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

  • 1
    $\begingroup$ Can you narrow it down to the one question, linked posts each having a question in its own thread are just fine though. $\endgroup$ Sep 8, 2021 at 17:41
  • $\begingroup$ ??? This supposedly barely plausible animal is very similar to real-life coral animals. They have a free-swimming planula larva. They obtain most of their energy from photosynthetic symbiotic algae. They use their tentacles to catch food (mostly as a source of protein, but secondarily as a supplement of energy). $\endgroup$
    – AlexP
    Sep 8, 2021 at 19:11
  • $\begingroup$ I actually modeled my creature on coral. I was not aware of the extent to which they rely on photosynthesis and was under the impression they gained most of their energy via eating. More significantly, I was looking for something larger and not symbiotic. $\endgroup$
    – legio1
    Sep 8, 2021 at 19:32
  • $\begingroup$ this is actually a good question in therms - it is clear and detailed enough, so as an answer can be quite good with real-world examples and such - idk why closers demand more focus it all sides of one coin. Vote to leave it open. $\endgroup$
    – MolbOrg
    Sep 9, 2021 at 20:11
  • $\begingroup$ @ARogueAnt. I'm not sure I see how this is really multiple questions. The only real question is the first (is this plausible?). The other two merely describe concerns I have that might make the answer no. I could have as easily phrase it: "Is something like this even remotely plausible? I'm concerned that photosynthesis will never provide a non-trivial portion of this planimal's energy and, given that, evolutionary pressure will cause it to disappear" $\endgroup$
    – legio1
    Sep 9, 2021 at 20:51

3 Answers 3


There are many organisms that do what you propose.




Cassiopea (upside-down jellyfish) is a genus of true jellyfish and the only members of the family Cassiopeidae.1 They are found in warmer coastal regions around the world, including shallow mangrove swamps, mudflats, canals, and turtle grass flats in Florida, and the Caribbean and Micronesia. The medusa usually lives upside-down on the bottom, which has earned them the common name. These jellyfish partake in a symbiotic relationship with photosynthetic dinoflagellates and therefore, must lay upside-down in areas with sufficient light penetration to fuel their energy source.[2] Where found, there may be numerous individuals with varying shades of white, blue, green and brown.

Cassiopea is one of several jellyfish that have onboard photosymbionts. The Golden Jellyfish is another good one if you are digging it - it is still motile as you propose, but I think Cassiopea looks cooler so I used its image. Many other creatures (sponges, molluscs, corals, worms etc) have free living early stages, then acquire wild photosymbionts and settle down for a life in the sun. Some stay motile even with the photosymbionts.

The photosymbionts provide carbon (sugar) and get a place to live in exchange. The organisms still catch some food which they use for protein - none are totally photosymbiotic which I think is because of the protein issue.

  • 2
    $\begingroup$ This answer is a good reminder that, when a living being can't do it alone, nature can always mash them up with something else to make something new and weirder. Like our little pals, the Mitochondrion. $\endgroup$
    – Mermaker
    Sep 8, 2021 at 20:07
  • $\begingroup$ So, I was under-ambitious? I was hoping to have this be a single organism rather than a symbiont but maybe I could roll back the evolutionary clock and have a primal form permanently incorporate photosynthesizing bacteria (the way earth plants did)? $\endgroup$
    – legio1
    Sep 9, 2021 at 15:59

It can be a plant like animal (an animal that evolved photosynthesis), an animal like plant (a plant that evolved the ability to move about freely), a symbiotic relationship between an animal and a plant, where the animal lives off of the energy that the plant produces, or an entirely different organism (colonial cyanobacteria).


You can consider the animals that are capable of kleptoplasty. Kleptoplasty is a symbiotic phenomenon whereby plastids, notably chloroplasts from algae, are sequestered by host organisms. https://en.wikipedia.org/wiki/Kleptoplasty

You can consider the species of Elysia, a genus of sea slugs. The longest known kleptoplastic association, which can last up to ten months, is found in Elysia chlorotica (the eastern emerald elysia).

enter image description here
An E. chlorotica individual consuming its obligate algal food Vaucheria litorea

Elysia sea slugs graze on algae and some species such as E. viridis and E. chlorotica hijack the chloroplasts for themselves. The chloroplasts end up lining the slug’s digestive tract, enabling the slugs to survive solely by photosynthesis for several months at a time. This association is crucial for the development and maturing of the slug.


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