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I'm designing an alien species with a somewhat unique life cycle. When these aliens reach maturity, both males and females start producing thousands of tiny spores filled with hydrogen gas that float into the planet's thicker atmosphere. After a while, the seeds come into contact with the ground and much like a plant they start growing roots. In just one year, the seed has grown into something resembling a flower, and just like flowers on earth, it produces something akin to nectar that attracts pollinators. Soon enough, the pollinators will tranfer the genetic material from the male flowers to the female flowers, and when that happens the female flower will undergo a metamorphosis. It will gradually become thicker, bulkier, and grow spikes from its stem, anything to drive away predators, while also changing its diet by becoming carnivorous. Meanwhile, underneath the plant's stem inside the ground, an embryo starts to develop, using the nutrients the plants collects from the soil through its roots. It takes time (2 to 4 years depending on soil, weather, predators, diseases e.t.c) but eventually the young will be developed enough to crawl outside the ground and into the world.

Is any of this possible, could an animalistic creature go through a stage in its life cycle where it is essentially a plant?

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    $\begingroup$ for an alien sure, animal and plant are groups of earth life, aliens plants and animals won't be in any way related to earth plants and animals and their biology. $\endgroup$
    – John
    Feb 18 at 17:57
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    $\begingroup$ You are not goitto believe what I've found: Euglenophytes is actually an algae but shares animal characteristics. $\endgroup$
    – user6760
    Feb 19 at 2:30
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    $\begingroup$ Rationale for the duplicate flag: The most upvoted answer for both is exactly the same. The Stack duplicate question policy aims to avoid exactly this kind of redundancy. To OP, this is no demerit on your participation to the Stack. Duplicate questions serve to funnel a broader range of search queries to the information they want. $\endgroup$ Feb 19 at 13:31

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What you are describing is not formally a plant. In fact it’s not a plant at all by most definitions of the term. The correct description is that it’s an animal that has a sessile phase during it’s lifecycle. ‘sessile’ is a biological term that refers to living things that cannot move (in the sense of changing their location) by themselves.

And the answer is a resounding YES. There are actually a lot of species that have this, including (but not limited to):

  • All medusozoans (jellyfish other than comb jellies, the early phases of their lifecycle are sessile, later phases are motile)
  • Most anthozoans (almost all species are sessile as adults but some are motile as juveniles, this includes sea anemones, corals, and a variety of other species that don’t have widely recognized common names)
  • All holometabolous insects (the pupal phase is sessile, this includes beetles, bees, ants, wasps, butterflies, moths, lacewings, antlions, and a majority of things called flies)
  • All cirrapedians (barnacles, the final stage of their lifecycle is sessile, early stages are motile)
  • All tunicates (final stage is sessile, early stages are motile)
  • All poriferans (final stage is sessile, early stages are motile)

That’s (probably) six instances of this evolving independently (there may be more that I don’t know of, and I may be incorrect about whether the cnidarian species evolved this independently and whether or not holometabolous insects all evolved from a common ancestor or not), which is a reasonably good sign that it’s a viable strategy. That said, the most common strategy is the reverse of what you want, so I’m not sure how well your exact idea would work. Keep in mind that being sessile is risky, it requires far less energy than being motile, but it also means you’re much more dependent on your environment for just about everything and you’re survival is at the whims of your predators.

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Copying word to word from my previous answer to a similar question:

It looks like the baseline of what you are looking for is the life cycle of jellyfishes with the difference of not taking place in water.

jellyfish life cycle

In most cases, adults release sperm and eggs into the surrounding water, where the unprotected eggs are fertilized and develop into larvae.

The planula is a small larva covered with cilia. When sufficiently developed, it settles onto a firm surface and develops into a polyp. The polyp generally consists of a small stalk topped by a mouth that is ringed by upward-facing tentacles. The polyps resemble those of closely related anthozoans, such as sea anemones and corals. The jellyfish polyp may be sessile, living on the bottom, boat hulls or other substrates, or it may be free-floating or attached to tiny bits of free-living plankton or rarely, fish or other invertebrates. Polyps may be solitary or colonial. Most polyps are only millimeters in diameter and feed continuously. The polyp stage may last for years.

