You cannot directly answer the question “What does a crab eat?” because the question is actually too general. For example, you can’t directly answer the question “What does a bird eat?” either because you don’t know what species the question is about.
So what a crab eats depends very much on which species you have but also on the age within this species. Adult crabs will eat different things than their young do.
So let’s start at the beginning.
You may also want to know what a chipmunk eats.
What does the larva of a crab eat?
A larva is a crab that has just hatched from an egg. It does not yet look at all like a crab but a bit like a shrimp, often floating on the surface of the water along with other plankton.
At this stage of life they often eat food floating in the water, this can be different kinds of algae but also other animals even smaller than themselves.
In studies in aquariums, there have been tests feeding microalgae and phytoplankton to the larvae of crabs and lobsters. Brine shrimp were also fed in the later stages of the larvae.
It is not known from many crab species how the development of larvae goes, as many species remain on the bottom so it is very difficult to do research on this.
What does a crab eat as an adult?
Many crab species are omnivorous and thus eat everything available. Depending on the habitat, they eat crustaceans, starfish, worms, shellfish, snails and fish. They can also eat plant foods such as fallen leaves, seaweed and other plant species. Crabs will also not bother to eat their own conspecifics, this is called cannibalism.
Since crabs are also scavengers they will not let food pass them by, which is why you can catch crabs quite easily with a trap that contains bait.
There are species of crabs that have somewhat more specialized eating patterns. For example, there are crabs that eat sand, not the sand itself but small food particles that are in between the grains of sand. The sand is worked back out and is visible as small balls of sand.
There are crabs that filter plankton from the water as a meal. It is possible for a crab to live commensal to another animal. In symbiosis, both organisms benefit from cooperation; in commensal ecology, one party benefits but the other does not suffer. An example is the Pea crab, which can live in a mussel or other shellfish.
How does a crab eat?
A crab has two claws on the front of its body which are perfect for picking up food and putting it in its mouth. Often one of the claws will be used to hold the food and the other claw will be used to tear off small pieces and bring them to the mouth. Some species grab pieces of food with both scissors and then work them in alternately. This scene looks just like a person very greedily wolfing down food.
Both are very fun to watch so it is not surprising that people enjoy keeping crabs in an aquarium. You can then get a very close look at how and what a crab eats.