Actually, if you want, I can flood you with a wall of sci-fi background explaining everything from the abovementioned energy problem, to the wormhole generation (just don't expect it to be included in any physics handbooks anytime soon
).
For now, suffice to say that the Xin ships are not entirely organic. Their biological proto-forms are matured withing gigantic subterranean caves which are excavated down on the planet and then filled with water. The caverns are completely isolated from the athmosphere and are kept within the layers of crust which are most suited for their existence. And even though there are some methods of temperature regulation involving massive geothermal capillary systems, the proto-forms themselves are incredibly tough and do not mind the extreme conditions of their habitats. After all, they have been designed to withstand the most hostile environment possible - long term exposure to vacuum of space, with all the perks like extreme temperature variations and radiation levels that encompany it.
After they mature, they are surgically fitted with integrated mechanical systems. For example, their energy comes from a gravity tidal wave oscillator (a mini-black hole container), their plasma weapons are also mechanical in nature, as is most of the outer coating of their hulls.
So basically, you are looking at big huge cyborg ships. 