They appear to be non humanoid, multiple limbed symbiont organisms (though only a minority are symbiont with a Leviathan). Besides this not much is known about their physiology, only that they have the ability to join with a Leviathan (a biomechanical living starship), have superior healing abilities to most humanoids and possibly limb regeneration and that they can live in excess of one thousand years (however if they join with a Leviathan this is shortened to roughly three hundred years).