After an interval and stimulated by seasonal or hormonal changes, the polyp may begin reproducing asexually by budding and, in the Scyphozoa, is called a segmenting polyp, or a scyphistoma. Budding produces more scyphistomae and also ephyrae. Budding sites vary by species; from the tentacle bulbs, the manubrium (above the mouth), or the gonads of hydromedusae. In a process known as strobilation, the polyp's tentacles are reabsorbed and the body starts to narrow, forming transverse constrictions, in several places near the upper extremity of the polyp. These deepen as the constriction sites migrate down the body, and separate segments known as ephyra detach. These are free-swimming precursors of the adult medusa stage, which is the life stage that is typically identified as a jellyfish. The ephyrae, usually only a millimeter or two across initially, swim away from the polyp and grow.

You might object that polyps are not plants, however keep in mind that until recent times corals, also made by polyps, were thought to be plants.

And jellyfish, though not being as active as other animals, have their actions defined in their nervous systems, like we do. We are not taught how to breathe or swallow, it's something hard coded in our brains.

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    $\begingroup$ you know, if the same answer fits, word-for-word, for two questions, they are duplicates. I'm flagging the least-voted question as such. Do you disagree, diamond? $\endgroup$ Feb 19 at 13:27
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Other posters have noted animals that have similar lifecycle to this, but plants also have a analogous process. It is called the alternation of generations. Basically, individuals who are diploid (have 2 sets of chromosomes) have children who are haploid (have 1 set of chromosomes), who in turn have children who are diploid. The weird part is that diploid and haploid members of the same species can look nothing alike. In most species one of the generations remains microscopic while the other is macroscopic, but in mosses both are macroscopic and one form even grows within the other. Granted, no plant species have a mobile, animal-like form, but that's why its called science-fiction.

If you wanted to base your alien's lifecycle on alternation of generations, this is how I would recommend doing it:
1: Diploid adult 'animal' releases haploid spores into the air.
2: Haploid spores land and grow into macroscopic 'plants'.
3: Adult haploid organisms release/ receive gametes through flower-like structures.
4: Received gametes fuse with organism's own gametes to form a diploid.
5: Diploid offspring receives nutrients from parent and grows inside parent's body.
6: Diploid offspring leaves parent as macroscopic 'animal'.
7: Return to step 1.

The main advantage for this setup is that the the mobile 'animal' generations do not need a mate to reproduce, so a single individual who travels to a new location can be the start of a new colony.

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  • $\begingroup$ I'd argue that the reverse makes more sense - haploid generations are motile as finding a suitable mate is easier when you move than relying on random encounters of spores. $\endgroup$ Feb 19 at 11:02
  • $\begingroup$ Maybe I did not explain this well enough. Each individual haploid spore will grow into an individual plant, assuming it lands in the the right conditions. That plant, called a gametophyte, will spent its entire life as a haploid. Spores do not need to encounter other spores to continue their lifecycle. $\endgroup$
    – E Tam
    Feb 19 at 23:52
  • $\begingroup$ I understand, my argument was that you could have haploid "animals", that choose mates based on fitness signals to create diploid spores which grow into diploid sessile "plants" which then grow haploid "animal" offspring without the need of mate. The advantage is that now a) a "plant" that ends up by chance on a secluded place won't die without offspring - and the offspring can travel to meet mates and b) mate choice can be informed by various signals and thus more "efficient". $\endgroup$ Feb 20 at 12:36
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If by plant stage, you mean a sessile stage rather than a photosynthetic one, it's possible. The tunicate, a marine invertebrate, does it.

One notes that it devours much of its brain after it roots in. (Not all, as the article points out.)

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