Just what do turtles eat? The answer to that depends upon the species of turtle. Though turtles in general eat mostly vegetation, some eat insects or small animals, and some eat marine life of one kind or another. Some turtles, like sea turtles, spend most of their time in salt water. Others spend a part of there time in freshwater streams or ponds, and part of the time on land. Still other species of turtle spend nearly all of their time on land. What do turtles eat? That has a lot to do with where they spend most of their time.
Turtles and tortoises are closely related. With very few exceptions, tortoises are herbivores, relying 100% or nearly so on plant life. Turtles on the other hand, tend to be either carnivores or omnivores. The turtles that spend much of their time in the water, the so-called aquatic turtles, tend to be carnivores. Turtles that spend half of their time, or nearly all of their time on land, are for the most part omnivores, with half of their diet consisting of vegetation, and the other half made up of insects, snails, slugs, worms, or anything slow enough to catch. Omnivores seem to be particularly fond of crickets, often a staple food for turtles in captivity.
No Cat Food, No Dog Food - What do turtles eat in captivity? An easy answer would be, they usually don't eat what they should be eating. There's more than a little truth to this. The biggest mistake some per owners make is to feed a pet turtle a little cat food or dog food, probably under the misguided assumption that the turtle in a pet, so pet food is what it needs. As one expert on turtles expressed it, "dogs are a species, cats are a species, but there are many species of turtles". The point is, different species of animals have different dietary needs, and though a turtle may be a pet, it is not a truly domesticated animal, meaning we don't always know exactly what we should be feeding it, unless we know exactly what species of turtle it is. Aquatic turtles for instance, are largely meat eaters, but some species, as they grow older, come to relay more and more on vegetation, or at least make a partial transition from carnivore to omnivore.
If You Don't Know What Your Turtles Eats - Unless you have a sea turtle as a pet, in which case you'll need to stock up on jellyfish, mollusks, and salt water vegetation, you might not know exactly what species your pet is (though you should know if it is aquatic or semi-aquatic). You may get by on fresh vegetables, clover and dandelion can be good choices, though some species actually prefer grass, and some insects, perhaps crickets, to see what your turtle's preferences are. If it eats mainly vegetation, that is where its preferences will lie. A good piece of advice is to avoid too much protein, which is not good for a turtle. Another good piece of advice is to make certain that whatever the turtle eats contains some calcium, otherwise provide some calcium supplements. A turtle needs plenty of calcium to keep its shell fit and healthy. Overfeeding is especially to be avoided. An overfed turtle is not a happy turtle, and may soon become a very sick turtle.
Summary - In summary, given all the different species of turtles in existence, the food they eat is quite wide ranging. For a given species however, the answer to what do turtles eat can be quite narrow, and a turtle owner really needs to know and understand the species to know what a given turtle's dietary needs are going to be.