The idea is basically the same as a planet. Each one is a self contained eco-system, the basic resources for which are garnered during the egg stage from the leftovers of the dead parents and surrounding environment. Eventually everything that can be used inside is, that is old age for them, and nothing can stop it or extend lifespan. In addition, the secondary source of energy is the sun. They evolved on a planet with no atmosphere composed of ice. This means very low electromagnetic field and high ambient radiation. Their bodies are capable of absorbing and harnessing the radiation. A sort of super photosynthesis that keeps their internal eco-systems running.
They are an very biologically complex species with an evolutionary timeline that when compared with earth would start with the earliest dinosaurs and end... now.
Note the species did not evolve on an airless void world, as the original atmosphere was liquid rich and very thick. The world slowly died due to the lack of an electromagnetic field and all other life was wiped out as the atmosphere was slowly torn away by solar winds and steadily dropping surface temperatures. They managed to evolve in a way to enable survival. Their planets Version of the cockroach.
Please pick them apart freely. I want to try and develop them as much as possible. Finding flaws or making suggestions would be quite helpfull.