Regarding the tibby's energy requirements, I think that could be part of their symbiotic relation with the land-plants. The land-plants provide energy(Thru the seedlings living in the tibby's lung/stomach) this allows the tibbys to reside in a environmental 'niche' other sea-critters avoid.
Over the course of a local year; the tibbys would provide safe-passage for the land-plants seed(Pollen?) to the oppisite hemisphere with the changing of the seasons.
The current sticking points are; I also want a more local, seasonal benefit to the land-plants and I'm still trying to work in where the sea-dwelling plants fit in. I'm thinking now the sea-plants are simply a life-stage of the land-plants.
The sea-plants use photosynthesis, ala earth plants, the land-dwelling stage lives up in mountains(above the 5000 foot level) using a chemosynthesis to break down minerals in the rocks they grow on. This relationship between tibbys, mountain+ sea stages of the plants takes advantage of specific behaviors/conditions of each, benefitting the whole.
I must confess; I didn't start out trying to make tibbys much of a part of my stories(LOL). I think of it as useful 'backgroud' work: I know when the colonists arrive on Zeta; they will quickly discover that they're not in Kansas anymore
The trick will be; the colonists must realize that wreckless behavior could seriously damage their new home. Those that learn from the eco-systems on Zeta will be the ones who accomplish the most. It won't be easy for them.....
Couple ending thoughts on tibbys themselves:
Being large can assist in maintaining body-temps, so they could be very large. I think also; that allows more room internally to be devoted to their terrarium-innards. I suspect the efficiency of said innards would increase if larger as well. They'll likely be slow-moving, not-too-agile critters, though. But they don't have to be; there are hardly any predators that can reach them.
Also; I see Tibbys having decended from land-dwelling creatures; as whales and seals seem to have on earth. Perhaps a relationship of some sort with the mountain-plants had already been established before tibbys ancestors found their niche.
I still have to figure out what killed off all the water-breathing critters, though.