Ok, I know the rather annoying mathematics of animals that rely on photosynthesis. But another question got me thinking about this and I had a thought: what if my planimal gets only some of its energy from photosynthesis?
Juvenile phase: The juvenile planimal is a very small organism that can swim but not strong enough to swim against the current. It looks something like a tiny jellyfish just a few millimeters across. It is pure animal, eating anything it can get its feeding tentacles on while flapping its mantle to swim.
Transformation: When the planimal reaches about 4cm across, it begins to transform into the adult phase. This involves inflating a gas pocket (like a Portuguese man o' war), losing swimming features, and signaling the previously dormant photosynthesizing cells on the top of its mantle to develop and start processing sunlight.
Adult phase: The adult planimal will grow to about 25-30cm across and will spend its life drifting wherever the current and wind take it. It retains its feeding tentacles and will continue to use them to snag prey which will provide a significant portion of its energy needs.
Is something like this even remotely plausible?
I'm concerned that photosynthesis will never provide a non-trivial portion of this planimal's energy. If that's the case, then it's possible that evolutionary pressure will cause it to disappear