Sea cucumbers eat plankton and other organic matter. Some position themselves in a current that brings a steady supply of food, and spend hours there. The tentacles are opened up and collect food in the current. The sea cucumber brings each tentacle to its mouth and licks it, while the other tentacles go on collecting food. Other sea cucumbers feed by sifting through sand using their tentacles.
Many sea cucumbers are poisonous.
If injured, sea cucumbers can kill fish in the same aquarium as them. The sea cucumber has an interesting and slightly disgusting way of defending itself. This either scares off or satisfies predators. The cucumber can then grow another set of internal organs. Some sea cucumbers secrete a very sticky substance as a defense mechanism.