I was playing around with my planimal idea when it occurred to me that my world could reinvent fish if I simply tweaked the starting point then applied neoteny (i.e., if they grew to maturity while retaining their motile juvenile form). But being as I’m somewhat allergic to the mundane, I decided to limit neoteny to just the male form. That gave me the following:
juvenile form: As a juvenile, my species is a relatively good subcarangiform swimmer (although turned 90° so the undulations are vertical) with several feeding tentacles and fleshy fins.
original adult form: As they transition to adult form, they inflate a gas pocket on their back and lose their swimming features (fins, tail, etc.). Their feeding tentacles get longer and more robust. In addition (because this started with my planimal design) they activate photosynthesizing cells on their upper surfaces. And, of course, they fully develop sexually.
APPLY MALE NEOTENY PATCH HERE
new adult form (male): Applying simple neoteny, the male form is a slightly larger version of the juvenile with a couple of changes. They are fully sexually mature (which is self-evident given they are adult form). They also get elongated pectoral and/or ventral fins with limited grasping ability to aid in mating.
new adult form (female): essentially unchanged from original adult form.
To my knowledge, no earth species has this type of sexual dimorphism. It’s possible that I’ve missed something but, given the amount I’ve read recently on sexual dimorphism, I would have expected someone to have mentioned this if it existed. So, assuming it does not exist…could it?
Could sexual dimorphism be realized as an extreme difference in motility?
I’m really trying to understand any challenges I need to consider so the more detail you can provide in any objections would be appreciated. Also, if I’m wrong and this does exist somewhere, please point me to the species.
Based on responses I'm thinking that maybe I should clarify my question. I attempt that below. Hopefully I don’t make things more confusing.
My goal here is a species that is radically sexually dimorphic but not with respect to size.
The adult male will essentially be a fish-like creature that swims the oceans looking for food and females. It will probably mate with multiple females over its life (and will likely try to mate with every female it encounters). Other than mating, the male is solitary (i.e., it will mate then move on).
The adult female, in contrast, will be a sessile floating creature somewhat like the Portuguese man-of-war (but not colonial) drifting with current and/or wind. It will use feeding tentacles to capture food underwater while also hosting photosynthesis on its upper side. It can also expect to mate with multiple partners over its life (it may or may not accept every male that comes along). I’m also using some form of sperm storage to increase genetic diversity. I haven’t worked out the details yet, but they will likely evolve to be able to attach to other females and form same-sex communities.
I’m using male-exclusive neoteny to get there. This can be seen as analogous to a sex-flipped version of the larviform female (i.e., the male retains what is essentially the larval/juvenile form).
Side Note: Probably not critical here but my world is a pure ocean world (no land, minimum depth of several kilometers). So there’s no worry about my females being blown onto shore since no shore exists.
Hopefully that clarifies my target here. If not, feel free to ask questions in the comments and I’ll keep trying to refine this.