeder, and it almost certainly does include some animal matter in its natural diet, there is not much else I can tell you about most cowries. I'd make two suggestions for you 1) always find out what an animal eats and how to care for it properly BEFORE you buy it in the future, and 2) keep a close eye on your new snail to make sure that it's not one of the species that has a penchant for eating your prize tank inhabitants....